Article

# Discovery of a Double Circumnuclear Ring and Minibar in the Starburst Galaxy M83

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## Abstract

Our (J-K) color observations of the central regions of the starburst galaxy M83 reveal a double circumnuclear ring. The main dust lanes spiral into the outer nuclear ring at a radius of 150 pc. The two rings may coincide with two inner Lindblad resonances. The main hot spots occur in an arc that is between the rings. A dust bar that is offset 90° from the primary stellar bar connects the outer nuclear ring to the inner nuclear ring at a radius of 50 pc and may provide the path for gas to flow to the central starburst.

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... The weighted average location of the kinematic centre from Knapen et al. (2010) is shown as a black plus. A schematic of the main structural components observed by Elmegreen et al. (1998) is overlaid and separated by colour. Red represents the dust lanes, blue is the outer circumnuclear ring, purple the inner circumnuclear ring, and green represents the narrow bar or 'bridge' connecting these latter two components. ...
... Sub-arcsecond angular resolution J and K band observations of the circumnuclear region of M83 show two prominent dust lanes (red dotted line in Fig. 1) spiralling into a circumnuclear dust ring at a galactocentric radius of a few hundred pc. As shown in Fig. 1, the outer circumnuclear ring (blue dotted line) is connected to an inner circumnuclear ring (purple dotted line) via a narrow inner bar or 'bridge' (green dotted line) perpendicular to the primary stellar bar (Elmegreen, Chromey & Warren 1998). The area between the two rings was identified as being a region of intense star formation. ...
... The sharp cut-off in the age distribution (Harris et al. 2001) would suggest the former is far more likely (Lamers, Gieles & Portegies Zwart 2005;Kruijssen et al. 2011). A majority of these star clusters are located within the star formation arc identified by Elmegreen et al. (1998); they are shown as blue crosses in Fig. 1. The population of clusters is highly asymmetric with respect to the optical nucleus (orange star), in the south-western space between the inner and outer circumnuclear rings. ...
Article
In the centres of the Milky Way and M83, the global environmental properties thought to control star formation are very similar. However, M83’s nuclear star formation rate (SFR), as estimated by synchrotron and Hα emission, is an order of magnitude higher than the Milky Way’s. To understand the origin of this difference we use ALMA observations of HCN (1 − 0) and HCO+ (1 − 0) to trace the dense gas at the size scale of individual molecular clouds (0.54″, 12pc) in the inner ∼500 pc of M83, and compare this to gas clouds at similar resolution and galactocentric radius in the Milky Way. We find that both the overall gas distribution and the properties of individual clouds are very similar in the two galaxies, and that a common mechanism may be responsible for instigating star formation in both circumnuclear rings. Given the considerable similarity in gas properties, the most likely explanation for the order of magnitude difference in SFR is time variability, with the Central Molecular Zone (CMZ) currently being at a more quiescent phase of its star formation cycle. We show M83’s SFR must have been an order of magnitude higher 5 − 7 Myr ago. M83’s ‘starburst’ phase was highly localised, both spatially and temporally, greatly increasing the feedback efficiency and ability to drive galactic-scale outflows. This highly dynamic nature of star formation and feedback cycles in galaxy centres means (i) modeling and interpreting observations must avoid averaging over large spatial areas or timescales, and (ii) understanding the multi-scale processes controlling these cycles requires comparing snapshots of a statistical sample of galaxies in different evolutionary stages.
... Sub-arcsecond angular resolution J and K band observations of the circumnuclear region of M83 show two prominent dust lanes (red dotted line in Figure 1) spiralling into a circumnuclear dust ring at a galactocentric radius of a few hundred pc. As shown in Figure 1, the outer circumnuclear ring (blue dotted line) is connected to an inner circumnuclear ring (purple dotted line) via a narrow inner bar or 'bridge' (green dotted line) perpendicular to the primary stellar bar (Elmegreen et al. 1998). The area between the two rings was identified as being a region of intense star formation. ...
... The sharp cutoff in the age distribution (Harris et al. 2001) would suggest the former is far more likely (Lamers et al. 2005;Kruĳssen et al. 2011). A majority of these star clusters are located within the star formation arc identified by Elmegreen et al. (1998), they are shown as blue crosses in Figure 1. The population of clusters is highly asymmetric with respect to the optical nucleus (orange star), in the south-western space between the inner and outer circumnuclear rings. . ...
... The weighted average location of the kinematic centre from Knapen et al. (2010) is shown as a black plus. A schematic of the main structural components observed by Elmegreen et al. (1998) is overlaid and separated by colour. Red represents the dust lanes, blue is the outer circumnuclear ring, purple the inner circumnuclear ring and green represents the narrow bar or 'bridge' connecting these latter two components. ...
Preprint
In the centres of the Milky Way and M83, the global environmental properties thought to control star formation are very similar. However, M83's nuclear star formation rate (SFR), as estimated by synchrotron and H-alpha emission, is an order of magnitude higher than the Milky Way's. To understand the origin of this difference we use ALMA observations of HCN (1-0) and HCO+ (1-0) to trace the dense gas at the size scale of individual molecular clouds (0.54", 12pc) in the inner ~500 pc of M83, and compare this to gas clouds at similar resolution and galactocentric radius in the Milky Way. We find that both the overall gas distribution and the properties of individual clouds are very similar in the two galaxies, and that a common mechanism may be responsible for instigating star formation in both circumnuclear rings. Given the considerable similarity in gas properties, the most likely explanation for the order of magnitude difference in SFR is time variability, with the Central Molecular Zone (CMZ) currently being at a more quiescent phase of its star formation cycle. We show M83's SFR must have been an order of magnitude higher 5-7 Myr ago. M83's starburst' phase was highly localised, both spatially and temporally, greatly increasing the feedback efficiency and ability to drive galactic-scale outflows. This highly dynamic nature of star formation and feedback cycles in galaxy centres means (i) modeling and interpreting observations must avoid averaging over large spatial areas or timescales, and (ii) understanding the multi-scale processes controlling these cycles requires comparing snapshots of a statistical sample of galaxies in different evolutionary stages.
... On the other hand, the central regions of M83 in the infrared are rather complex. The J – K images of the inner region show two non-concentric circumnuclear dust rings, which are associated with two inner Lindblad resonances (Elmegreen et al. 1998). These two rings are connected by an inner bar, almost perpendicular to the main stellar bar. ...
... The Brγ emission is mainly associated with the inner starforming arc toward the west side of the FoV. The global velocity gradient of ∼60 km s −1 from northwest to southeast is consistent with an inflow of gas along the spiral arms and through the inner bar (Elmegreen et al. 1998; Crosthwaite et al. 2002; Fathi et al. 2008) to the photometric center of the galaxy. Superimposed on this, there is a ring that is on and around aperture A inFigure 4. One of the most remarkable properties of this ring-like feature is that it shows no velocity gradient and has a low velocity dispersion that, together, argue against it being a dynamical structure. ...
Article
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The barred grand-design spiral M83 (NGC 5236) is one of the most studied galaxies given its proximity, orientation, and particular complexity. Nonetheless, many aspects of the central regions remain controversial, conveying our limited understanding of the inner gas and stellar kinematics, and ultimately of the nucleus evolution. In this work, we present AO VLT-SINFONI data of its central ~235 × 140 pc with an unprecedented spatial resolution of ~0.2 arcsec, corresponding to ~4 pc. We have focused our study on the distribution and kinematics of the stars and the ionized and molecular gas by studying the Paα and Brγ emission in detail, the H2 1-0S(1) line at 2.122 μm, and the [Fe II] line at 1.644 μm, together with the CO absorption bands at 2.293 μm and 2.323 μm. Our results reveal a complex situation where the gas and stellar kinematics are totally unrelated. Supernova explosions play an important role in shaping the gas kinematics, dominated by shocks and inflows at scales of tens of parsecs that make them unsuitable to derive general dynamical properties. We propose that the location of the nucleus of M83 is unlikely to be related to the off-center "optical nucleus." The study of the stellar kinematics reveals that the optical nucleus is a gravitationally bound massive star cluster with M dyn = (1.1 ± 0.4) × 107M ☉, formed by a past starburst. The kinematic and photometric analysis of the cluster yield that the stellar content of the cluster is well described by an intermediate age population of log T(yr) = 8.0 ± 0.4, with a mass of M (7.8 ± 2.4) × 106M ☉.
... M83 (see also Figure 7) is a grand-design spiral with Hubble type SAB(s)c and a central starburst [40]. The two COS spectra were taken at this central region, for two UV sources separated by about 150 pc (see Figure 9). ...
Preprint
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Spectroscopic observations of a massive star formation in the ultraviolet and their interpretation are reviewed. After a brief historical retrospective, two well-studied resolved star clusters and the surrounding H II regions are introduced: NGC 2070 in the Large Magellanic Cloud and NGC 604 in M33. These regions serve as a training set for studies of more distant clusters, which can no longer be resolved into individual stars. Observations of recently formed star clusters and extended regions in star-forming galaxies in the nearby universe beyond the Local Group are presented. Their interpretation relies on spectral synthesis models. The successes and failures of such models are discussed, and future directions are highlighted. I present a case study of the extraordinary star cluster and giant H II region in the blue compact galaxy II Zw 40. The review concludes with a preview of two upcoming Hubble Space Telescope programs: ULLYSES, a survey of massive stars in nearby galaxies, and CLASSY, a study of massive star clusters in star-forming galaxies.
... The columns are as follows: (1) our adopted name; (2) the name given by Bresolin et al. (2009, H II-4 corresponds to multiple H II regions in their study); (3) field identification; (4, 5) H II region position (J2000 equinox); (6) aperture radius in arcsec; (7, 8) dust-corrected H α fluxes in units of 10 −15 erg cm −2 s −1 -measured from our CTIO images (column 7) using the circular aperture specified by columns 4-6 and measured from the Bresolin et al. (2009) slit spectroscopy (column 8); (9) the log of the ionizing photon rate required to produce the flux given in column (7) in units of photons s −1 ; (10, 11) the mass and spectral type of a single main-sequence O star that can provide the ionizing flux listed in table (9) infrared (Brand, Coulson & Zealey 1981;Comte 1981;Rouan et al. 1996;Calzetti et al. 1999), intense continuum emission spanning the electromagnetic spectrum (e.g. Talbot, Jensen & Dufour 1979;Trinchieri, Fabbiano & Palumbo 1985;Sukumar, Klein & Graeve 1987;Bohlin et al. 1990;Gallais et al. 1991;Rouan et al. 1996;Elmegreen, Chromey & Warren 1998;Buat et al. 2002;Soria & Wu 2002;Vogler et al. 2005) with a spectrum dominated by young stellar populations (Bohlin et al. 1983;Heckman et al. 2001;Wofford, Leitherer & Chandar 2011), large quantities of dense molecular gas (Walker et al. 1993;Mauersberger et al. 1999;Dumke et al. 2001;Lundgren et al. 2004;Muraoka et al. 2007), and numerous young blue star clusters (Bohlin et al. 1990;Larsen 1999;Larsen & Richtler 2000;Harris et al. 2001;Chandar et al. 2010;Andrews et al. 2014), some with estimated masses up to those seen in Galactic globular clusters (Lundgren et al. 2004). Some areas in M83's central region are highly dust obscured, perhaps even the true nucleus (Díaz et al. 2006;Rodrigues et al. 2009; however cf. ...
