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Interstellar dust properties of M51 from AKARI mid-infrared images
Abstract
Using mid-infrared (MIR) images of four photometric bands of the Infrared
Camera (IRC) onboard the AKARI satellite, S7 (7 um), S11 (11 um), L15 (15 um),
and L24 (24 um), we investigate the interstellar dust properties of the nearby
pair of galaxies M51 with respect to its spiral arm structure. The arm and
interarm regions being defined based on a spatially filtered stellar component
model image, we measure the arm-to-interarm contrast for each band. The
contrast is lowest in the S11 image, which is interpreted as that among the
four AKARI MIR bands the S11 image best correlates with the spatial
distribution of dust grains including colder components, while the L24 image
with the highest contrast traces warmer dust heated by star forming activities.
The surface brightness ratio between the bands, i.e. color, is measured over
the disk of the main galaxy, M51a, at 300 pc resolution. We find that the
distribution of S7/S11 is smooth and well traces the global spiral arm pattern
while L15/S11 and L24/S11 peak at individual HII regions. This result indicates
that the ionization state of PAHs is related to the spiral structure.
Comparison with observational data and dust models also supports the importance
of the variation in the PAH ionization state within the M51a disk. However, the
mechanism driving this variation is not yet clear from currently available data
sets. Another suggestion from the comparison with the models is that the PAH
fraction to the total dust mass is higher than previously estimated.
... In addition to temporal variation, ARs also show spatial variation in FIP bias. The highest FIP bias values are observed at AR loop footpoints (Baker et al. 2013), indicating that this is where the fractionation process takes place (as proposed by Laming 2004). Traces of high FIP bias are observed along some of the AR loops, indicating plasma starting to mix along loops (Baker et al. 2013). ...
... The highest FIP bias values are observed at AR loop footpoints (Baker et al. 2013), indicating that this is where the fractionation process takes place (as proposed by Laming 2004). Traces of high FIP bias are observed along some of the AR loops, indicating plasma starting to mix along loops (Baker et al. 2013). Flux cancellation along the AR main polarity inversion line (PIL), along with the associated flux rope formation, leads to lower FIP bias levels (Baker et al. 2013(Baker et al. , 2022. ...
... Traces of high FIP bias are observed along some of the AR loops, indicating plasma starting to mix along loops (Baker et al. 2013). Flux cancellation along the AR main polarity inversion line (PIL), along with the associated flux rope formation, leads to lower FIP bias levels (Baker et al. 2013(Baker et al. , 2022. Lower FIP bias levels were also found in the part of an AR where two failed eruptions occurred . ...
The chemical composition of the solar corona is different from that of the solar photosphere, with the strongest variation being observed in active regions (ARs). Using data from the Extreme Ultraviolet (EUV) Imaging Spectrometer (EIS) on Hinode, we present a survey of coronal elemental composition as expressed in the first ionization potential (FIP) bias in 28 ARs of different ages and magnetic flux content, which are at different stages in their evolution. We find no correlation between the FIP bias of an AR and its total unsigned magnetic flux or age. However, there is a weak dependence of FIP bias on the evolutionary stage, decreasing from 1.9 to 2.2 in ARs with spots to 1.5–1.6 in ARs that are at more advanced stages of the decay phase. FIP bias shows an increasing trend with average magnetic flux density up to 200 G, but this trend does not continue at higher values. The FIP bias distribution within ARs has a spread between 0.4 and 1. The largest spread is observed in very dispersed ARs. We attribute this to a range of physical processes taking place in these ARs, including processes associated with filament channel formation. These findings indicate that, while some general trends can be observed, the processes influencing the composition of an AR are complex and specific to its evolution, magnetic configuration, or environment. The spread of FIP bias values in ARs shows a broad match with that previously observed in situ in the slow solar wind.
... This galaxy is also known as the whirlpool galaxy and is interacting with a companion galaxy, M51b or NGC 5195. We adopt parameters of M51 used in Egusa et al. (2013) and list them in Table 1. Their notation of two spiral arms, arm1 being to extend to the opposite side of M51b and arm2 being connected to M51b, is also adopted. ...
... In order to avoid picking up peaks in interarm regions, we define arm regions based on the position of stellar arms. As an initial step, the arm definition from Egusa et al. (2013), which is based on a stellar brightness map from Mentuch Cooper et al. (2012), is adopted. This definition is presented by the thin dashed contours in Figure 2. We regrid the Σ star phase diagram so that the radial interval becomes 4 ′′ and create azimuthal profile at each radius. ...
... Each image is normalized by its azimuthal average at each radius in order to emphasize the spiral arm structure. Thin dashed contours indicate the arm definition byEgusa et al. (2013), while thick solid contours indicate new definition adopted in this paper (see §2.4). ...
Theoretical studies on the response of interstellar gas to a gravitational potential disc with a quasi-stationary spiral arm pattern suggest that the gas experiences a sudden compression due to standing shock waves at spiral arms. This mechanism, called a galactic shock wave, predicts that gas spiral arms move from downstream to upstream of stellar arms with increasing radius inside a corotation radius. In order to investigate if this mechanism is at work in the grand-design spiral galaxy M51, we have measured azimuthal offsets between the peaks of stellar mass and gas mass distributions in its two spiral arms. The stellar mass distribution is created by the spatially resolved spectral energy distribution fitting to optical and near infrared images, while the gas mass distribution is obtained by high-resolution CO and HI data. For the inner region (r < 150"), we find that one arm is consistent with the galactic shock while the other is not. For the outer region, results are less certain due to the narrower range of offset values, the weakness of stellar arms, and the smaller number of successful offset measurements. The results suggest that the nature of two inner spiral arms are different, which is likely due to an interaction with the companion galaxy.
... Similarly, the intensity of galactic cosmic rays should be very smooth on the relevant scales, as was observed before and after the event under consideration. Burlaga et al. (2013) reported an observation by V1 of a weak shock or compression in the interstellar magnetic field near the end of 2012 November. This magnetic-field compression was accompanied by plasma-wave activity which was observed by the PWS on V1. ...
... For this reason, we think it possible that the cosmic ray event observed in 2013 was a consequence of a disturbance, similar to that reported by Burlaga et al. (2013) originating in the heliosphere, propagating out into the interstellar medium. We note that there was no apparent cosmic-ray disturbance associated with the 2012 November event. ...
... We next consider a plausible disturbance, similar to that reported by Burlaga et al. (2013), propagating out from the heliopause in the interstellar medium. Only the collisionless interstellar plasma with a density of ≈.08 cm −3 is relevant here. ...
We present a possible explanation for the transient increase in the galactic cosmic ray flux observed by Voyager 1 (V1) beyond the heliopause, in 2013 March. We suggest that this disturbance may be caused by a propagating disturbance in the interstellar magnetic field, of heliospheric origin. A model in which a magnetic disturbance, propagating outward from the heliosphere into the very-local interstellar plasma, affects the galactic cosmic rays is presented. We also discuss the possibility that this event is related to the plasma-wave event observed some 25 days later by the PWS experiment on V1.
... For several pointings, we have confirmed that these factors are also applicable to extended objects (e.g. Egusa et al., 2013). ...
... The toolkit already contains modules to create a template for the earthshine light and to subtract it from affected frames, which have been confirmed to work successfully for several pointings (e.g. Egusa et al., 2013). ...
We have been working on data processing and calibration of AKARI/IRC images from pointed observations. As of September 2014, a data package for each pointing only contains raw data and quick-look data, so that users have to process them using the toolkit by themselves. We plan to change this situation and to provide science-ready data sets, which are easy-to-use for non-AKARI experts. For Phase 1&2, we have updated dark and flat calibrations, and also the toolkit itself to produce images more reliable and easier to use. A new data package includes fully calibrated images with WCS information. We released it for about 4000 pointings at the end of March 2015.
The formation of methyl formate (CH3OCHO) upon electron irradiation of mixed ices of carbon monoxide (CO) and methanol (CH3OH) has been monitored by post-irradiation thermal desorption spectrometry (TDS). The energy dependence of the product yields obtained with electron energies between 3 and 18 eV was studied. These energies are characteristic of secondary electrons that are released in vast numbers under the effect of ionizing radiation. Our results reveal that the reactions leading to methyl formate are initiated by a number of different electron-molecule interactions that produce CH3O˙ radicals. Dissociative electron attachment (DEA) to CH3OH around 5.5 eV and neutral dissociation (ND) above 7 eV release CH3O˙ radicals that can add to CO to initiate a reaction sequence leading to formation of methyl formate. Around 10 eV, DEA to CO yields an oxygen radical anion that reacts with CH3OH to also produce CH3O˙ radicals. Alternatively, CH3OH can also release H˙ radicals upon both DEA and ND. These can also add to CO to form HCO˙ radicals as an intermediate to formaldehyde (H2CO), which was also investigated to unravel the reaction mechanisms leading to formation of methyl formate. The recombination of HCO˙ and CH3O˙ as minority radical species is considered as an alternative but less probable pathway to the formation of methyl formate. To the best of our knowledge, this is the first study showing considerable contributions of DEA to the formation of methyl formate in CH3OH containing ices. Thus, our study has important implications for current astrochemical models.