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Using Hubble Space Telescope ACS/WFC data we present the photometry and spatial distribution of resolved stellar populations of four fields within the extended ultraviolet disc (XUV disc) of M83. These observations show a clumpy distribution of main-sequence stars and a mostly smooth distribution of red giant branch stars. We constrain the upper end of the initial mass function (IMF) in the outer disc using the detected population of main-sequence stars and an assumed constant star formation rate (SFR) over the last 300 Myr. By comparing the observed main-sequence luminosity function to simulations, we determine the best-fitting IMF to have a power-law slope α = −2.35 ± 0.3 and an upper mass limit M_u = 25⁺¹⁷₋₃M⊙⁠. This IMF is consistent with the observed H α emission, which we use to provide additional constraints on the IMF. We explore the influence of deviations from the constant SFR assumption, finding that our IMF conclusions are robust against all but strong recent variations in SFR, but these are excluded by causality arguments. These results, along with our similar studies of other nearby galaxies, indicate that some XUV discs are deficient in high-mass stars compared to a Kroupa IMF. There are over one hundred galaxies within 5 Mpc, many already observed with HST, thus allowing a more comprehensive investigation of the IMF, and how it varies, using the techniques developed here.
... M83 (see also Figure 7) is a grand-design spiral with Hubble type SAB(s)c and a central starburst [40]. The two COS spectra were taken at this central region, for two UV sources separated by about 150 pc (see Figure 9). ...
Article
Full-text available
Spectroscopic observations of a massive star formation in the ultraviolet and their interpretation are reviewed. After a brief historical retrospective, two well-studied resolved star clusters and the surrounding H II regions are introduced: NGC 2070 in the Large Magellanic Cloud and NGC 604 in M33. These regions serve as a training set for studies of more distant clusters, which can no longer be resolved into individual stars. Observations of recently formed star clusters and extended regions in star-forming galaxies in the nearby universe beyond the Local Group are presented. Their interpretation relies on spectral synthesis models. The successes and failures of such models are discussed, and future directions are highlighted. I present a case study of the extraordinary star cluster and giant H II region in the blue compact galaxy II Zw 40. The review concludes with a preview of two upcoming Hubble Space Telescope programs: ULLYSES, a survey of massive stars in nearby galaxies, and CLASSY, a study of massive star clusters in star-forming galaxies.
... Recent theory predicts that gas accumulates at these radii owing to a minimum in the shear that is responsible for its radial transport (Krumholz & Kruijssen 2015). Similarly sized and shaped central concentrations of gas are frequently seen in the centers of barred galaxies, including nearby galaxies like IC 342 (Ishizuki et al. 1990), M83 (Elmegreen et al. 1998), the starburst NGC 253 (García-Burillo et al. 2000), and the Seyfert 2 galaxy NGC 4945 (Chou et al. 2007). In the CMZ, the distribution of this gas is asymmetric in several ways, with more gas found both at positive longitudes and on the near side of this ring, which is suggested to be due to some combination of instabilities in inflowing gas (Sormani et al. 2018) and tidal compression at different points along the orbit (Longmore et al. 2013b). ...
Article
We present an analysis of gas densities in the central R = 300 pc of the Milky Way, focusing on three clouds: GCM -0.02-0.07 (the 50 km s⁻¹ cloud), GCM -0.13-0.08 (the 20 km s⁻¹ cloud), and GCM 0.25+0.01 (the "Brick"). Densities are determined using observations of the J = (3-2), (4-3), (5-4), (10-9), (18-17), (19-18), (21-20), and (24-23) transitions of the molecule HC3N. We find evidence of at least two excitation regimes for HC3N and constrain the low-excitation component to have a density less than 10⁴ cm⁻³ and the high-excitation component to have a density between 10⁵ and 10⁶ cm⁻³. This is much less than densities of 10⁷ cm⁻³ that are found in Sgr B2, the most actively star-forming cloud in the Galactic center. This is consistent with the requirement of a higher-density threshold for star formation in the Galactic center than is typical in the Galactic disk. We are also able to constrain the column density of each component in order to determine the mass fraction of "dense" (n > 10⁵ cm⁻³) gas for these clouds. We find that this is ∼15% for all three clouds. Applying the results of our models to ratios of the (10-9) and (3-2) line across the entire central R = 300 pc, we find that the fraction of dense (n > 10⁴ cm⁻³) gas increases inward of a radius of ∼140 pc, consistent with the predictions of recent models for the gas dynamics in this region. Our observations show that HC3N is an excellent molecule for probing the density structure of clouds in the Galactic center. © 2018. The American Astronomical Society. All rights reserved..
... [1,3] suggests that the compact object has been ejected from the optical nucleus (ON), which is itself off-set with respect to the galaxy's kine- matic center. The ON has a mass of (1 -4) ×10 6 M ʘ [4,5] and it presents a velocity dispersion [5,6] that is com- patible with the presence of a super-massive black hole with M SMBH ≤ 10 6 M ʘ or a spatially unresolved mass concentration located at its centre. ...
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We present results of the $^{12}$CO (1--0) mosaic observations of the nearby barred-spiral galaxy M83 obtained with the Atacama Large Millimeter/submillimeter Array (ALMA). The total flux is recovered by combining the ALMA data with single-dish data obtained using the Nobeyama 45-m telescope. The combined map covers a $\sim$13 kpc$^{2}$ field that includes the galactic center, eastern bar, and spiral arm with a resolution of \timeform{2''.03} $\times$ \timeform{1''.1} ($\sim$45 pc $\times$ $\sim$25 pc). With a resolution comparable to typical sizes of giant molecular clouds (GMCs), the CO distribution in the bar and arm is resolved into many clumpy peaks that form ridge-like structures. Remarkably, in the eastern arm, the CO peaks form two arc-shaped ridges that run along the arm and exhibit a distinct difference in the activity of star formation: the one on the leading side has numerous HII regions associated with it, whereas the other one on the trailing side has only a few. To see whether GMCs form stars with uniform star formation efficiency (SFE) per free-fall time (SFEff), GMCs are identified from the data cube and then cross-matched with the catalog of HII regions to estimate the star formation rate for each of them. 179 GMCs with a median mass of 1.6 $\times$ 10$^{6}$ $M_{\odot}$ are identified. The mass-weighted average SFEff of the GMCs is $\sim$9.4 $\times$ 10$^{-3}$, which is in agreement with models of turbulence regulated star formation. Meanwhile, we find that SFEff is not universal within the mapped region. In particular, one of the arm ridges shows a high SFEff with a mass-weighted value of $\sim$2.7 $\times$ 10$^{-2}$, which is higher by more than a factor of 5 compared to the inter-arm regions. This large regional variation in SFEff favors the recent interpretation that GMCs do not form stars at a constant rate within their lifetime.
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We present observations of the nearby barred starburst galaxy M83 (NGC 5236), with the new Fabry-Perot interferometer GHαFaS mounted on the 4.2 m William Herschel Telescope on La Palma. The unprecedented high-resolution observations, of 16 pc FWHM−1, of the Hα-emitting gas cover the central 2 kpc of the galaxy. The velocity field displays the dominant disk rotation with signatures of gas inflow from kpc scales down to the nuclear regions. At the inner inner Lindblad resonance radius of the main bar and centered at the dynamical center of the main galaxy disk, a nuclear (5.5 ± 0.9) × 108 M☉ rapidly rotating disk with scale length of 60 ± 20 pc has formed. The nuclear starburst is found in the vicinity as well as inside this nuclear disk, and our observations confirm that gas spirals in from the outer parts to feed the nuclear starburst, giving rise to several star formation events at different epochs, within the central 100 pc radius of M83.
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We investigate the ionization structure of the nebular gas in M83 using the line diagnostic diagram, [O III](5007??)/H? versus [S II](6716??+6731??)/H?, with the newly available narrowband images from the Wide Field Camera 3 (WFC3) of the Hubble Space Telescope (HST). We produce the diagnostic diagram on a pixel-by-pixel (02 × 02) basis and compare it with several photo- and shock-ionization models. We select four regions from the center to the outer spiral arm and compare them in the diagnostic diagram. For the photoionized gas, we observe a gradual increase of the log ([O III]/H?) ratios from the center to the spiral arm, consistent with the metallicity gradient, as the H II regions go from super-solar abundance to roughly solar abundance from the center out. Using the diagnostic diagram, we separate the photoionized from the shock-ionized component of the gas. We find that the shock-ionized H? emission ranges from ~2% to about 15%-33% of the total, depending on the separation criteria used. An interesting feature in the diagnostic diagram is a horizontal distribution around log ([O III]/H?) ? 0. This feature is well fit by a shock-ionization model with 2.0 Z ? metallicity and shock velocities in the range of 250-350 km s?1. A low-velocity shock component, <200 km s?1, is also detected and is spatially located at the boundary between the outer ring and the spiral arm. The low-velocity shock component can be due to (1) supernova remnants located nearby, (2) dynamical interaction between the outer ring and the spiral arm, and (3) abnormal line ratios from extreme local dust extinction. The current data do not enable us to distinguish among those three possible interpretations. Our main conclusion is that, even at the HST resolution, the shocked gas represents a small fraction of the total ionized gas emission at less than 33% of the total. However, it accounts for virtually all of the mechanical energy produced by the central starburst in M83.