The mid-infrared spectrum contains rich diagnostics to probe the physical properties of galaxies, among which the pervasive emission features from polycyclic aromatic hydrocarbons (PAHs) offer promising means of estimating the star formation rate (SFR) relatively immune from dust extinction. This paper investigates the effectiveness of PAH emission as a SFR indicator on subkiloparsec scales by studying the Spitzer/IRS mapping-mode observations of the nearby grand-design spiral galaxy M51. We present a new approach of analyzing the spatial elements of the spectral data cube that simultaneously maximizes spatial resolution and spatial coverage, while yielding reliable measurements of the total, integrated 5–20 μ m PAH emission. We devise a strategy of extracting robust PAH emission using spectra with only partial spectral coverage, complementing missing spectral regions with properly combined mid-infrared photometry. We find that in M51 the PAH emission correlates tightly with the extinction-corrected far-ultraviolet, near-ultraviolet, and H α emission, from scales of ∼0.4 kpc close to the nucleus to 6 kpc out in the disk of the galaxy, indicating that PAH serves as an excellent tracer of SFR over a wide range of galactic environments. But regional differences exist. Close to the active nucleus of M51 the 6.2 μ m feature is weaker, and the overall level of PAH emission is suppressed. The spiral arms and the central star-forming region of the galaxy emit stronger 7.7 and 8.6 μ m PAH features than the inter-arm regions.
Isomers with the formula C3H2O have intrigued and puzzled astronomers and astrochemists for many years, with propynal and cyclopropenone, but not propadienone, known to be interstellar. However, there is a severe lack of laboratory spectra of the solid phases of these compounds with which to investigate their interstellar chemistry. Here we present the first infrared spectra of amorphous and crystalline forms of propynal, HCC—C(O)H, at multiple temperatures. Band positions are tabulated and band strengths are derived in terms of absorption coefficients and integrated intensities. Optical constants are calculated for amorphous propynal, refractive indices are measured, and densities are estimated. Three laboratory astrochemistry applications are described, including a new spectral identification in an earlier paper. It is shown that propynal's C|$\equiv$|C infrared absorbance is about 30 000 per cent stronger than the corresponding feature in acetylene. This band's intensity and spectral position make it an attractive candidate for astronomical searches involving interstellar ices.
The H-addition reactions on the icy interstellar grains may play an important role into the formation of complex organic molecules. In the present work we propose a comparative study of H2CO + H, CH3CH2CHO + H and CH3OCHO + H solid state reactions at 10 K under interstellar conditions in order to characterize the main reaction pathways involved into the hydrogenation processing of CHO functional group. We show that the most probable mechanism for the formation of alcohols under non-energetic conditions through the saturation of CHO group corresponds to the attachment of the H atom to the CH group with noticeable variations of the energy barriers for each studied reaction. These energy barriers have been calculated to be 8.3, 14.6 and 32.7 kJ/mol for H2CO + H, CH3CH2CHO + H and CH3OCHO + H, respectively. The coupling of the experimental and theoretical analysis proves that while the simplest aldehyde, formaldehyde, is easily reduced into methanol, methylformate and propanal behave differently under H-bombardments but they cannot be a source of alcohol formation through H-addition reactions. Consequently, for the formation of alcohols larger than CH3OH, other chemical pathways should be taken into account, probably energetic processing such as the photolysis of interstellar ices analogues containing C-, H- and O- bearing compounds or the coupling of the H-addition reaction and photon-irradiation on species with a CHO functional group.
We present $Spitzer$/IRS mid-infrared spectral maps of the Galactic star-forming region M17 as well as IRSF/SIRIUS Br$\gamma$ and Nobeyama 45-m/FOREST $^{13}$CO ($J$=1--0) maps. The spectra show prominent features due to polycyclic aromatic hydrocarbons (PAHs) at wavelengths of 6.2, 7.7, 8.6, 11.3, 12.0, 12.7, 13.5, and 14.2 $\mu$m. We find that the PAH emission features are bright in the region between the HII region traced by Br$\gamma$ and the molecular cloud traced by $^{13}$CO, supporting that the PAH emission originates mostly from photo-dissociation regions. Based on the spatially-resolved maps, we examine spatial variations of the PAH properties in detail. As a result, we find that the interband ratio of PAH 7.7 $\mu$m/PAH 11.3 $\mu$m varies locally near M17SW, but rather independently of the distance from the OB stars in M17, suggesting that the ionization degree of PAHs is mainly controlled by local conditions rather than the global UV environments determined by the OB stars in M17. We also find that the interband ratios of the PAH 12.0 $\mu$m, 12.7 $\mu$m, 13.5 $\mu$m, and 14.2 $\mu$m features to the PAH 11.3 $\mu$m feature are high near the M17 center, which suggests structural changes of PAHs through processing due to intense UV radiation, producing abundant edgy irregular PAHs near the M17 center.
The Japanese infrared astronomical satellite AKARI performed ∼4000 pointed observations for 16 months until the end of August
2007, when the telescope and instruments were cooled by liquid helium. Observation targets include solar system objects, Galactic
objects, local galaxies, and galaxies at cosmological distances. We describe recent updates on calibration processes of near-
and mid-infrared images taken by the Infrared Camera (IRC), which has nine photometric filters covering 2–27 μm continuously.
Using the latest data reduction toolkit, we created calibrated and stacked images from each pointed observation. About 90%
of the stacked images have a position accuracy better than 1${^{\prime\prime}_{.}}$5. Uncertainties in aperture photometry estimated from a typical standard sky deviation of stacked images are a factor of
∼2–4 smaller than those of AllWISE at similar wavelengths. The processed images, together with documents such as process logs,
as well as the latest toolkit are available online.
The reaction mechanisms between azacyclopropenylidene and R-H (R = F, OH, NH2, CH3) have been systematically investigated employing the second-order Møller-Plesset perturbation theory method in order to better understand the reactivity of azacyclopropenylidene with R-H compounds. We have investigated the reaction mechanisms and obtained the potential energy surface of these reactions. The mechanisms of the four reactions are identical to each other. Based on the calculated results, there are three steps along the reaction pathways of azacyclopropenylidene and R-H. The first step is insertion of azacyclopropenylidene into the R-H bond to form the cyclicimine intermediate. The second step is the H-transfer process to form the cyclic carbene intermediate. The third step is the ring-opening process to form ketenimine. Based on the calculated energy barrier the second step is the rate-determining step.
In August 2012, Voyager 1 crossed the heliopause at a distance of 121.5 AU from the Sun. It is argued that the spacecraft entered a region in the outer heliosheath that had the characteristics of a plasma depletion layer. Observed plasma parameters at the heliopause, properties of plasma depletion layers, and some assumptions are used to derive a set of plasma parameters on both sides of the heliopause. Using the density, temperature, and magnetic field magnitude on each side, the corresponding plasma beta and Alfven Mach number (in the outer heliosheath) are derived. These plasma parameters are used to demonstrate that the plasma depletion process is occurring in the outer heliosheath adjacent to the heliopause and these parameters are used to determine if lower hybrid waves are generated locally and if magnetic reconnection is occurring locally at the location of the Voyager 1 crossing. Reconnection may not be an effective source of superthermal electrons at the heliopause, based on the small Alfven speeds there (VA ≤ 100 km/s) and an empirical connection between electron heating and Alfven speed found in inner solar system studies.
We present high angular resolution observations of the HCN(1-0) emission (at
~1" or ~34 pc), together with CO J = 1-0, 2-1, and 3-2 observations, toward the
Seyfert 2 nucleus of M51 (NGC 5194). The overall HCN(1-0) distribution and
kinematics are very similar to that of the CO lines, which have been indicated
as the jet-entrained molecular gas in our past observations. In addition, high
HCN(1-0)/CO(1-0) brightness temperature ratio of about unity is observed along
the jets, similar to that observed at the shocked molecular gas in our Galaxy.