Article
M83 is one of the nearest galaxies with enhanced nuclear star formation, and it presents one of the best opportunities to study the kinematics and physical properties of a circumnuclear starburst. Our three-dimensional spectroscopy data in the R band confirm the presence of a secondary nucleus or mass concentration (previously suggested by Thatte and coworkers). We determine the position of this hidden nucleus, which would be more massive than the visible one and was not detected in the optical Hubble Space Telescope images due, probably, to the strong dust extinction. Using a Keplerian approximation, we estimated for the optical nucleus a mass of (5.0 ± 0.8) × 106 M/ sin i (r < 15), and for the hidden nucleus, located 4'' ± 1'' to the northwest (position angle of 271° ± 15°) of the optical nucleus, a mass of (1.00 ± 0.08) × 107 M/ sin i (r < 15). The emission-line ratio map also unveils the presence of a second circumnuclear ring structure, previously discovered by IR imaging (Elmegreen and coworkers). The data allow us to resolve the behavior of the interstellar medium inside the circumnuclear ring and around the binary mass concentration.
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The center of M83, a barred starburst galaxy with a double nucleus, has been observed in the CO (J = 2-1) and CO (J = 3-2) lines with the Submillimeter Array. The molecular gas shows a distribution and kinematics typical for barred galaxies at ~1 kpc radii but reveals unusual kinematics around the double nucleus in the central ~300 pc. Our CO velocity data show that the visible nucleus in M83 is at least 3'' (65 pc) away from the galaxy's dynamical center, which most likely coincides with the center of symmetry previously determined in the K band and is suggested to host another nucleus. We discovered high-velocity molecular gas associated with the visible off-center nucleus and also found a steep velocity gradient across it. We attribute these features to a gas disk rotating around the off-center nucleus, which may be the remnant of a small galaxy accreted by M83. The dynamical mass of this component is estimated to be 3 × 108 M☉ within a radius of 40 pc. The dynamical perturbation from the off-center nucleus may have played a key role in shaping the lopsided starburst.
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Near-infrared observations of NGC 2997 and NGC 6951 reveal complete rings of symmetrically placed hot spots, which are star-forming complexes. The masses of the complexes range from a few times 104 to a few times 105 M in both galaxies. The regular hot-spot spacings of about 200 pc in a ring 55 pc thick in NGC 2997 and 380 pc in a ring 150 pc thick in NGC 6951 are consistent with formation by a large-scale gravitational instability in the rings. Visual extinction is estimated to be up to 3 mag around the ring in NGC 2997 and up to 5 mag in NGC 6951, as dust lanes spiral in and cross the ring. Hubble Space Telescope observations reveal smaller super–star clusters within the large complexes. These clusters have power-law luminosity functions with an exponent of approximately -2 in both galaxy rings, the same as for star-forming complexes in the disks of normal spiral galaxies.
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Two-dimensional kinematics of the central region of M 83 (NGC 5236) were obtained through three-dimensional NIR spectroscopy with the Gemini South telescope. The spatial region covered by the integral field unit (~5'' × 13'' or ~90 × 240 pc) was centered approximately at the center of the bulge isophotes and oriented southeast-northwest. The Paβ emission at half-arcsecond resolution clearly reveals spider-like diagrams around three centers, indicating the presence of extended masses, which we describe in terms of Satoh distributions. One of the mass concentrations is identified as the optical nucleus (ON), another as the center of the bulge isophotes, similar to the CO kinematical center (KC), and the third as a condensation hidden at optical wavelengths (hidden nucleus, HN), coincident with the largest lobe in 10 μm emission. We run numerical simulations that take into account ON, KC, and HN and four more clusters, representing the star-forming arc at the southwest of the optical nucleus. We show that ON, KC, and HN suffer strong evaporation and merge in 10-50 Myr. The star-forming arc is scattered in less than one orbital period, also falling into the center. Simulations also show that tidal striping boosts the external shell of the condensations to their escape velocity. This fact might lead to an overestimation of the mass of the condensations in kinematical observations with spatial resolution smaller than the condensations' apparent sizes. Additionally, the existence of two ILR resonances embracing the chain of H II regions, claimed by different authors, might not exist due to the similarity of the masses of the different components and the fast dynamical evolution of M 83 central 300 pc.
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We present a photometric catalog of 45 massive star clusters in the nuclear starburst of M83 (NGC 5236), observed with the Hubble Space Telescope Wide Field Planetary Camera 2, in both broadband (F300W, F547M, and F814W) and narrowband (F656N and F487N) filters. By comparing the photometry with theoretical population synthesis models, we estimate the age and mass of each cluster. We find that over 75% of the star clusters more massive than 2 × 104 M in the central 300 pc of M83 are less than 10 Myr old. Among the clusters younger than 10 Myr and more massive than 5 × 103 M, 70% are between 5 and 7 Myr old. We list an additional 330 objects that are detected in our F300W images but not in the shallower F547M and F814W images. The clusters are distributed throughout a semicircular annulus that identifies the active region in the galaxy core, between 50 and 130 pc from the optical center of M83. Clusters younger than 5 Myr are preferentially found along the northwestern end of the semicircular annulus, and the northwest region also corresponds to the highest equivalent widths of the Hα emission [EW(Hα)] values. The cluster ages are consistent with a burst of star formation that began 10 Myr ago. However, the current data cannot exclude a longer duration of activity, because the star clusters may dissolve on a ~10 Myr timescale. We discuss both possible scenarios.
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The Fanaroff-Riley II radio source J133658.3–295105, which is also an X-ray source, appears to be projected onto the disk of the barred-spiral galaxy M83 at about 60'' from the galaxy's optical nucleus. J133658.3–295105 and its radio lobes are aligned with the optical nucleus of M 83 and two other radio sources, neither of which are supernova remnants or H II regions. Due to this peculiar on-the-sky projection, J133658.3–295105 was previously studied by Gemini+GMOS optical spectroscopy, which marginally revealed the presence of Hα in emission receding at 130 km s–1 with respect to the optical nucleus. In this Letter, we reanalyze the Chandra spectroscopy carried out in 2000. We show that J133658.3–295105 presents an Fe Kα emission line at a redshift of z = 0.018. This redshift is compatible with a black hole at the distance of M 83. We discuss similarities to the recently reported micro-quasar in NGC 5408. This finding reinforces the kicked-off black hole scenario for J133658.3–295105.
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A combination of Hubble Space Telescope Wide Field Planetary Camera 2 (WFPC2) and Near-Infrared Camera Multiobject Spectrograph (NICMOS) images are used to investigate the gas/dust and stellar structure inside the central 300 pc of the nearby active galaxy NGC 4303. The NICMOS H-band (F160W) image reveals a bright core and a nuclear-elongated barlike structure of 250 pc in diameter. The bar is centered on the bright core, and its major axis is oriented in projection along the spin axis of the nuclear gaseous rotating disk recently detected. The V-H (F606W-F160W) image reveals a complex gas/dust distribution with a two-arm spiral structure of about 225 pc in radius. The southwestern arm is traced by young star-forming knots while the northeastern arm is detected by the presence of dust lanes. These spirals do not have a smooth structure, but rather they are made of smaller flocculent spirals or filament-like structures. The magnitudes and colors of the star-forming knots are typical of clusters of young stars with masses of (0.5-1) × 105 M☉ and ages of 5-25 million years. The overall structure of the nuclear spirals as well as the size, number, and masses of the star-forming knots are explained in the context of a massive gaseous nuclear disk subject to self-gravitational instabilities and to the gravitational field created by the nuclear bar. According to the model, the gaseous disk has a mass of about 5 × 107 M☉ inside a radius of 400 pc, the bar has a radius of 150 pc and a pattern speed of ~0.5 Myr-1, and the average mass accretion rate into the core (R < 8 pc) is ~0.01 M☉ yr-1 for about 80 Myr.
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We present a new optical imaging survey of supernova remnants in M83, using data obtained with the Magellan I 6.5m telescope and IMACS instrument under conditions of excellent seeing. Using the criterion of strong [S II] emission relative to Halpha, we confirm all but three of the 71 SNR candidates listed in our previous survey, and expand the SNR candidate list to 225 objects, more than tripling the earlier sample. Comparing the optical survey with a new deep X-ray survey of M83 with Chandra, we find 61 of these SNR candidates to have X-ray counterparts. We also identify an additional list of 46 [O III] -selected nebulae for follow-up as potential ejecta-dominated remnants, seven of which have associated X-ray emission that makes them strong candidates. Some of the other [O III]-bright objects could also be normal ISM-dominated supernova remnants with shocks fast enough to doubly ionize oxygen, but with Halpha and [S II] emission faint enough to have been missed. A few of these objects may also be H II regions with abnormally high [O III] emission compared with the majority of M83 H II regions, compact nebulae excited by young Wolf-Rayet stars, or even background AGN. The supernova remnant Halpha luminosity function in M83 is shifted a factor of ~ 4.5x higher than for M33 supernova remnants, indicative of a higher mean ISM density in M83. We describe the search technique used to identify the supernova remnant candidates and provide basic information and finder charts for the objects.
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We present virial mass estimates of young massive clusters (YMCs) in the starburst galaxies NGC1140 and M83, determined from high spectral resolution VLT echelle spectroscopy and high spatial resolution Hubble Space Telescope imaging. The survivability of such clusters is important in testing the scenario that YMCs are potentially proto-globular clusters. As young clusters, they lie in the domain in which dynamical masses appear to overestimate true cluster masses, most likely due to the clusters not being virialised. We find that the dynamical mass of NGC1140-1 is approximately ten times greater than its photometric mass. We propose that the most likely explanation for this disparity is the crowded environment of NGC1140-1, rather than this being solely due to a lack of virial equilibrium. KeywordsGalaxies: individual: NGC1140-Galaxies: starburst-Galaxies: star clusters-Stars: Wolf-Rayet
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The central ≈ 80×200 pc of the barred spiral galaxy M83 (NGC 5236) has been observed with the GEMINI-S+CIRPASS configuration which produced 490 spectra with a spectral resolving power of 3200, centered at 1.3 microns, oriented NW-SE. We determine the kinematics of this region with 0.36″ sampling and sub-arcsec resolution. Disk-like motions have been detected in Paβ at parsec scales around: a) the optical nucleus, b) the center of the external K-band isophotes coincident with that of the CO velocity map both also tracing the center of the bulge, and c) a hidden condensation located at ( R ,θ)=(158pc, 301 o ). The present resolution allows to detect other minor whirls, not discussed here. The disk around (a) has a radius of ∼8 pc and the two around (b) and (c) can be traced approximately up to 50–60 pc from their kinematical centers. The rotation curves have been fitted by Satoh like spheroids+disk indicating masses of ≈4× 10 ⁶ M ⊙, 60– 70× 10 ⁶ M ⊙ and 30– 40× 10 ⁶ M ⊙ respectively. Limit to the masses of central BHs can be set by supposing that the central unresolved line broadening inside each condensations is dominated by the BH as far as allowed by the central error bars. The BH +Satoh models were smoothed with a 9 pc Gaussian. The upper mass limit derived for the BH is for (a) ∼ 10 ⁶ / sin( i )M⊙, and for (b) and (c) 0.2–1.0× 10 ⁶ / sin( i )M⊙. Many questions arise from this interesting nucleus: are we witnessing a unique phenomenon or simply a barred galaxy with ongoing strong SF in our backyard? Does each one of the condensations host a BH indeed? or is there only one at the bulge center? Which is the fate of this complex scenario?