These results strongly indicate that both diffuse and dense gases are entrained
by the jets and outflowing from the AGN. The channel map of HCN(1-0) at the
systemic velocity shows a strong emission right at the nucleus, where no
obvious emission has been detected in the CO lines. The HCN(1-0)/CO(1-0)
brightness temperature ratio at this region reaches >2, a value that cannot be
explained considering standard physical/chemical conditions. Based on our
calculations, we suggest infrared pumping and possibly weak HCN masing, but
still requiring an enhanced HCN abundance for the cause of this high ratio.
This suggests the presence of a compact dense obscuring molecular gas in front
of the nucleus of M51, which remains unresolved at our ~1" (~34 pc) resolution,
and consistent with the Seyfert 2 classification picture.
The electronic spectrum of protonated coronene in the gas phase was measured at vibrational and rotational temperatures of 15 K in a 22-pole ion trap. The (1) 1A′ ← X 1A′ electronic transition of this larger polycyclic aromatic hydrocarbon cation has an origin band maximum at 14 383.8 ± 0.2 cm–1 and shows distinct vibrational structure in the (1) 1A′ state. Neither the origin nor the strongest absorptions to the blue coincide with known diffuse interstellar bands, implying that protonated coronene is not a carrier.
To investigate whether the Sun's global meridional circulation is primarily mechanically or thermally driven, we solve a hydrodynamical model to determine what meridional flow pattern is most likely to occur for the Sun's relatively well-observed differential rotation. We include Coriolis forces and turbulent Reynolds stresses, and hold the inclusion of thermodynamics for a future work. We find that the steady-state solution from this model is sensitive to the magnitude of the density decline with radius, yielding a single flow cell with poleward surface flow for density declines of less than about a factor of 4 within the convection zone, but two cells, with a reversed circulation in high latitudes, for all higher density declines, including that for the adiabatically stratified solar convection zone. This two-celled pattern is virtually independent of the magnitude of the decline of the turbulent viscosity with depth within the convection zone. For a solar-like density increase with depth, two cells are found for a wide variety of differential rotations, including that closest to solar observations. We compare our meridional circulation with axisymmetric convective rolls that occur in 3D global convective simulations, which tend to align with rotation axis. For solar-like turbulent viscosity, the meridional flow speeds are much larger than observed, implying that an additional physical mechanism is needed that works against the meridional flow. Candidates include negative buoyancy forces, anisotropic small-scale turbulent diffusion of momentum or heat and latitude-dependent radial heat flux by solar convection influenced by rotation. The role of these additional mechanisms in determining flow speed and in producing latitudinal and radial structures in the meridional circulation will be investigated in a future paper.
DustEM computes the extinction and the emission of interstellar dust
grains heated by photons. It is written in Fortran 95 and is jointly
developed by IAS and CESR. The dust emission is calculated in the
optically thin limit (no radiative transfer) and the default spectral
range is 40 to 108 nm. The code is designed so dust properties can
easily be changed and mixed and to allow for the inclusion of new grain
physics.
We present a study of H II regions in M51 using the Hubble Space Telescope Advanced Camera for Surveys images taken as part of the Hubble Heritage Program. We have cataloged about 19,600 H II regions in M51 with H? luminosity in the range of L = 1035.5-1039.0?erg?s?1. The H? luminosity function of H II regions (H II LF) in M51 is well represented by a double power law with its index ? = ?2.25 ? 0.02 for the bright part and ? = ?1.42 ? 0.01 for the faint part, separated at a break point L = 1037.1?erg?s?1. This break was not found in previous studies of M51 H II regions. Comparison with simulated H II LFs suggests that this break is caused by the transition of H II region ionizing sources, from low-mass clusters (with ~103M
?, including several OB stars) to more massive clusters (including several tens of OB stars). The H II LFs with L < 1037.1?erg?s?1are found to have different slopes for different parts in M51: the H II LF for the interarm region is steeper than those for the arm and the nuclear regions. This observed difference in H II LFs can be explained by evolutionary effects: H II regions in the interarm region are relatively older than those in the other parts of M51.
We have mapped 6 and 2 cm continuum emission from the nuclear regions of
ten normal spiral galaxies (Maffei 2, NGC 2403, M81, NGC 4236, NGC 4258,
NGC 4736, NGC 4826, M51, and M101) using the Very Large Array (VLA). The
galaxies were chosen because they are nearby (approximately less than or
equal to 5 Mpc) and likely to have bright continuum emission. Matching
beamsizes of approximately 1.5 sec allow the construction of spectral
index maps from which the relative amounts of thermal bremsstrahlung and
nonthermal synchrotron emission are estimated. Bright radio continuum is
detected within the inner arcminute of eight of the 10 galaxies. Even at
arcsecond resolution, corresponding to sizescales of 10-30 pc,
nonthermal synchrotron emission from supernova remnants (SNRs) dominates
the 6 cm continuum fluxes. High intensities suggest that this nonthermal
component originates in a large population of young SNRs associated with
recent star formation. Candidates for radio supernovae appear in several
galaxies, including a candidate for SN 1986L in M83. Weak nonthermal
core sources, similar to the Galactic center source Sgr A*, may be
present in four of the galaxies. The estimated thermal fluxes indicate
that many of the galaxies have significant Lyman continuum fluxes in
their cores, NLyc approximately 1051 - 1053/s,
corresponding to OB luminosities of 2 x 108 solar
luminosity to 5 x 109 solar luminosity, generally consistent
with observed far-infrared luminosities. Recent star formation appears
to dominate both infrared and radio continuum fluxes in these galaxies.
We present maps of the separated thermal and nonthermal emission for the
starburst sources in Maffei 2 and M83.
We have obtained the first images of the 6.75 and 15μm continuum
emission in the Whirlpool galaxy with ISOCAM, the infrared camera of the
ISO (Kesslet et al., 1996A&A...315L..27K). These data reveal the
well-developed, spiral arms of the galaxy, in addition to diffuse
emission between arms and in the outer regions of the disk. There is a
striking correspondence with the 15μm emission and the Hα
emission over the galaxy. We tighten this link by looking at the
properties of the individual HII regions and show that the 15μm
emission traces massive star formation and is not affected by extinction
as is Hα. The data show systematic mid-infrared color variations
perpendicular to the arms, which we interpret as two distinct dust
components contributing to the mid-infrared emission. We examine the
implication of these color variations in light of our current
understanding of dust at these wavelength.
High-resolution mid-infrared spectra are presented for 155 nuclear and extranuclear regions from the Spitzer Infrared Nearby Galaxies Survey (SINGS). The fluxes for nine atomic forbidden and three molecular hydrogen mid-infrared emission lines are also provided, along with upper limits in key lines for infrared-faint targets. The SINGS sample shows a wide range in the ratio of [S III] 18.71 μm/[S III] 33.48 μm, but the average ratio of the ensemble indicates a typical interstellar electron density of 300-400 cm–3 on ~23'' × 15'' scales and 500-600 cm–3 using ~11'' × 9'' apertures, independent of whether the region probed is a star-forming nuclear, a star-forming extranuclear, or an active galactic nuclei (AGN) environment. Evidence is provided that variations in gas-phase metallicity play an important role in driving variations in radiation field hardness, as indicated by [Ne III] 15.56 μm/[Ne II] 12.81 μm, for regions powered by star formation. Conversely, the radiation hardness for galaxy nuclei powered by accretion around a massive black hole is independent of metal abundance. Furthermore, for metal-rich environments AGN are distinguishable from star-forming regions by significantly larger [Ne III] 15.56 μm/[Ne II] 12.81 μm ratios. Finally, [Fe II] 25.99 μm/[Ne II] 12.81 μm versus [Si II] 34.82 μm/[S III] 33.48 μm also provides an empirical method for discerning AGN from normal star-forming sources. However, similar to [Ne III] 15.56 μm/[Ne II] 12.81 μm, these mid-infrared line ratios lose their AGN/star-formation diagnostic powers for very low metallicity star-forming systems with hard radiation fields.
With the goal of investigating the degree to which the MIR emission traces the SFR, we analyze Spitzer 8 and 24 μm data of star-forming regions in a sample of 33 nearby galaxies with available HST NICMOS images in the Paα (1.8756 μm) emission line. The galaxies are drawn from the SINGS sample and cover a range of morphologies and a factor ~10 in oxygen abundance. Published data on local low-metallicity starburst galaxies and LIRGs are also included in the analysis. Both the stellar continuum-subtracted 8 μm emission and the 24 μm emission correlate with the extinction-corrected Paα line emission, although neither relationship is linear. Simple models of stellar populations and dust extinction and emission are able to reproduce the observed nonlinear trend of the 24 μm emission versus number of ionizing photons, including the modest deficiency of 24 μm emission in the low-metallicity regions, which results from a combination of decreasing dust opacity and dust temperature at low luminosities. Conversely, the trend of the 8 μm emission as a function of the number of ionizing photons is not well reproduced by the same models. The 8 μm emission is contributed, in larger measure than the 24 μm emission, by dust heated by nonionizing stellar populations, in addition to the ionizing ones, in agreement with previous findings. Two SFR calibrations, one using the 24 μm emission and the other using a combination of the 24 μm and Hα luminosities (Kennicutt and coworkers), are presented. No calibration is presented for the 8 μm emission because of its significant dependence on both metallicity and environment. The calibrations presented here should be directly applicable to systems dominated by ongoing star formation.
We present a detailed analysis of the radial distribution of dust properties in the SINGS sample, performed on a set of ultraviolet (UV), infrared (IR), and H I surface brightness profiles, combined with published molecular gas profiles and metallicity gradients. The internal extinction, derived from the total-IR (TIR)-to-far-UV (FUV) luminosity ratio, decreases with radius, and is larger in Sb-Sbc galaxies. The TIR-to-FUV ratio correlates with the UV spectral slope β, following a sequence shifted to redder UV colors with respect to that of starbursts. The star formation history (SFH) is identified as the main driver of this departure. Both L TIR/L FUV and β correlate well with metallicity, especially in moderately face-on galaxies. The relation shifts to redder colors with increased scatter in more edge-on objects. By applying physical dust models to our radial spectral energy distributions, we have derived radial profiles of the total dust mass surface density, the fraction of the total dust mass contributed by polycyclic aromatic hydrocarbons (PAHs), and the intensity of the radiation field heating the grains. The dust profiles are exponential, their radial scale length being constant from Sb to Sd galaxies (only ~10% larger than the stellar scale length). Many S0/a-Sab galaxies have central depressions in their dust radial distributions. The PAH abundance increases with metallicity for 12 + log(O/H) < 9, and at larger metallicities the trend flattens and even reverses, with the SFH being a plausible underlying driver for this behavior. The dust-to-gas ratio is also well correlated with metallicity and therefore decreases with galactocentric radius. Although most of the total emitted IR power (especially in the outer regions of disks) is contributed by dust grains heated by diffuse starlight with a similar intensity as the local Milky Way radiation field, a small amount of the dust mass (~1%) is required to be exposed to very intense starlight in order to reproduce the observed fluxes at 24 μm, accounting for ~10% of the total integrated IR power.
Massive star formation occurs in giant molecular clouds (GMCs); an understanding of the evolution of GMCs is a prerequisite to develop theories of star formation and galaxy evolution. We report the highest-fidelity observations of the grand-design spiral galaxy M51 in carbon monoxide (CO) emission, revealing the evolution of GMCs vis-a-vis the large-scale galactic structure and dynamics. The most massive GMCs (giant molecular associations (GMAs)) are first assembled and then broken up as the gas flow through the spiral arms. The GMAs and their H2 molecules are not fully dissociated into atomic gas as predicted in stellar feedback scenarios, but are fragmented into smaller GMCs upon leaving the spiral arms. The remnants of GMAs are detected as the chains of GMCs that emerge from the spiral arms into interarm regions. The kinematic shear within the spiral arms is sufficient to unbind the GMAs against self-gravity. We conclude that the evolution of GMCs is driven by large-scale galactic dynamics—their coagulation into GMAs is due to spiral arm streaming motions upon entering the arms, followed by fragmentation due to shear as they leave the arms on the downstream side. In M51, the majority of the gas remains molecular from arm entry through the interarm region and into the next spiral arm passage.
We report the results of an [O III] λ5007 survey for planetary nebulae (PNs) in three spiral galaxies: M101 (NGC 5457), M51 (NGC 5194/5195), and M96 (NGC 3368). By comparing on-band/off-band [O III] λ5007 images with images taken in Hα and broadband R, we identify 65, 64, and 74 PN candidates in each galaxy, respectively. From these data, an adopted M31 distance of 770 kpc, and the empirical planetary nebula luminosity function (PNLF), we derive distances to M101, M51, and M96 of 7.7 ± 0.5, 8.4 ± 0.6, and 9.6 ± 0.6 Mpc. These observations demonstrate that the PNLF technique can be successfully applied to late-type galaxies and can provide an important overlap between the Population I and Population II distance scales. We also discuss some special problems associated with using the PNLF in spiral galaxies, including the effects of dust and the possible presence of [O III] bright supernova remnants.
Far-ultraviolet to far-infrared images of the nearby galaxy NGC 5194 (M51a), from a combination of space-based (Spitzer, GALEX, and Hubble Space Telescope) and ground-based data, are used to investigate local and global star formation and the impact of dust extinction. The Spitzer data provide unprecedented spatial detail in the infrared, down to sizes ~500 pc at the distance of NGC 5194. The multiwavelength set is used to trace the relatively young stellar populations, the ionized gas, and the dust absorption and emission in H II-emitting knots, over 3 orders of magnitude in wavelength range. As is common in spiral galaxies, dust extinction is high in the center of the galaxy (AV ~ 3.5 mag), but its mean value decreases steadily as a function of galactocentric distance, as derived from both gas emission and stellar continuum properties. In the IR/UV-UV color plane, the NGC 5194 H II knots show the same trend observed for normal star-forming galaxies, having a much larger dispersion (~1 dex peak to peak) than starburst galaxies. We identify the dispersion as due to the UV emission predominantly tracing the evolved, nonionizing stellar population, up to ages ~50-100 Myr. While in starbursts the UV light traces the current star formation rate (SFR), in NGC 5194 it traces a combination of current and recent past SFRs. Possibly, mechanical feedback from supernovae is less effective at removing dust and gas from the star formation volume in normal star-forming galaxies than in starbursts because of the typically lower SFR densities in the former. The application of the starburst opacity curve for recovering the intrinsic UV emission (and deriving SFRs) in local and distant galaxies appears therefore appropriate only for SFR densities 1 M☉ yr-1 kpc-2. Unlike the UV emission, the monochromatic 24 μm luminosity is an accurate local SFR tracer for the H II knots in NGC 5194, with a peak-to-peak dispersion of less than a factor of 3 relative to hydrogen emission line tracers; this suggests that the 24 μm emission carriers are mainly heated by the young, ionizing stars. However, preliminary results show that the ratio of the 24 μm emission to the SFR varies by a factor of a few from galaxy to galaxy; this variation needs to be understood and carefully quantified before the 24 μm luminosity can be used as an SFR tracer for galaxy populations. While also correlated with star formation, the 8 μm emission is not directly proportional to the number of ionizing photons; it is overluminous, by up to a factor of ~2, relative to the galaxy's average in weakly ionized regions and is underluminous, by up to a factor of ~3, in strongly ionized regions. This confirms earlier suggestions that the carriers of the 8 μm emission are heated by more than one mechanism.
The Spitzer Space Telescope was used to study the mid- to far-infrared properties of NGC 300 and to compare dust emission to Hα to elucidate the heating of the interstellar medium (ISM) and the star formation cycle at scales smaller than 100 pc. The new data allow us to discern clear differences in the spatial distribution of 8 μm dust emission with respect to 24 μm dust and to H II regions traced by Hα light. The 8 μm emission highlights the rims of H II regions, and the 24 μm emission is more strongly peaked in star-forming regions than 8 μm. We confirm the existence and approximate amplitude of interstellar dust emission at 4.5 μm, detected statistically in Infrared Space Observatory (ISO) data, and conclude it arises in star-forming regions. When averaging over regions larger than ~1 kpc, the ratio of Hα to aromatic feature emission in NGC 300 is consistent with the values observed in disks of spiral galaxies. The mid- to far-infrared spectral energy distribution of dust emission is generally consistent with pre-Spitzer models.
For a sample of 43 nearby, late-type galaxies, we have investigated the radial variation of both the current star formation rate and the dust-induced UV light attenuation. To do this we have cross-correlated IRAS images and GALEX observations for each of these galaxies and compiled observations of the gas (CO and H I) and metal-abundance gradients found in the literature. We find that attenuation correlates with metallicity. We then use the UV profiles, corrected for attenuation, to study several variants of the Schmidt law and conclude that our results are compatible with a simple law similar to the one of Kennicutt extending smoothly to lower surface densities, but with considerable scatter. We do not detect an abrupt break in the UV light at the threshold radius derived from Hα data (at which the Hα profile shows a break and beyond which only a few H II regions are usually found). We interpret the Hα sudden break not as a change in the star formation regime (as often suggested), but as the vanishingly small number of ionizing stars corresponding to low levels of star formation.