Conference Paper
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The luminosity profile of M 83 bulge can be traced by a de Vaucouleurs' law between ≈ 200 pc and ≈ 800 pc. The inner part can be fitted by a n = -1/2 Sérsic profile. Also the IR (J - K) color shows difference between the periphery and the central part of the bulge, both properties indicating the presence of a pseudobulge. Previous Gemini-S 3-D, Paβ spectroscopy of the central ≈ 5″×13″ revealed spider like diagrams indicating disk like motion around three extended masses identified respectively with the optical nucleus (ON), with the center of the bulge isophotes, similar to the CO kinematical center (KC), and with a condensation hidden at optical wavelengths (HN), coincident with the largest lobe in 10 μm emission, most probably a cannibalized satellite. Numerical simulations show that they suffer strong evaporation and they would merge engulfing also the star forming arc in few hundred Myr, increasing the mass at the kinematical center by a factor o five or more. Upper mass limit of putative Black Holes associated to ON, KC and HN are a few ten thousand to a million solar masses. GMOS+Gemini imaging and spectroscopy of a chain of radio sources has yield no optical high redshift counterparts. This radio sources are aligned with ON, neither associated to SN nor to HII regions and might point to an older similar phenomenon, which left behind a kick-off spur.
Conference Paper
The dusty circumnuclear environments of many spiral galaxies hide important clues for understanding the tight correlation between the supermassive black hole and the host galaxy bulge mass. Two of the nearest spiral galaxies with nuclear starburst host off-centered compact cores associated with the peak of the infrared emission.These compact objects are lightweight, up to a few million solar masses. Their locations imply that they are triggering the present star formation within the circumnuclear disk. Numerical modeling implies that the giant star clusters formed in the process evaporate in a few system revolutions (due to the strong asymmetric gravitational potential). Therefore the off-center compact object scenario provides a model in which the dynamically cold gaseous component is rapidly converted into a dynamically hot stellar component. Meanwhile only a small fraction of the gas contributes to the growth of the off-centered compact object.
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We analyze archival HST/STIS/FUV-MAMA imaging and spectroscopy of 13 compact star clusters within the circumnuclear starburst region of M83, the closest such example. We compare the observed spectra with semi-empirical models, which are based on an empirical library of Galactic O and B stars observed with IUE, and with theoretical models, which are based on a new theoretical UV library of hot massive stars computed with WM-Basic. The models were generated with Starburst99 for metallicities of Z = 0.020 and Z = 0.040, and for stellar initial mass functions (IMFs) with upper mass limits of 10, 30, 50, and 100 M ☉. We estimate the ages and masses of the clusters from the best-fit model spectra and find that the ages derived from the semi-empirical and theoretical models agree within a factor of 1.2 on average. A comparison of the spectroscopic age estimates with values derived from HST/WFC3/UVIS multi-band photometry shows a similar level of agreement for all but one cluster. The clusters have a range of ages from about 3 to 20 Myr and do not appear to have an age gradient along M83's starburst. Clusters with strong P-Cygni profiles have masses of a few×104M ☉, seem to have formed stars more massive than 30 M ☉, and are consistent with a Kroupa IMF from 0.1to100 M ☉. Field regions in the starburst lack P-Cygni profiles and are dominated by B stars.
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We report new near-IR integral field spectroscopy of the central starburst region of the barred spiral galaxy M83 obtained with Cambridge Infra-Red Panoramic Survey Spectrograph (CIRPASS) on Gemini-South, which we analyse in conjunction with GHαFaS Fabry–Perot data, an Anglo-Australian Telescope (AAT) IRIS2 Ks-band image, and near- and mid-IR imaging from the Hubble and Spitzer space telescopes. The bulk of the current star formation activity is hidden from optical view by dust extinction, but is seen in the near- and mid-IR to the north of the nucleus. This region is being fed by inflow of gas through the bar of M83, traced by the prominent dust lane entering into the circumnuclear region from the north. An analysis of stellar ages confirms that the youngest stars are indeed in the north-west. A gradual age gradient, with older stars further to the south, characterizes the well-known star-forming arc in the central region of M83, and is consistent with a stable scenario where gas inflow into the central regions is facilitated by the galactic bar. Detailed analyses of the Paβ ionized gas kinematics and near-IR imaging confirm that the kinematic centre coincides with the photometric centre of M83, and that these are offset significantly, by about 3 arcsec or 60 pc, from the visible nucleus of the galaxy. We discuss two possible options, the first of which postulates that the kinematic and photometric centre traces a galaxy nucleus hidden by a substantial amount of dust extinction, in the range AV= 3–10 mag. By combining this information with kinematic results and using arguments from the literature, we conclude that such a scenario is, however, unlikely, as is the existence of other ‘hidden’ nuclei in M83. We thus concur with recent authors and favour a second option, in which the nucleus of the galaxy is offset from its kinematic and photometric centre. This is presumably a result of some past interaction, possibly related to the event which lies at the origin of the disturbance of the outer disc of the galaxy. We find some indications for a disturbance in the Hα velocity field which would confirm the influence of the m= 1 perturbation in the gravitational potential, but note that further high-quality stellar kinematic data are needed to confirm this scenario.
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We present the most complete atlas of nuclear rings to date. We include 113 rings found in 107 galaxies, six of which are elliptical galaxies, five are highly inclined disc galaxies, 18 are unbarred disc galaxies, and 78 are barred disc galaxies. Star-forming nuclear rings occur in 20% of disc galaxies with types between T=-3 and T=7. We aim to explore possible relationships between the size and morphology of the rings and various galactic parameters. We produce colour index and structure maps, as well as Halpha and Paalpha continuum-subtracted images from HST archival data. We derive ellipticity profiles from H-band 2MASS images in order to detect bars and find their metric parameters. We measure the non-axisymmetric torque parameter, Qg, and search for correlations between bar, ring metric parameters, and Qg. Our atlas of nuclear rings includes star-forming and dust rings. Nuclear rings span a range from a few tens of parsecs to a few kiloparsecs in radius. Star-forming nuclear rings can be found in a wide range of morphological types, from S0 to Sd, with a peak in the distribution between Sab and Sb, and without strong preference for barred galaxies. Dust nuclear rings are found in elliptical and S0 galaxies. For barred galaxies, the maximum radius that a nuclear ring can reach is a quarter of the bar radius. We found a nearly random distribution of PA offsets between nuclear rings and bars. There is some evidence that nuclear ring ellipticity is limited by bar ellipticity. We confirm that the maximum relative size of a star-forming nuclear ring is inversely proportional to the non-axisymmetric torque parameter, Qg, and that the origin of nuclear rings, even the ones in non-barred hosts, are closely linked to the existence of dynamical resonances. Comment: Accepted for publication in MNRAS. A full resolution version of the manuscript with high resolution figures can be found at http://www.iac.es/folleto/research/preprints/
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We present Gemini optical imaging and spectroscopy of the radio source J 133658.3-295105. Thisd source has been suggested to be the core of an FR II radio source with two detected lobes. J 133658.3-295105 and its lobes are aligned with the optical nucleus of M 83 and with three other radio sources at the M 83 bulge outer region. These radio sources are neither supernova remnants nor H II regions. This curious configuration prompted us to try to determine the distance to J 133658.3-295105. We detected Hα emission redshifted by ≈ 130 km s–1 with respect to an M 83 H II region 25 east-southeast of the radio source. We do not detect other redshifted emission lines of an optical counterpart down to m i = 22.2 ± 0.8. Two different scenarios are proposed: the radio source is at z ≥ 2.5, a much larger distance than the previously proposed lower limit z ≥ 1.0, or the object was ejected by a gravitational recoil event from the M 83 nucleus. This nucleus is undergoing a strong dynamical evolution, judging from previous three-dimensional spectroscopy.
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We have performed smoothed particle hydrodynamics (SPH) simulations to study the response of the central kiloparsec region of a gaseous disk to the imposition of nonaxisymmetric bar potentials. The model galaxies are composed of the three axisymmetric components (halo, disk, and bulge) and a non-axisymmetric bar. These components are assumed to be invariant in time in the frame corotating with the bar. The potential of spherical $\gamma$-models of Dehnen is adopted for the bulge component whose density varies as $r^{-\gamma}$ near the center and $r^{-4}$ at larger radiiand hence, possesses a central density core for $\gamma = 0$ and cusps for $\gamma > 0$. Since the central mass concentration of the model galaxies increases with the cusp parameter $\gamma$, we have examined here the effect of the central mass concentration by varying the cusp parameter $\gamma$ on the mechanism responsible for the formation of the symmetric two-armed nuclear spirals in barred galaxies. Our simulations show that the symmetric two-armed nuclear spirals are formed by hydrodynamic spiral shocks driven by the gravitational torque of the bar for the models with $\gamma = 0$ and 0.5. On the other hand, the symmetric two-armed nuclear spirals in the models with $\gamma=1$ and 1.5 are explained by gas density waves. Thus, we conclude that the mechanism responsible for the formation of the symmetric two-armed nuclear spirals in barred galaxies changes from the hydrodynamic shocks to the gas density waves when the central mass concentration increases from $\gamma = 0$ to 1.5.