Using infrared imaging from the Herschel Space Observatory, observed as part of the Very Nearby Galaxies Survey, we investigate the spatially resolved dust properties of the interacting Whirlpool galaxy system (NGC 5194 and NGC 5195), on physical scales of ~1 kpc. Spectral energy distribution modeling of the new infrared images in combination with archival optical and near- through mid-infrared images confirms that both galaxies underwent a burst of star formation ~370-480 Myr ago and provides spatially resolved maps of the stellar and dust mass surface densities. The resulting average dust-to-stellar mass ratios are comparable to other spiral and spheroidal galaxies studied with Herschel, with NGC 5194 at log (M
dust/M
) = –2.5 ± 0.2 and NGC 5195 at log (M
dust/M
) = –3.5 ± 0.3. The dust-to-stellar mass ratio is constant across NGC 5194 suggesting the stellar and dust components are coupled. In contrast, the mass ratio increases with radius in NGC 5195 with decreasing stellar mass density. Archival mass surface density maps of the neutral and molecular hydrogen gas are also folded into our analysis, revealing a fairly constant gas-to-dust mass ratio, 94 ± 17 across the system. Somewhat surprisingly, we find the dust in NGC 5195 is heated by a strong interstellar radiation field (ISRF), over 20 times that of the ISRF in the Milky Way, resulting in relatively high characteristic dust temperatures (~30 K). This post-starburst galaxy contains a substantial amount of low-density molecular gas and displays a gas-to-dust ratio (73 ± 35) similar to spiral galaxies. It is unclear why the dust in NGC 5195 is heated to such high temperatures as there is no star formation in the galaxy and its active galactic nucleus is 5-10 times less luminous than the one in NGC 5194, which exhibits only a modest enhancement in the amplitude of its ISRF.
We present the results of near- to mid-infrared slit spectroscopic
observations (2.55--13.4 um) of the diffuse emission toward nine positions in
the Large Magellanic Cloud with the Infrared Camera (IRC) on board AKARI. The
target positions are selected to cover a wide range of the intensity of the
incident radiation field. The unidentified infrared bands at 3.3, 6.2, 7.7, 8.6
and 11.3 um are detected toward all the targets, and ionized gas signatures:
hydrogen recombination lines and ionic forbidden lines toward three of them. We
classify the targets into two groups: those without the ionized gas signatures
(Group A) and those with the ionized signatures (Group B). Group A includes
molecular clouds and photo-dissociation regions, whereas Group B consists of
HII regions. In Group A, the band ratios of I(3.3)/I(11.3), I(6.2)/I(11.3),
I(7.7)/$I(11.3) and $I(8.6)/$I(11.3) show positive correlation with the IRAS
and AKARI colors, but those of Group B do not follow the correlation. We
discuss the results in terms of the polycyclic aromatic hydrocarbon (PAH) model
and attribute the difference to the destruction of small PAHs and an increase
in the recombination due to the high electron density in Group B. In the
present study, the 3.3 um band provides crucial information on the size
distribution and/or the excitation conditions of PAHs and plays a key role in
the distinction of Group A from B. The results suggest the possibility of the
diagram of I(3.3)/I(11.3) v.s. $I(7.7)/$I(11.3) as an efficient diagnostic tool
to infer the physical conditions of the interstellar medium.
We present sensitive and high angular resolution CO(1-0) data obtained by the Combined Array for Research in Millimeter-wave Astronomy (CARMA) observations toward the nearby grand-design spiral galaxy M 51. The angular resolution of 0.7" corresponds to 30 pc, which is similar to the typical size of Giant Molecular Clouds (GMCs), and the sensitivity is also high enough to detect typical GMCs. Within the 1' field of view centered on a spiral arm, a number of GMC-scale structures are detected as clumps. However, only a few clumps are found to be associated with each Giant Molecular Association (GMA), and more than 90% of the total flux is resolved out in our data. Considering the high sensitivity and resolution of our data, these results indicate that GMAs are not mere confusion of GMCs but plausibly smooth structures. In addition, we have found that the most massive clumps are located downstream of the spiral arm, which suggests that they are at a later stage of molecular cloud evolution across the arm and plausibly are cores of GMAs. By comparing with H-alpha and Pa-alpha images, most of these cores are found to have nearby star forming regions. We thus propose an evolutionary scenario for the interstellar medium, in which smaller molecular clouds collide to form smooth GMAs at spiral arm regions and then star formation is triggered in the GMA cores. Our new CO data have revealed the internal structure of GMAs at GMC scales, finding the most massive substructures on the downstream side of the arm in close association with the brightest H II regions. Comment: accepted for publication in ApJ
We present a revised method for simultaneous determination of the pattern speed and star formation timescale of spiral galaxies, its application, and results for CO and Ha images of nearby spiral galaxies. Out of 13 galaxies, we were able to derive the 2 parameters for 5 galaxies. We categorize them as "C" galaxies, and find (1) The corotation radius is close to the edge of the CO data, and is about half of the optical radius for 3 galaxies. (2) The star formation timescale is roughly consistent with the free-fall time of typical molecular clouds, which indicates that the gravitational instability is the dominant mechanism triggering star formation in spiral arms. (3) The timescale is found to be almost independent of surface density of molecular gas, metallicity, or spiral arm strengths. The number of "C" galaxies and the quality of CO data, however, are not enough to confirm these relationships. We also find that 2 other galaxies show no offsets between CO and Ha, although their arms are clearly traced, and categorize them as "N" galaxies. The presence of a bar could account for this feature, since these 2 galaxies are both barred. With one galaxy excluded from our analysis due to its poor rotation curve, offsets of the remaining 5 galaxies are found to be ambiguous. We categorize them as "A" galaxies. The possible reasons for this ambiguity are (1) the density wave is weaker, and/or (2) observational resolution and sensitivity are not enough to detect the spiral arms and their offsets clearly. The former is supported by our finding that the arm strengths of "A" galaxies are slightly weaker than that of "C" galaxies. [abridged] Comment: 53 pages, 9 tables, 18 figures. Accepted to ApJ. Some figures are downgraded. See http://www.astro.caltech.edu/~fegusa/ms_offset.pdf for original version
AKARI, the first Japanese satellite dedicated to infrared astronomy, was launched on 2006 February 21, and started observations in May of the same year. AKARI has a 68.5 cm cooled telescope, together with two focal-plane instruments, which survey the sky in six wavelength bands from the mid- to far-infrared. The instruments also have the capability for imaging and spectroscopy in the wavelength range 2 - 180 micron in the pointed observation mode, occasionally inserted into the continuous survey operation. The in-orbit cryogen lifetime is expected to be one and a half years. The All-Sky Survey will cover more than 90 percent of the whole sky with higher spatial resolution and wider wavelength coverage than that of the previous IRAS all-sky survey. Point source catalogues of the All-Sky Survey will be released to the astronomical community. The pointed observations will be used for deep surveys of selected sky areas and systematic observations of important astronomical targets. These will become an additional future heritage of this mission. Comment: 13 pages, 4 figures, and 3 tables. Accepted for publication in the AKARI special issue of the Publications of the Astronomical Society of Japan
We present maps of dust emission at 7 microns and 15 microns/7 microns intensity ratios of selected regions in 43 spiral galaxies observed with ISOCAM. This atlas is a complement to studies based on these observations, dealing with star formation in centers of barred galaxies and in spiral disks. It is accompanied by a detailed description of data reduction and an inventory of generic morphological properties in groups defined according to bar strength and HI gas content. Comment: 38 pages, 51 figure files, uses aa.cls, apj.bst, graphicx.sty and natbib.sty -- accepted for publication in A&A
We present the results of a systematic study of mid-IR spectra of Galactic regions, Magellanic HII regions, and galaxies of various types (dwarf, spiral, starburst), observed by the satellites ISO and Spitzer. We study the relative variations of the 6.2, 7.7, 8.6 and 11.3 micron features inside spatially resolved objects (such as M82, M51, 30 Doradus, M17 and the Orion Bar), as well as among 90 integrated spectra of 50 objects. Our main results are that the 6.2, 7.7 and 8.6 micron bands are essentially tied together, while the ratios between these bands and the 11.3 micron band varies by one order of magnitude. This implies that the properties of the PAHs are remarkably universal throughout our sample, and that the relative variations of the band ratios are mainly controled by the fraction of ionized PAHs. In particular, we show that we can rule out both the modification of the PAH size distribution, and the mid-infrared extinction, as an explanation of these variations. Using a few well-studied Galactic regions (including the spectral image of the Orion Bar), we give an empirical relation between the I(6.2)/I(11.3) ratio and the ionization/recombination ratio G0/ne.Tgas^0.5, therefore providing a useful quantitative diagnostic tool of the physical conditions in the regions where the PAH emission originates. Finally, we discuss the physical interpretation of the I(6.2)/I(11.3) ratio, on galactic size scales. Comment: Accepted by the ApJ, 67 pages, 70 figures
The Infrared Camera (IRC) is one of two focal-plane instruments on the AKARI satellite. It is designed for wide-field deep
imaging and low-resolution spectroscopy in the near–to mid-infrared (1.8–26.5$\mu$m) in the pointed observation mode of AKARI. The IRC is also operated in the survey mode to make an All-Sky Survey at 9 and
18$\mu$m. It comprises three channels. The NIR channel (1.8–5.5$\mu$m) employs a 512 $\times$ 412 InSb array, whereas both the MIR-S (4.6–13.4$\mu$m) and MIR-L (12.6–26.5$\mu$m) channels use 256 $\times$ 256 Si:As impurity band conduction arrays. Each of the three channels has a field-of-view of about $10^\prime \times 10^\prime$, and they are operated simultaneously. The NIR and MIR-S share the same field-of-view by virtue of a beam splitter. The MIR-L
observes the sky about 25$^\prime$ away from the NIR/MIR-S field-of-view. The IRC gives us deep insights into the formation and evolution of galaxies, the evolution
of planetary disks, the process of star-formation, the properties of interstellar matter under various physical conditions,
and the nature and evolution of solar system objects. The in-flight performance of the IRC has been confirmed to be in agreement
with the pre-flight expectation. This paper summarizes the design and the in-flight operation and imaging performance of the
IRC.