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We combine VLT/ISAAC NIR spectroscopy with archival HST/WFPC2 and HST/NICMOS imaging to study the central 20"x20" of M83. Our NIR indices for clusters in the circumnuclear star-burst region are inconsistent with simple instantaneous burst models. However, models of a single burst dispersed over a duration of 6 Myrs fit the data well and provide the clearest evidence yet of an age gradient along the star forming arc, with the youngest clusters nearest the north-east dust lane. The long slit kinematics show no evidence to support previous claims of a second hidden mass concentration, although we do observe changes in molecular gas velocity consistent with the presence of a shock at the edge of the dust lane. Comment: Accepted for publication in MNRAS. 24 pages, 11 figures
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(Abridged) Aims: We obtained VLT/FLAMES+UVES high-resolution, fibre-fed spectroscopy (FFS) of five young massive clusters in M83 (NGC 5236). This forms the basis of a pilot study testing the feasibility of using FFS to measure the velocity dispersions of several clusters simultaneously, in order to determine their dynamical masses; Methods: We adopted two methods for determining the velocity dispersion of the star clusters: cross-correlating the cluster spectrum with the template spectra and minimising a chi^2 value between the cluster spectrum and the broadened template spectra. Cluster 805 in M83 was chosen as a control to test the reliability of the method, through a comparison with the results obtained from a standard echelle VLT/UVES spectrum obtained by Larsen & Richtler; Results: We find no dependence of the velocity dispersions measured for a cluster on the choice of red giant versus red supergiant templates, nor on the method adopted. We measure a velocity dispersion of sigma_los = 10.2+/-1.1 km/s for cluster 805 from our FFS. Our FLAMES+UVES velocity dispersion measurement gives M_vir = (6.6+/-1.7)e5 M_sun, consistent with previous results. This is a factor of ~3 greater than the cluster's photometric mass, indicating a lack of virial equilibrium. However, based on its effective star formation efficiency, the cluster is likely to virialise, and may survive for a Hubble time, in the absence of external disruptive forces; Conclusions: We find that reliable velocity dispersions can be determined from FFS. The advantages of observing several clusters simultaneously outweighs the difficulty of accurate galaxy background subtraction, providing that the targets are chosen to provide sufficient S/N ratios, and are much brighter than the galaxy background. Comment: 10 pages, 4 figures, accepted by A&A
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We present the main physical properties of very young stellar populations seen with the Far Ultraviolet Spectroscopic Explorer in 24 individual starbursts. These characteristics have been obtained using the evolutionary spectral synthesis technique in the far-ultraviolet range with the LavalSB code. For each starburst, quantitative values for age, metallicity, initial mass function slope, stellar mass and internal extinction have been obtained and discussed in details. Limits of the code have been tested. One main conclusion is that most starbursts (and probably all of them) cannot be represented by any continuous star formation burst in the far ultraviolet. Also, quantitative values of various optical diagnostics related to these stellar populations have been predicted. Underlying stellar populations, dominated by B-type stars, have been detected in NGC 1140, NGC 4449 and possibly NGC 3991. We characterized the young stellar populations of less than 5 Myr in Seyfert 2 nuclei.
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High-resolution near-infrared (J and K band) images of the inner 50" of the hotspot galaxies NGC 3351, NGC 3504, and NGC 5248 were used to measure colors of star-forming regions. The hotspot colors are consistent with ages of approximately 107 yr. Near-infrared (MR) peaks in NGC 5248 are coincident with published HST UV peaks. In NGC 3351, MR hotspots coincide with published Hα peaks, which are offset from CO twin peaks. In addition, bright K-band emission exists without optical or gas peak counterparts, and reveals the underlying inner Lindblad resonance ring and older star formation sites. Published CO maps of NGC 3351 were used to determine that the Q stability parameter is less than 1.4 in the ring, consistent with star formation. In NGC 3504, a ring of star formation is revealed in the (J- K) map at the location of the inner inner Lindblad resonance, with a two-arm spiral originating outside of the ring. Circumnuclear star formation rates are 0.4 Msun yr-1 in NGC 3351 and 1.5Msun yr-1 in NGC 5248; these rates may scale with galaxy Hubble type and total mass. We also observed the barred spirals NGC 4123, 4303, 5350, and 5921. None of these show evidence of circumnuclear rings in the MR.
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The diameters D_c_ of the largest star-forming complexes in 67 Magellanic spiral and irregular galaxies and 16 blue compact dwarf (BCD) galaxies are found to scale approximately with the square root of the galaxy luminosity for each type, i.e., smaller galaxies have proportionately smaller star-forming regions. This is the same relation as for the largest complexes in bright spiral galaxies found previously, although Sm/Im galaxies have complexes that, on average, are a factor of 2 larger than the extrapolation for spiral galaxies at the same absolute magnitude, and the BCD galaxies have complexes that are ~2 times larger than those typical of the Sm/Im galaxies at the same absolute magnitude. These results are consistent with the interpretation that the largest complexes form at the gravitational length scale in a marginally stable interstellar medium with a nearly constant velocity dispersion c ~ 5-10 km s^-1^. The luminosity scaling is then the result of higher average total densities in smaller galaxies compared with the outer regions of giant spirals. This total density correlation is shown using published H I line widths and optical galaxy sizes. The implication of these results is that star formation begins when the ratio of the gas density ρ to the total density (gas + stars + dark matter) exceeds several tenths. If star formation lasts for a time scaling with (Grho_)^-1/2^ ~ D_c_/c, then the main morphological differences between star formation in galaxies of various sizes can be explained: large galaxies have large star complexes that form groups of OB associations slowly for up to 50 Myr; small galaxies have small complexes (in terms of absolute size) that form dense associations quickly, in bursts spanning less than 5 Myr.
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Given that dust and molecular gas tend to concentrate at the locations of either shocks or orbit crowding, the study of the dust morphology of barred spirals can give insight into the kinematics of the galaxies. In order to do this, the authors present a sample of barred galaxies for which they have obtained near infrared and optical photometry and BIMA interferometer observations of CO emission. The authors obtain the dust extinction and relative scale height of the dust and stars using a radiative transfer model to interpret the photometry. One finds that the scale height of the stars compared to the dust is generally larger in the central regions compared to further out in the bar dust lanes. The morphology of the dust extinction regions closely resembles the distribution of CO emission. An analysis of a larger sample of galaxies with IR data indicates that dust column densities appear larger in the nuclear regions than in the bar dust lanes.
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The Br-alpha (4.05 micron) and Br-gamma (2.17 micron) recombination lines of ionized hydrogen have been observed in five positions in the nucleus of M83 and detected in two. The results show that young OB stars are the major source of energy in the nucleus and that the extinction toward the young stellar region is high and spatially patchy. The Brackett lines are anomalously strong relative to the thermal radio flux. Possible explanations for this unexpected results are very low electron temperatures or a population of compact H II regions in the nucleus. In the latter case, it is suggested that the starburst must be extremely short-lived (10,000 yr) and not likely to be sustained.
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We present ISOCAM images (1.5arcsec resolution), in the Brα line and in the continuum at 4μm, of the starburst nuclear region in the nearby galaxy M 83. The starburst is found to lie: a) along arc joining the 2μm sources at S and SW of the nucleus to the NW 10μm peak, a possible indication of a propagating star formation; b) at the nucleus itself and at a bridge, linking the arc to the nucleus, that could trace a gaseous bar. The strong 4μm continuum emission cannot be free-free and probably corresponds to the continuum emission associated with the Very-Small-Grain/PAH component of the dust.
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An improved algorithm for extracting various types of symmetries from galaxy images is introduced. It preserves the nonlinear shapes of the spiral arms without introducing false interarm components, and it displays the m = 2, 3, 4, etc. subcomponents in whole galaxies all at once to avoid confusion with localized star formation and foreground stars. The algorithm is applied to 18 galaxies with various types of spirals. Conventional Fourier transform techniques are also used to study the arm structures. Underlying three-arm spirals are found to be common in the present sample, and a few galaxies have a complex four- or five-arm system, too. To check whether this structure is sensible, the termination radii of the two-, three-, and four-arm spirals are compared to the corresponding resonance radii predicted from theory. The symmetric components in nearly all of the galaxies are found to end at their outer wave-epicycle resonances.
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Basic dynamical mechanisms that produce an amplification of the accretion rate of gas clouds into the central regions of barred galaxies, and their subsequent effects on the evolution of barred galaxies, are discussed. Weakly dissipative orbits, representing gas clouds, are computed in a barred galaxy model with a central mass concentration such as a black hole, or a secondary small inner bar. Amplified accretion across resonances that is especially rapid inside the inner Lindblad resonance, large excursions outside the galactic plane, and the existence of nontrivial attractors like strange (chaotic) attractors or limit cycles are found. The underlying physical mechanisms are, in general, due to the presence of broad horizontal and vertical resonances through which weakly dissipative particles can rapidly traverse. The principal physical implications of these findings are discussed.
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Ten H I clouds with masses larger than 10 exp 8 solar masses in the interacting galaxies IC 2163/NGC 2207 are identified. Twenty-eight other interacting pairs of galaxies with large knots or star formation structures in their optical images are also tabulated. It is suggested that interactions can lead to the formation of greater than 10 solar mass clouds and young stellar regions in the outer parts of galactic disks, and that some of these regions may become gravitationally bound dwarf galaxies if they are ejected in tidal arms. It is proposed that the key to the origin of clouds of greater than 10 exp 8 solar mass in interacting systems lies in the high velocity dispersion of the interstellar gas. Numerical N-body simulations of the interacting galaxies suggest that the complete detachment of an unbound dwarf galaxy requires a companion mass comparable to or larger than the galaxy mass.
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New optical and near-infrared (NIR) K-band images of the inner 3 kpc region of the nearby Virgo spiral M100 (NGC 4321) display remarkable morphological changes with wavelength. While in the optical the light is dominated by circumnuclear zone of enhanced star formation, the morphological features in the 2.2 micrometer image correspond to a newly discovered kiloparsec-size stellar bar and a pair of leading arms situated inside an ovally shaped region. Analysis of the K image confirms its symmetry: only a very small percentage of the flux, some 5%, is emitted in antisymmetric structures. This indicates that the overall morphology observed in the NIR is dominated by a global density wave. Making a first-order correction for the presence of localized dust extinction in the K light using the I-K image, we find that the observed leading arm morphology is not caused or enhanced but in fact slightly hidden by dust. Possible mechanisms responsible for the optical and NIR morphology are discussed, and tests are proposed to discriminate between them. Our dynamical conclusions are supported with an evolutionary stellar population model reproducing the observed optical and NIR colors in a number of star-forming zones. We argue that the observed morphology is compatible with the presence of a pair of inner Lindblad resonances in the region and show this explicitly in an accompanying paper by detailed numerical modeling. The phenomena observed in NGC 4321 may provide insight into physical process leading to central activity in galaxies.
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We present UV (2300 Ang) images, obtained with the HST Faint Object Camera, of the central 20'' of five galaxies containing circumnuclear star-forming rings. The five galaxies are from a well-defined sample of 103 normal, nearby galaxies we have observed with HST. At the HST resolution (0.05''), the rings break up into discrete star-forming clumps, each composed of many luminous and compact ($R \ltorder 5$ pc) star clusters. These objects are similar to those that have been recently reported in colliding and starburst galaxies, and in several other circumnuclear rings. A large fraction, 15%--50%, of the UV emission originates in these compact clusters. Compact clusters therefore may be the preferred mode of star formation in starburst environments. For one galaxy, NGC 2997, we measure the UV-optical colors of the individual clusters using an archival HST WFPC2 image at 6000 Ang . Comparing the colors and luminosities to starburst population synthesis models, we show that the clusters are less than 100 Myr old and have masses of at least a few $10^3 M_{\odot}$, with some as high as $10^5 M_{\odot}$. The UV extinction to those clusters that are detected in the UV is at most a factor of 10. In NGC 2997, the limits on the masses and the ages of the young clusters indicate that these objects will remain bound and evolve into globular clusters. The luminosity function of the clusters in the rings is similar in shape to those measured for super star clusters in other star-forming galaxies, and extends to luminosities lower by several orders of magnitude. All five of the UV-detected circumnuclear rings occur in barred or weakly barred spiral galaxies of type Sc or earlier. None of the five rings has an active nucleus at its center, arguing against a direct correspondence between circumnuclear star formation and nuclear Comment: LaTex file, 7 Postscript figures, tables, aaspp4.sty, and epsf.sty included. Accepted to The Astronomical Journal, June 1996 issue
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We present detailed JHK surface photometry with ellipse fits of 13 galaxies selected from previous optical observations as likely candidates for having a secondary bar or a triaxial bulge within the primary bar. We have found 7 double-barred galaxies, 3 double-barred galaxies with an additional intermediate structure with twisted isophotes, and 3 galaxies with a bar and central twisted isophotes. A global analysis of the structural parameter characteristics in the I- and K-bands is presented. Various numerical models of galaxies with bars within bars are also analysed using the ellipse fitting technique and compared to the observations. A thorough review of the possible hypotheses able to explain this phenomenon is given with emphasis on the most likely ones.
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With its nearly face-on orientation and relatively close distance, the barred spiral galaxy M83 = NGC 5236 is one of the best objects for studies of nuclear starbursts. High-resolution images taken with the Planetary Camera on HST combined with ultraviolet spectra obtained by the IUE reveal that the nuclear starburst is actually a collection of over twenty young star clusters, each comparable to the prototypical starburst 30 Doradus in the LMC.
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The author reports on a conference at the La Palma Observatory, held from 8 - 12 November 1993.
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Observational data on 2367 galaxies with systemic velocities less than 3000 km/s are compiled in tables, in a catalog designed to complement the Nearby Galaxies Atlas of Tully and Fisher (1987). Included are 1053 galaxies from the Shapley-Ames sample (Sandage, 1978) and 1314 galaxies from the all-sky neutral hydrogen survey of Fisher and Tully (1981) and Reif et al. (1982). The main table lists data on all of the galaxies, while information on group membership and supercluster complexes is presented separately.
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Numerical hydrodynamic calculations have been carried out in order to investigate the response of a non-self-gravitating gaseous component to a rotating weak oval distortion in the axisymmetric gravitational field of a disk. The purpose of these time-dependent calculations was to determine the form of the steady-state gas density and flow distributions which would develop in the rotating frame of the oval distortion. The calculations reveal that the gas responds to the presence of the oval distortion by forming an open two-arm trailing spiral wave rotating with the angular speed of the oval distortion. The gas flow pattern is similar in form to predicted gas flow through small pitch-angle gravitational self-consistent spiral waves, although the deviations from uniform circular motion are large (approximately 75 km/s in regions of low gas density). These results may be understood from an investigation of periodic particle orbits in the rotating frame of the bar. Such an investigation implies that the presence of all three resonance regions (inner Lindblad, corotation, and outer Lindblad) within the gaseous disk is a necessary condition for such a spiral response.
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The Tully-Fisher relation is used to probe dark matter (DM) in the optical regions of spiral galaxies. By establishing it at several different isophotal radii in an appropriate sample of 58 galaxies with good B-band photometry and rotation curves, it is shown that some of its attributes (such as scatter, residuals, nonlinearity, and bias) dramatically decrease moving from the disk edge inward. This behavior challenges any mass model which assumes no DM or a luminosity-independent DM mass fraction interior to the optical radius of spiral galaxies.
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UBVR surface photometry of the bright southern spiral galaxy NGC 5236 (M83) is presented. Calibrated maps of UBVR and H-alpha surface brightness and colors are compared with models of evolving mixes of stellar populations in order to establish a measurable drift velocity relative to some feature for the young associations supposedly formed in the wake of a spiral density wave. The color and morphological information is used to remove the effects of recent star formation from the radial profile of surface brightness. A search for systematic color gradients across and behind the spiral arms reveals no systematic gradient within the arms and no significant gradients behind the arms in those regions completely free of stars less than 50 million years old. The lack of a systematic color gradient within the arms is shown to be in agreement with expectations from density-wave theory when proper allowance is made for noncircular orbits and imperfect synchrony of star formation.
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A rotation curve for the Galaxy within the solar circle is found using data from the fourth Galactic quadrant. The observations are consistent with a picture in which the Galaxy rotates with a circular velocity of 209 + or - 2 km/s between Galactocentric distances of 0.24 and 1.0 solar radius. Clouds associated with a part of the Carina spiral arm exhibit an average rotation velocity 12 km/s higher than expected, implying an apparent anomalous motion. An excess in velocity of about the same magnitude observed at the tangent direction of the Centaurus arm is attributed to streaming and internal motions of large molecular complexes. Similar phenomena found in the Sagittarius and Scutum arms by northern observers strongly support the hypothesis that Carina is the continuation of the Sagittarius arm and Centaurus is that of the Scutum arm.
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This new, much enlarged catalogue has been made necessary by the explosive growth of extragalactic astronomy over the last 15 years. With data on more than 23,000 galaxies, it includes all galaxies with apparent diameters larger than one arc minute, magnitudes brighter than about magnitude 15.5, and redshifts not larger than 15,000 km/sec, as well as many other objects of interest. Vol. I contains the explanations and references, Vol. II and III contain the catalogue proper. The catalogue gives for each galaxy its position, names, type and luminosity class, optical diameter and axis ratio, position angle, optical, infrared, and neutral hydrogen magnitudes, optical color indices and surface brightnesses, neutral hydrogen indices and line widths, and radial velocities.
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CO (J = 1-0) line observations of the bar and nucleus of the intermediate barred spiral galaxy M83 were made with the 45-m telescope of the Nobeyama Radio Observatory with a 16-arcsec beam. The observed area was 3.8 kpc x 1.1 kpc, which covered the optical bar. A ridge of CO emission is found on the leading edge of the bar and is traced by straight dust lanes. In the mapped field, about 40 percent of the CO line flux is concentrated within a radius of 0.4 kpc around the center of the galaxy.
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Multi-aperture JHKL colours on the new SAAO photometric system are presented for samples of Seyfert 1, Seyfert 2 and H II-region galaxies. Comparisons are made with the colours of ‘ordinary’ galaxies obtained using the same system. From the tightness of their distribution in the infrared 2-colour diagrams, Seyfert 1s are inferred to be a relatively homogeneous group whose colours can be explained by the addition of nuclei with power-law spectral distributions to ordinary background galaxies. H II region galaxies are like ordinary ones affected to some degree by high dust content, giving rise to reddening and/or blackbody emission. Seyfert 2s form an intermediate category, in some cases being like Seyfert 1s with obscuration and in others being like H II galaxies. The properties of H II-region galaxies in the U–B, V–K diagram are discussed. Most galaxies of this class appear to have had star formation events involving up to 10 per cent of their mass within the last 108 yr. Most Seyfert 2s can be similarly interpreted except that the events would have to have occurred much longer ago.
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We have obtained new and archival Hubble Space Telescope V-band images of kiloparsec-sized circumnuclear star-forming rings in the barred spiral galaxies NGC 1097 and NGC 6951, both of which contain active nuclei. The images resolve the rings into two-armed spirals composed of bright knots located on the outer edges of prominent dust lanes. The two-armed ring morphology seen in these two galaxies appears to be common in barred spirals and is in accord with the results of simulations of bar-driven inflow. In both galaxies, circumnuclear star formation is tightly confined to the ring: in the region between the ring and the nucleus there is an intricate spiral pattern of narrow dust lanes, but no bright knots are visible. Miniature gaseous or stellar bars interior to nuclear rings have been suggested as mechanisms for transporting gas to active nuclei, but it is not clear from the optical morphology whether either of these mechanisms may be at work in these two galaxies. The young star clusters in the rings are extremely compact, with mean effective radii of 2.5 pc in NGC 1097 and <= 4 pc in NGC 6951. Without correcting for extinction, the clusters have absolute V magnitudes of up to - 12.6 mag; with a highly uncertain extinction correction the brightest clusters have luminosities of M_v_ = - 14 to - 15 mag. Such high luminosities and small radii indicate that these objects are examples of "super star clusters" which may be newly formed globular clusters like those found recently in merging galaxies. The centers of barred galaxies may therefore be common sites for the present-day formation of globular clusters. The image of NGC 1097 also contains the type II supernova 1992bd 12 days prior to its discovery in ground-based observations.
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The barred spiral galaxy M83 has been observed at 1465 and 4885 MHz with the VLA. The author finds significant emission in the bar, which is coincident with the narrow, linear, offset dust lanes visible there. This emission is nonthermal and moderately polarized. The author also finds spiral-arm structure, composed of H II regions and extended nonthermal features associated with observed dust lanes. These observations are consistent with the interpretation that the bar and nonthermal spiral-arm emission are due to shock enhancement of the underlying disk emission.
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Millimeter and submillimeter-wave molecular line observations were used to probe the interstellar medium in the central few kpc of three nearby starburst galaxies: M82, IC 342, and M83. The molecular mediums associated with starburst activity in these galaxies appear to be composed of multiple components. The starbursts in M82 and M83 have similar characteristics. The starburst occurring in IC 342 is much more subdued, with many of the derived cloud properties similar to that observed in the central kiloparsec of the Milky Way. A correlation is observed between the cloud size and the star formation efficiency in each galaxy; the higher the efficiency, the smaller the cloud size. There also appears to be a correlation between star formation efficiency and gas surface density.