We report on mid- and far-IR Spitzer observations of 7 nearby dusty elliptical galaxies by using the Multiband Imaging Photometer
for Spitzer (MIPS) and Infrared Spectrograph (IRS). Our sample galaxies are known to contain excessive amounts of interstellar
dust against sputtering destruction in hot plasma filling the interstellar space of elliptical galaxies. In order to study
the origin and the properties of the excess dust in the hot plasma, we selected galaxies with a wide range of X-ray luminosities,
but similar optical luminosities for our Spitzer Guest Observers (GO1) program. The 7 galaxies were detected at the MIPS 24$\mu$m, 70$\mu$m, and 160$\mu$m bands; the far- to mid-IR flux ratios of relatively X-ray-bright elliptical galaxies are lower than those of X-ray-faint
galaxies. From the IRS spectra, polycyclic aromatic hydrocarbon (PAH) emission features were detected significantly from 5
of the 7 galaxies; the emission intensities are weaker as the X-ray luminosity of the galaxy is larger. We have found a correlation
between the far- to mid-IR flux ratio and the equivalent width of the PAH emission feature. We have obtained an apparent spatial
correspondence between the mid-IR and X-ray distributions in the outer regions for the three X-ray-brightest galaxies in our
sample. Possible interpretations for our observational results are discussed.
We present a detailed mid-infrared study of the nearby, face-on spiral galaxy M83 based on ISOCAM data. M83 is a unique case study, since a wide variety of MIR broad-band filters as well as spectra, covering the wavelength range of 4 to 18\mu m, were observed and are presented here. Emission maxima trace the nuclear and bulge area, star-formation regions at the end of the bar, as well as the inner spiral arms. The fainter outer spiral arms and interarm regions are also evident in the MIR map. Spectral imaging of the central 3'x3' (4 kpc x 4 kpc) field allows us to investigate five regions of different environments. The various MIR components (very small grains, polycyclic aromatic hydrocarbon (PAH) molecules, ionic lines) are analyzed for different regions throughout the galaxy. In the total 4\mu m to 18\mu m wavelength range, the PAHs dominate the luminosity, contributing between 60% in the nuclear and bulge regions and 90% in the less active, interarm regions. Throughout the galaxy, the underlying continuum emission from the small grains is always a smaller contribution in the total MIR wavelength regime, peaking in the nuclear and bulge components. The implications of using broad-band filters only to characterize the mid-infrared emission of galaxies, a commonly used ISOCAM observation mode, are discussed. We present the first quantitative analysis of new H-alpha and 6cm VLA+Effelsberg radio continuum maps of M83. The distribution of the MIR emission is compared with that of the CO, HI, R band, H-alpha and 6cm radio. A striking correlation is found between the intensities in the two mid-infrared filter bands and the 6cm radio continuum. To explain the tight mid-infrared-radio correlation we propose the anchoring of magnetic field lines in the photoionized shells of gas clouds. Comment: 22 pages, 15 figures. Accepted for publication in A&A
Abstract This review surveys the observed properties of interstellar dust grains: the wavelength-dependent extinction of starlight, including absorption features, from UV to infrared; optical luminescence; infrared emission; microwave emission; optical, UV, and X-ray scattering by dust; and polarization of starlight and of infrared emission. The relationship between presolar grains in meteorites and the interstellar grain population is discussed. Candidate grain materials and abundance constraints are considered. A dust model consisting of amorphous silicate grains, graphite grains, and polycyclic aromatic hydrocarbons is compared with observed emission and scattering. Some issues concerning evolution of interstellar dust are discussed.
The aromatic features in M101 were studied spectroscopically and photometrically using observations from all three instruments on the Spitzer Space Telescope. The global SED of M101 shows strong aromatic feature (commonly called PAH feature) emission. The spatially resolved spectral and photometric measurements of the aromatic feature emission show strong variations with significantly weaker emission at larger radii. We compare these variations with changes in the ionization index (as measured by [Ne III]/[Ne II] and [S IV/S III], which range from 0.03 to 20 and 0.044 to 15, respectively) and metallicity [expressed as log(O/H)+12, which ranges from 8.1 to 8.8]. Over these ranges, the spectroscopic equivalent widths of the aromatic features from seven H II regions and the nucleus were found to correlate better with ionization index than metallicity. This implies that the weakening of the aromatic emission in massive star-forming regions is due primarily to processing of the dust grains in these environments, not to differences in how they form. The behavior of the aromatic feature versus ionization index correlation can be described as a constant equivalent width until a threshold in ionization index is reached ([Ne III]/[Ne II]~1), above which the equivalent widths decrease with a power-law dependence. This behavior is also seen for the starburst galaxy sample presented in the companion study by Engelbracht and coworkers, which expands the range of [Ne III]/[Ne II] ratios to 0.03-25 and log(O/H)+12 values to 7.1-8.8. The form of the correlation explains seemingly contradictory results present in the literature. The behavior of the ratios of different aromatic features versus ionization index does not follow the predictions of existing PAH models of the aromatic features, implying a more complex origin of the aromatic emission in massive star-forming regions.
Physical dust models which include a PAH component are quite successful in reproducing the measured extinction vs. wavelength in the Milky Way, the infrared (IR) emission observed from Milky Way regions and other galaxies, and a number of other observational constraints. Many of the adopted PAH properties are necessarily highly idealized. The observed variations in the 7.7 mum/11.3 mum band ratio can be reproduced by varying the PAH ionization balance. Changing the spectrum of the starlight heating the PAHs affects the overall strength of the PAH emission (relative to total IR), but has relatively little effect on the 7.7 mum/11.3 mum band ratio.
Mid-infrared images frequently suffer artifacts and extended point-spread functions (PSFs). We investigate the characteristics of the artifacts and the PSFs in images obtained with the infrared camera (IRC) on board AKARI at four mid-infrared bands of the S7 (7 mum), S11 (11 mum), L15 (15 mum), and L24 (24 mum). Removal of the artifacts significantly improves the reliability of the reference data for flat-fielding at the L15 and L24 bands. A set of models of the IRC PSFs is also constructed from on-orbit data. These PSFs have extended components that come from diffraction and scattering within the detector arrays. We estimate the aperture correction factors for point sources and the surface brightness correction factors for diffuse sources. We conclude that the surface brightness correction factors range from 0.95 to 0.8, taking account of the extended component of the PSFs. To correct for the extended PSF effects for the study of faint structures, we also develop an image reconstruction method, which consists of the deconvolution with the PSF and the convolution with an appropriate Gaussian. The appropriate removal of the artifacts, improved flat-fielding, and image reconstruction with the extended PSFs enable us to investigate detailed structures of extended sources in IRC mid-infrared images.