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This paper discusses data on young star groups that imply the existence of two distinct scales of star formation. The largest scale is that of the star complexes which we assume to be the initial scale, connected with the gas superclouds. The smaller scale is for the well known OB associations and young clusters, which relate to the elementary scale of star formation, that connected with individual giant molecular clouds. An intermediate scale, that of aggregates (groups of a few associations), probably also exists and corresponds to clusters of a few GMCs. The great majority of associations and young clusters are found inside huge star complexes, which include individual older stars (plausible former members of dissolved associations). This hierarchical structure of young star groups, together with the generally different luminosities of individual stars within the different scale aggregates explain the very different estimates of sizes of these groups (all of which have previously been called "associations") in galaxies at different distances and therefore resolution. Concentrations of gas clouds, young associations, and stars within vast complexes are consistent with a top-down scenario of star formation, implying that superclouds producing star complexes are the initial structures, which formed faster than clouds of smaller scale in gaseous galactic disks and especially within spiral density waves, mainly owing to large scale gravitational instability. Sheared star complexes form a flocculent spiral structure without older stars in spiraling patches. Probably all young galactic disks consist of units with masses of about 10^7^ in solar units. Most of the specimens of star complexes- the huge groups of clusters, associations, and high luminosity stars in our Galaxy, luminous patches in unresolved galaxies, and the vast star clouds in nearby galaxies-are not accidental agglomerations but physical entities deserving careful investigation.
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We present CO (J = 1 → 0) interferometry at 2".5 resolution and Hα CCD observations of the circumnuclear starburst region of the barred spiral galaxy NGC 3504. The CO emission is centrally peaked, extends over a region 16" (1.6 kpc) in diameter, and is relatively azimuthally symmetric. The CO radial distribution is well fitted by an exponential with a scale length of 2".3 (220 pc). This simple distribution is surprisingly unusual for the center of a galaxy. The velocity field is consistent with purely circular motions. Gas comprises ˜40% of the dynamical mass within a radius of 100 pc (1"), if the "standard" CO-H2 relationship is assumed. If isothermal and self-gravitating, the circumnuclear gas disk has a scale height of only 5-10 pc, and a spatially averaged proton density of 104 cm-3 at radii less than 300 pc. The rotation curve and the dust-lane morphology indicate the presence of an outer inner Lindblad resonance (OILR) at a radius of ˜5", and an inner inner Lindblad resonance (IILR) at a radius of ˜2". The starburst and most of the circumnuclear gas disk seem to be located between the OILR and the IILR. The maximum value of Ω-κ/2 is nearly twice as large as the bar pattern speed of the large-scale bar, and the OILR and the IILR are well separated, and these may be important dynamical differences between NGC 3504 and nonstarburst barred galaxies. The rate of high-mass star formation per unit gas mass, as traced by the ratio of Hα to CO emission, is uniformly high over the portion of the rotation curve which is nearly solid body, and drops by a factor of ˜4 where the rotation curve turns over and flattens out. Since the CO radial distribution is not ringlike despite the fact that gas is being consumed more rapidly in the center, we believe that the starburst in NGC 3504 is in an early phase of its evolution. The Toomre Q stability parameter is approximately constant at 0.9±0.2 throughout the circumnuclear molecular gas disk, so the simple gravitational instability theory is consistent with ongoing star formation. The radial variation in the cloud growth timescale predicted from a Toomre instability is similar to the radial variation in the gas depletion timescale derived from the Hα/CO ratio, although the timescales differ by a factor of ˜103. Either star formation is surprisingly inefficient, or the cloud collapse timescale is longer than the instability growth timescale We propose that the behavior of star formation for a given value of Q is strongly influenced by the strength of tidal shear, which can help control the star formation rate via the cloud destruction rate. In the central 300 pc of NGC 3504, where the rotation curve is nearly solid body and where the starburst is most intense, a lump of gas with Q ≃ 1 has a density much greater than that which is susceptible to tidal shear. However, 400-600 pc from the center of NGC 3504, where the rotation curve is nearly flat, a lump of gas with Q ≃ 1 has a density close to the range where tidal shear can shred it. A combination of tidal shear and gravitational instability theory can explain why starbursts evolve from the inside out, why evolved starbursts have rings of gas where the rotation curve turns over, and why star formation and the gas supply are regulated to maintain Q ≃ 1 where rotation curves are nearly flat, but may be unregulated where rotation curves are nearly solid body.
Article
Starbursts in inner Lindblad resonance rings are proposed to result from gravitational instabilities that fragment the ring into several bound clouds. Each cloud forms a separate star cluster or hot spot after further energy dissipation and collapse. A linear instability analysis including accretion and an azimuthal magnetic field suggests that the ring instability occurs only after a critical density is reached, which presumably follows a relatively long epoch of gas accretion from bar or spiral torques. The critical density is very high in the inner regions because the Coriolis and tidal forces are high. Typical densities are greater than 100/cu cm, depending on the inner Lindblad resonance radius, rotation curve, accretion rate, and other parameters. The rapid star formation in the starburst follows from the high density at the expected rate epsilon omega rho for local efficiency per cloud epsilon, instability growth rate omega, and density rho. Most of the high rate comes from the density dependence of omega(rho)rho, but the efficiency epsilon could also increase if the ambient velocity dispersion is high in the ring.
Article
New interferometric observations of the molecular gas within the early type barred spiral galaxy NGC 3351 (M95) have revealed a nuclear molecular gas bar about 1 kpc long, that is aligned perpendicular to the large scale stellar bar. The mass of gas associated with the nuclear molecular gas bar is about 1.8 x 10 exp 8 solar masses representing about 25 percent of the total molecular gas mass. NGC 3351 is associated with other distinguishing morphological characteristics including a nuclear ring of H II regions, which brought NGC 3351 early notoriety as a 'hotspot nucleus', and a much larger ring of H II regions that encircles the stellar bar. Collectively, the distinctive morphology of the neutral and ionized medium within NGC 3351 can be understood in terms of resonances associated with a stellar bar pattern rotating at an angular velocity of about 60 km/s kpc.
Article
The distances of eight probable or possible bright members of the Centaurus group of galaxies are derived from secondary and tertiary indicators and from redshifts. The best determined distance is that of M83, and this group centered at supergalactic coordinates L = 154 deg and B = -2 deg covers an elongated area parallel to the supergalactic plane, and has a depth of approximately 2 Mpc in the line of sight. The Fourcade-Figueroa galaxy is probably a background object and the Circinus galaxy is probably a field galaxy, but additional members between it and the group may be hidden near the galactic plane. The velocity dispersion may be interpreted as evidence for stability (bound group) or instability (expanding association or accidental grouping). The presence of three early-type systems in a group dominated by late-type spirals and magellanic irregulars is unusual and suggestive of a chance encounter.
Article
The Catalog of Southern Ringed Galaxies (CSRG) is a comprehensive compilation of diameters, axis ratios, relative bar position angles, and morphologies of inner and outer rings, pseudorings, and lenses in 3692 galaxies south of declination -17 deg. The purpose of the catalog is to evaluate the idea that these ring phenomena are related to orbital resonances with a bar or oval in galaxy potentials. The catalog is based on visual inspection of most of the 606 fields of the Science Research Council (SRC) IIIa-J southern sky survey, with the ESO-B, ESO-R, and Palomar Sky surveys used as auxiliaries when needed for overexposed core regions. The catalog is most complete for SRC fields 1-303 (mostly south of declination -42 deg). In addition to ringed galaxies, a list of 859 mostly nonringed galaxies intended for comparison with other catalogs is provided. Other findings from the CSRG that are not based on statistics are the identification of intrinsic bar/ring misalignment; bars which underfill inner rings; dimpling of R'1 pseudorings; pointy, rectangular, or hexagonal inner or outer ring shapes; a peculiar polar-ring-related system; and other extreme examples of spiral structure and ring morphology.
Article
Images of molecular gas in spiral galaxies show two types of bar structure: large-scale bars, which are correlated with the presence of stellar bars, and generally follow the spiral structure, and nuclear “bars” of a few hundred parsecs in size, which show no direct correlation with the presence of a stellar bar. The dynamical origin of nuclear molecular bars is unclear, although they are consistent with radial inflow of gas into the nucleus, and subsequent evolution of the gas structure.
Article
The response of a gaseous disk to rotating stellar bars of various strengths and pattern speeds is studied using a particle scheme in which the gas 'clouds' move as free particles except where their trajectories cross. Inelastic collisions are introduced to treat the shocks which occur at these locations. The arm-to-interarm density contrast and the pitch angle of the induced spiral arms both increase with increasing bar strength. A widening region exists in which stable periodic orbits are difficult to find until the region encompasses the whole annulus between corotation (CR) and the outer Lindblad resonance. Stars moving in quasi-stochastic orbits originating in this region form an exponential tail in the surface density profile of the stellar disk. As the pattern speed is lowered, more structure appears inside CR which may give rise to the inner rings found near the ends of bars in SB systems.
Article
The metric properties of nuclear rings, pseudorings, spirals, and bars are discussed in the context of resonance theory and are used to gauge the range of sizes for such features. The nuclear rings and pseudorings of strongly barred SB and SAB galaxies have a wide range in linear diameter; the mean diameter is about 1.1 kpc for a sample of 20 objects. We suggest that there are clear analogs of the nuclear rings of strongly barred spirals in both weakly barred (SAB) and nonbarred (SA) spirals. The size ratios of nuclear rings with respect to outer rings and pseudorings, as well as morphological characteristics, are best explained if outer rings and pseudorings are linked to the outer Lindblad resonance, and nuclear rings, pseudorings, and spirals are linked to the inner Lindblad resonance. Nuclear bars are often associated with nuclear rings and spirals, but also can exist independently of such features. We present a list of 13 double-barred galaxies and discuss the significance of the phase angle of the nuclear bar with respect to the primary bar.