We present the burst ages for young stellar populations in a sample of six nearby (<10 Mpc) spiral galaxies using a differential pixel-based analysis of the ionized gas emission. We explore this as an alternative approach for connecting large-scale dynamical mechanisms with star formation processes in disc galaxies, based on burst ages derived from the Hα to far-UV (FUV) flux ratio. Images of each galaxy in Hα were taken with Taurus Tunable Filter and matched to FUV imaging from GALEX. The resulting flux ratio provides a robust measure of relative age across the disc which we discuss in terms of the large-scale dynamical motions. Systematic effects such as a variable initial mass function, non-solar metallicities, variable star formation histories (SFHs) and dust attenuation have been used to derive estimates of the systematic uncertainty.
The resulting age maps show a wide range of patterns outside of those galaxies with the strongest spiral structure, confirming the idea that star formation is driven one by several processes, largely determined by the individual circumstances of the galaxy. Generally, grand design spirals such as M74, M100 and M51 exhibit age gradients across the main spiral arms, with the youngest star formation regions along the central and inner edges. Likewise, in the dominant star-forming complex of IC 2574 or the ring of M94, the most recent star formation is centrally confined to the regions of star formation activity. In M63 and M74 galaxy-wide trends emerge, contrary to the spiral structure in these galaxies, suggesting that spiral density waves are not the dominant driver in some cases. We argue that despite appearances, galaxy morphology is not an absolute discriminator of the SFH of an individual galaxy, nor of the processes triggering it. We conclude that Hα-to-FUV flux ratios are a relatively direct way to probe burst ages across galaxies and infer something of their dynamical histories, provided that sources of systematics are properly taken into account.
We present results on mid-infrared imaging and spectroscopic observations of the nearby late-type spiral NGC6946 with the
Infrared Camera on board AKARI. Based on mid-infrared imaging with the S7 (7$\mu$m) and S11(11$\mu$m) bands, we found that the S7/S11 ratios have larger values in the arm region, containing many star-forming regions, than in the interarm. Slit spectra of
an interarm and a star-forming region show a series of unidentified infrared (UIR) bands at 6.2, 7.7, 8.6, and 11.2$\mu$m. The strengths of the 6.2, 7.7, and 8.6$\mu$m bands are larger relative to the 11.2$\mu$m band in the star-forming region, than in the interarm, even if the interstellar extinction of $A_V=3$mag towards the star-forming region is taken into account. The increase in the 6.2$\mu$m and 7.7$\mu$m features relative to the 11.2$\mu$m feature is consistent with the ionization model of PAHs. The ratio of the UIR features to the plateau emission under the
7.7$\mu$m and 8.6$\mu$m features and/or the ratio of the 7.6$\mu$m/7.8$\mu$m components in the 7.7$\mu$m feature increase in the star-forming region compared to the interarm. This variation can be accounted for by the hypothesis
that the photo-evaporation of PAH clusters produce small free-flying PAHs in the star-forming region. The ratios of 6.2$\mu$m/11.2$\mu$m, 7.7$\mu$m/11.2$\mu$m, and 8.6$\mu$m/11.2$\mu$m may be tools to measure the star-formation activity in remote galaxies if these ratios and their variations are well examined
and established for a large sample.
The absolute photometric calibration of imaging observations with the Infrared Camera (IRC) aboard the AKARI satellite was
performed by monitoring the same stars regularly and by observing a set of standard stars. By our monitoring observations,
we confirmed that all channels of the IRC were stable to within 4% and that their sensitivities were constant until the liquid
helium was exhausted. Using the data of these repeated observations, we evaluated the intrinsic errors as a function of the
brightnesses of objects and found that the errors increase rapidly toward fainter objects. We also checked the consistency
between short and long exposure times, and confirmed that the data sampling had been executed as designed. Finally, by comparing
the estimated in-band flux densities and the observed data values of standard stars, we obtained conversion factors to the
absolute flux densities of all the band/exposure configurations. Their absolute uncertainties are estimated to be less than
6%.
Using the new capabilities of Spitzer and extensive multiwavelength data from SINGS, it is now possible to study the infrared properties of star formation in nearby galaxies down to scales equivalent to large H II regions. We are therefore able to determine what fraction of large, infrared-selected star-forming regions in normal galaxies are highly obscured and address how much of the star formation we miss by relying solely on the optical portion of the spectrum. Employing a new empirical method for deriving attenuations of infrared-selected star-forming regions, we investigate the statistics of obscured star formation on 500 pc scales in a sample of 38 nearby galaxies. We find that the median attenuation is 1.4 mag in Hα and that there is no evidence for a substantial subpopulation of uniformly highly obscured star-forming regions. The regions in the highly obscured tail of the attenuation distribution (AHα 3) make up only ~4% of the sample of nearly 1800 regions, although very embedded infrared sources on the much smaller scales and lower luminosities of compact and ultracompact H II regions are almost certainly present in greater numbers. The highly obscured cases in our sample are generally the bright, central regions of galaxies with high overall attenuation but are not otherwise remarkable. We also find that a majority of the galaxies show decreasing radial trends in Hα attenuation. The small fraction of highly obscured regions seen in this sample of normal, star-forming galaxies suggests that on 500 pc scales the timescale for significant dispersal or breakup of nearby, optically thick dust clouds is short relative to the lifetime of a typical star-forming region.
We present a full-sky 100 μm map that is a reprocessed composite of the COBE/DIRBE and IRAS/ISSA maps, with the zodiacal foreground and confirmed point sources removed. Before using the ISSA maps, we remove the remaining artifacts from the IRAS scan pattern. Using the DIRBE 100 and 240 μm data, we have constructed a map of the dust temperature so that the 100 μm map may be converted to a map proportional to dust column density. The dust temperature varies from 17 to 21 K, which is modest but does modify the estimate of the dust column by a factor of 5. The result of these manipulations is a map with DIRBE quality calibration and IRAS resolution. A wealth of filamentary detail is apparent on many different scales at all Galactic latitudes. In high-latitude regions, the dust map correlates well with maps of H I emission, but deviations are coherent in the sky and are especially conspicuous in regions of saturation of H I emission toward denser clouds and of formation of H2 in molecular clouds. In contrast, high-velocity H I clouds are deficient in dust emission, as expected.
We observed M51 at three frequencies, 1.4 GHz (20 cm), 4.9 GHz (6 cm), and 8.4 GHz (3.6 cm), with the Very Large Array and the Effelsberg 100 m telescope to obtain the highest quality radio continuum images of a nearby spiral galaxy. These radio data were combined with deconvolved Spitzer IRAC 8 μm and MIPS 24 μm images to search for and investigate local changes in the radio-IR correlation. Utilizing wavelet decomposition, we compare the distribution of the radio and IR emission on spatial scales between 200 pc and 30 kpc. We show that the radio-IR correlation is not uniform across the galactic disk. It presents a complex behavior with local extrema corresponding to various galactic structures, such as complexes of H II regions, spiral arms, and interarm filaments, indicating that the contribution of the thermal and non-thermal radio emission is a strong function of environment. In particular, the relation of the 24 μm and 20 cm emission presents a linear relation within the spiral arms and globally over the galaxy, while it deviates from linearity in the interarm and outer regions as well in the inner region, with two different behaviors: it is sublinear in the interarm and outer region and overlinear in the central 3.5 kpc. Our analysis suggests that the changes in the radio/IR correlation reflect variations of interstellar medium properties between spiral arms and interarm region. The good correlation in the spiral arms implies that 24 μm and 20 cm are tracing recent star formation, while a change in the dust opacity, "Cirrus" contribution to the IR emission and/or the relation between the magnetic field strength and the gas density can explain the different relations found in the interarm, outer, and inner regions.
Mid-infrared (MIR; 12-60 μm) diffuse emission in the Galactic plane and the Carina Nebula is investigated relative to the far-infrared (FIR; ≥100 μm) emission. Observations show that the ratio of the 12 μm emission to the total FIR intensity is more or less constant in the Galactic plane but exhibits a slight decrease followed by an increase as the FIR color becomes bluer in the Carina Nebula. The constancy is compatible with predictions from models of stochastic heating of very small grains or infrared fluorescence of polycyclic aromatic hydrocarbons. The decrease can be attributed to a weakening of the unidentified infrared band emission in strong radiation fields. Contrarily, the ratio of the 25 and 60 μm emission to FIR intensity increases linearly with field strength, which is incompatible with the model predictions. The Carina Nebula data show a much bluer FIR color than the Galactic plane, whereas the ratio of MIR to FIR emission is in a range similar to that of the plane. We interpret these characteristics in terms of a superposition of emission from dust grains in various radiation field strengths. The linear increase can be accounted for by an increasing contribution from emission by dust grains in strong radiation environments, whereas the shift in FIR color between the Galactic plane and the Carina Nebula can be attributed to different contributions from grains in weak radiation fields. Other possibilities, such as the effect of multiphoton processes, variations in the incident radiation spectrum, and possible contributions from iron grains, have also been examined, but none can account for the observations consistently. Applications of the present model to external galaxies are also discussed.