Article
Combined spectral and spatial analyses of the central region of the nearby spiral galaxy M83 shows the presence of two different emission components. One component has a spatial size of 2.′7, as defined by the rms radius of the emission region, and has an emission temperature of about 5 keV when it is approximated with a thermal-bremsstrahlung model. The other component has a much smaller radius with an upper limit of 1.′0 rms radius, and has a lower emission temperature of about 0.4 keV. The X-ray luminosities of the two components are, respectively, 2.1 × 1039 erg s−1 (soft component) and 4.8 × 1039 erg s−1 (hard component) in the 0.−10 keV band (for the distance of 3.75 Mpc). The soft, lower-temperature component is consistent to be emission from the hot gas originating from the star-burst activity in the galaxy. On the other hand, the hard component is consistent with being a collection of binary X-ray sources in the bulge region.
Article
The determination of the luminosities and ages of stellar populations in an external spiral galaxy is considered, and results are presented for the spiral galaxy M83. Procedures used in the deconvolution of the light from the spheroid or bulge, old stellar disk (age greater than 100 million years) and young stellar disk (age less than 100 million years) components of the galaxy are presented, and results are indicated in the form of maps of the luminosity, age and mass distributions of the young population. It is pointed out that although star formation is seen to occur in almost all of the 27 pc resolution pixels within the inner spiral arms, most of the luminosity is confined to associations in aggregates a few hundred pc in size. The ages and positions of these associations indicate a mechanism which globally triggers the formation of massive stars in a well-defined, symmetric two-armed spiral pattern, acts locally in a largely stochastic manner, and terminates after a few tens of millions of years. Results also suggest that the initial mass function in the arms of M83 cannot be represented by a power law over the customary mass range with normal population I stars.
Article
The flow of gas in and around the bars or ovals of modeled barred galaxies is studied, and it is shown that it is intimately linked to the properties of the periodic orbits. Simulations show that the density of the gas in and around the bar region is slow, except for the center and two narrow lanes which are the loci of shocks. Such shocks form if the x1 periodic orbits have either loops or large curvature values at their apocenters. The form of the shock loci depends on the number of parameters characterizing the bar and disk potential. In order for shocks to be offset from the bar major axis toward its leading side, the x2 and x3 families must not only exist but also cover a sufficient extent along the bar major axis. The existence of offset shock loci of the observed shape puts strict constraints on the values the various model parameters may take. For low values of the bar axial ratio or the bar quadrupole moment the shock loci are curved, with their concave sides towards the bar major axis. Substantial inflow is found in models with strong shocks.
Article
Molecular hydrogen is thought to form from atomic hydrogen on the surface of dust grains in the interstellar medium, and to be dissociated primarily by ultraviolet radiation from hot stars1. This process has been modelled on the 1 pc length scale for the case of dense interstellar clouds2 and near newly formed ionizing stars3. Observational evidence that molecular hydrogen is indeed dissociated in shells around young OB stars in the Galaxy has been presented recently4–6. We suggest here that the same dissociation process occurs on the kpc scale in active star-forming regions of some galaxies, and that this dissociation may strongly affect the observed morphology of atomic hydrogen in spiral arms.
Article
This paper presents a detailed photometric and kinematic study of the well-known Seyfert 2 galaxy NGC 3081, one of the best examples of a resonance ring barred galaxy in the sky. Improved optical images compared to previous studies reveal that NGC 3081 is a classic R1R galaxy, a type that shows a distinctive outer ring/pseudoring pattern at large radii that can be linked to orbit families at the outer Lindblad resonance (OLR). Together with an exceptionally strong inner ring and a blue nuclear ring, NGC 3081 has the rare distinction of having all four of the main types of resonance rings that have been predicted by test-particle models of barred spirals. Near-infrared imaging of NGC 3081 reveals clear old rings connected to the inner ring and the R1 outer ring. Objective comparison of the B- and H-band positions of the inner ring indicates no significant difference in shape, major-axis position angle, or major-axis radius between the two passbands, in spite of the different stellar populations each band emphasizes. Imaging Fabry-Perot interferometry provides an intriguing picture of star formation in the galaxy and of the dynamics of the system. Hα emission is strong in the inner ring and is confined to a bounded elliptical annulus of diffuse emission whose ellipticity increases from the inner edge to the outer edge. A few H II regions are connected to the strong R1-type outer ring, particularly just off the major axis of the inner ring where "dimples," typical of the R1 morphology, are found. There is bright emission in an oval zone in the inner 10'' radius, but no enhancement of emission is associated with the blue-light nuclear ring. Since most of the emission is connected with the intrinsically oval inner ring and the bright center, the global dynamics of the galaxy are uncertain. Nevertheless, using a combination of the gravitational potential derived from the H-band light distribution and a simple halo model, we estimate the pattern speed and the location of all major resonances in the disk of NGC 3081. We find that NGC 3081 fits extremely well into recent ideas of barred galaxy dynamics.
Article
Photometric, morphological and spectral grounds suggest that the peculiar nuclei regions of NGC 1097, 1672, 2997, 5236, and 7552 are composed basically of a normal nucleus surrounded by several large but otherwise normal (T e 8000 K,N e 1000/cm3) H II regions. The stellar components are also normal although in some cases (NGC 1097, 1672, 1808 and 7552) there is a larger contribution of late type stars.
Article
The southwestern bar-spiral arm transition zone in M83 is been studied in CO, H-alpha, H I, red light, and the radio continuum. A massive molecular gas complex in the heart of the transition zone is composed or two principal components which have the morphology and kinematics expected from orbit crowding, where gas on highly elliptical orbits form the bar region converges with gas on more circular orbits from the spiral arm region. Three mechanisms for the origin of the orbit crowding are investigated, and it is proposed that the crowding is due primarily to density wave streaming motions caused by the bar and spiral arms. The inner CO component is partially coincident with a region of highly polarized radio continuum emission which precedes the H-alpha spiral arm by 15-25 arcsec, indicating that it lies on or just downstream from a shock front. This suggests that the bar gas approaching the transition zone is shocked and explains the ridge of dense gas seen upstream from the spiral arm.
Article
A high-resolution interferometric map of the CO emission on the eastern spiral arm of M83 is presented. The detected emission originates in about five unresolved components located parallel but about 300 pc downstream from the dust lane which lies along the inner edge of the spiral arm. All the CO components in the map but one are located within 130 pc of an H II region and may represent emission from locally heated gas. The lack of CO emission on the dust lane indicates that the dense molecular gas does not pile up here in M83. Remarkable differences between the molecular gas distributions in M83 and the spiral arms or M51, where CO emission peaks on the dust lane, is attributed to the difference in the strength of their density waves. The observations of M83 are consistent with the model of Elmegreen in which diffuse gas is compressed at the shock front, producing the dust lane at the inner edge of the spiral arm while dense giant molecular clouds pass through the front and form a broad distribution on the arm.
Article
We present the results of an extensive grid of evolutionary synthesis models for populations of massive stars. The parameter space has been chosen to correspond to conditions typically found in objects like giant H II regions, H II galaxies, blue compact dwarf galaxies, nuclear starbursts, and infrared luminous starburst galaxies. The models are based on the most up-to-date input physics for the theory of stellar atmospheres, stellar winds, and stellar evolution. A population of massive stars is not only important in terms of its output of radiation but also via its deposition of mechanical energy. The output of radiative and mechanical luminosity is compared at various starburst epochs. In a supernova dominated instantaneous starburst, the mechanical luminosity can be as large as almost 10% of the total radiative luminosity. This occurs when most massive O stars have disappeared, and the synthetic spectrum in the optical and near-ultraviolet is dominated by B and A stars. During this epoch, the output of ionizing radiation below 912 A becomes very small, as indicated by a very large Lyman discontinuity and a very small ratio of ionizing over mechanical luminosity. We discuss the relevance of these results for the interpretation of starburst galaxies, active galactic nuclei, and the energetics of the interstellar medium.
Article
A mechanism, applicable to AGN and nuclear starburst galaxies in which there is accretion onto a supermassive black hole (SBH), is proposed which brings in gas from large to small scales by successive dynamical instabilities. On the large scale, a stellar bar sweeps the interstellar medium into a gaseous disk a few hundred pc in radius. Under certain conditions, this disk can become unstable again, allowing material to flow inwards until turbulent viscous processes control angular-momentum transport. This flow pattern may feed viscosity-driven accretion flows around an SBH or lead to the formation of an SBH if none was present initially.
Article
A detailed comparison of the distribution of 60-micron IR and 20-cm radio continuum emission within NGC 5236 (M83) and NGC 6946 is presented. In the disks of both galaxies, a slow decrease of the 60-micron-to-20-cm ratio with increasing radius is observed. Values of this ratio within the central regions are enhanced by roughly a factor of 3 with respect to the outer disks, whereas the corresponding enhancement in radio surface brightness is at least a factor of 20. The 100-micron-to-20-cm ratio shows a similar but shallower gradient. To account for these observations, it is suggested that spiral galaxies are characterized by an IR disk with a shorter scale length than that of the radio continuum disk, the latter being smeared as a result of cosmic-ray propagation.
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The IR properties of galaxies with star-formation-dominated central IR luminosity are discussed, summarizing the results of recent observational investigations. Consideration is given to the constituents of star-forming galaxies (dust in emission, photoionized gas, and shock-heated gas), the IR morphology (IR luminosity and young stars and NIR mapping to see beyond dust), the relationship between IR and radio emission, M82 as a typical object, and the implications of the observations for theoretical models of star formation. It is concluded that star formation in these galaxies will exhaust the available gas supply in much less than a Hubble time. Diagrams, graphs, sample images, and tables of numerical data are provided.
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We investigate the dynamical response of stellar orbits in a rotating barred galaxy potential to the perturbation by a nuclear gaseous ring. The change in 3D periodic orbit families is examined as the gas accumulates between the inner Lindblad resonances. It is found that the phase space allowable to the x2 family of orbits is substantially increased and a vertical instability strip appears with the growing mass of the ring. A significant distortion of the x1 orbits is observed in the vicinity of the ring, which leads to the intersection between orbits with different values of the Jacobi integral. We also examine the dependence of the orbital response to the eccentricity and alignment of the ring with the bar. Misalignment between an oval ring and a bar can leave observational footprints in the form of twisted near-infrared isophotes in the vicinity of the ring. It is inferred that a massive nuclear ring acts to weaken and dissolve the stellar bar exterior to the ring, whereas only weakly affecting the orbits interior to the inner Lindblad resonances. Consequences for gas evolution in the circumnuclear regions of barred galaxies are discussed as well. Comment: 27 pages, 11 postscript figures included, latex using aastex 4.0, uuencoded compressed tar file, to appear in ApJ