We explore variations in dust emission within the edge-on Sd spiral galaxy NGC 4631 using 3.6-160 μm Spitzer Space Telescope data and 450-850 μm JCMT data with the goals of understanding the relation between PAHs and dust emission, studying the variations in the colors of the dust emission, and searching for possible excess submillimeter emission compared to what is expected from dust models extrapolated from far-infrared wavelengths. The 8 μm PAH emission correlates best with 24 μm hot dust emission on 1.7 kpc scales, but the relation breaks down on 650 pc scales, possibly because of differences in the mean free paths between photons that excite the PAHs and photons that heat the dust and possibly because the PAHs are destroyed by the hard radiation fields within some star formation regions. The ratio of 8 μm PAH emission to 160 μm cool dust emission appears to vary as a function of radius. The 70 μm/160 μm and 160 μm/450 μm flux density ratios are remarkably constant even though the surface brightnesses vary by factors of 25, which suggests that the emission is from dust heated by a nearly uniform radiation field. Globally, we find an excess of 850-1230 μm emission relative to what would be predicted by dust models. The 850 μm excess is highest in regions with low 160 μm surface brightnesses, although the magnitude depends on the model fit to the data. We rule out variable emissivity functions or ~4 K dust as the possible origins of this 850 μm emission, but we do discuss the other possible mechanisms that could produce the emission.
We present a sample of low-resolution 5-38 μm Spitzer IRS spectra of the inner few square kiloparsecs of 59 nearby galaxies spanning a large range of star formation properties. A robust method for decomposing mid-infrared galaxy spectra is described and used to explore the behavior of PAH emission and the prevalence of silicate dust extinction. Evidence for silicate extinction is found in ~ of the sample, at strengths that indicate that most normal galaxies undergo AV 3 mag averaged over their centers. The contribution of PAH emission to the total infrared power is found to peak near 10% and extend up to ~20% and is suppressed at metallicities Z Z☉/4, as well as in low-luminosity AGN environments. Strong interband PAH feature strength variations (2-5 times) are observed, with the presence of a weak AGN and, to a lesser degree, increasing metallicity shifting power to the longer wavelength bands. A peculiar PAH emission spectrum with markedly diminished 5-8 μm features arises among the sample solely in systems with relatively hard radiation fields harboring low-luminosity AGNs. The AGNs may modify the emitting grain distribution and provide the direct excitation source of the unusual PAH emission, which cautions against using absolute PAH strength to estimate star formation rates in systems harboring active nuclei. Alternatively, the low star formation intensity often associated with weak AGNs may affect the spectrum. The effect of variations in the mid-infrared spectrum on broadband infrared surveys is modeled and points to more than a factor of 2 uncertainty in results that assume a fixed PAH emission spectrum, for redshifts z = 0-2.5.
This is the first part of an Hα kinematics follow-up survey of the Spitzer Infrared Nearby Galaxies Survey (SINGS) sample. The data for 28 galaxies are presented. The observations were done on three
different telescopes with Fabry—Perot of New Technology for the Observatoire du mont Megantic (FaNTOmM), an integral field
photon-counting spectrometer, installed in the respective focal reducer of each telescope. The data reduction was done through
a newly built pipeline with the aim of producing the most homogenous data set possible. Adaptive spatial binning was applied
to the data cubes in order to get a constant signal-to-noise ratio across the field of view. Radial velocity and monochromatic
maps were generated using a new algorithm, and the kinematical parameters were derived using tilted-ring models.
With AKARI, we have systematically performed near- to far-infrared (NIR–FIR) observations of the interstellar medium (ISM) in our Galaxy and nearby galaxies as one of AKARI mission programs called ISMGN (ISM in our Galaxy and Nearby galaxies). Our scientific objective is to increase our knowledge on the properties of the ISM exposed to an extensive range of environments, the processing, evolution, and destruction of interstellar dust, and their connection with physical conditions of interstellar gas and star-forming activity. Our Galaxy contains ideal laboratories for probing the life cycle of the ISM, while nearby external galaxies provide a much wider range of physical conditions. We present the contents of the AKARI observations relevant to our ISM studies, together with some results demonstrative of the AKARI uniqueness obtained during the AKARI cold mission phase (Phases 1 and 2). For the ISM in our Galaxy and the LMC, we have performed spectroscopic observations of several regions intensively in the NIR to FIR, including detailed studies of 14 Galactic and 21 LMC SNRs. We have observed about 60 nearby galaxies, for many of which NIR to FIR images in the 10 photometric bands centered at the wavelengths of 3, 4, 7, 11, 24, 15, 65, 90, 140, and as well as 2–14 μm low-resolution spectra are obtained. Our data, especially the 11 and imaging, the FIR 4-band imaging, and the NIR spectral data will be complementary to the Spitzer data of nearby galaxies such as those from the SINGS legacy program. We further refer to our on-going observations in the AKARI post-helium mission phase (Phase 3).
Large polycyclic aromatic hydrocarbon (PAH) molecules carry the infrared (IR) emission features that dominate the spectra of most galactic and extragalactic sources. This review surveys the observed mid-IR characteristics of these emission features and summarizes laboratory and theoretical studies of die spectral characteristics of PAHs and the derived intrinsic properties of emitting interstellar PAHs. Dedicated experimental studies have provided critical input for detailed astronomical models that probe the origin and evolution of interstellar PAHs and their role in the universe. The physics and chemistry of PAHs are discussed, emphasizing the contribution of these species to the photoelectric heating and the ionization balance of the interstellar gas and to the formation of small hydrocarbon radicals and carbon chains. Together, these studies demonstrate that PAHs are abundant, ubiquitous, and a dominant force in the interstellar medium of galaxies.
In the past, reflection nebulae have provided an astrophysical laboratory well suited for the study of the reflection properties of interstellar dust grains at visual and ultraviolet wavelengths. The present investigation is concerned with observations which were begun with the objective to extend to near-infrared wavelengths the study of grains in reflection. Observations of three classical visual reflection nebulae were conducted in the wavelength range from 1.25 to 2.2 microns, taking into account NGC 7023, 2023, and 2068. All three nebulae were found to have similar near-infrared colors, despite widely different colors of their illuminating stars. The brightness level shown by two of the nebulae at 2.2 microns was too high to be easily accounted for on the basis of reflected light. Attention is given to a wide variety of possible emission mechanisms.
A comprehensive study of the PAH hypothesis is presented, including the interstellar, IR spectral features which have been attributed to emission from highly vibrationally excited PAHs. Spectroscopic and IR emission features are discussed in detail. A method for calculating the IR fluorescence spectrum from a vibrationally excited molecule is described. Analysis of interstellar spectrum suggests that the PAHs which dominate the IR spectra contain between 20 and 40 C atoms. The results are compared with results from a thermal approximation. It is found that, for high levels of vibrational excitation and emission from low-frequency modes, the two methods produce similar results. Also, consideration is given to the relationship between PAH molecules and amorphous C particles, the most likely interstellar PAH molecular structures, the spectroscopic structure produced by PAHs and PAH-related materials in the UV portion of the interstellar extinction curve, and the influence of PAH charge on the UV, visible, and IR regions.
We present a quantitative model for the infrared emission from dust in the diffuse interstellar medium. The model consists of a mixture of amorphous silicate grains and carbonaceous grains, each with a wide size distribution ranging from molecules containing tens of atoms to large grains > 1 um in diameter. We assume that the carbonaceous grains have polycyclic aromatic hydrocarbon (PAH)-like properties at very small sizes, and graphitic properties for radii a > 50 A. On the basis of recent laboratory studies and guided by astronomical observations, we propose "astronomical" absorption cross sections for use in modeling neutral and ionized PAHs from the far ultraviolet to the far infrared. We also propose modifications to the far-infrared emissivity of "astronomical silicate". We calculate energy distribution functions for small grains undergoing "temperature spikes" due to stochastic absorption of starlight photons, using realistic heat capacities and optical properties. Using a grain size distribution consistent with the observed interstellar extinction, we are able to reproduce the near-IR to submillimeter emission spectrum of the diffuse interstellar medium, including the PAH emission features at 3.3, 6.2, 7.7, 8.6, and 11.3um. The model is compared with the observed emission at high Galactic latitudes as well as in the Galactic plane, as measured by COBE and IRTS. We calculate infrared emission spectra for our dust model heated by a range of starlight intensities, and we provide tabulated dust opacities (extended tables available at http://www.astro.princeton.edu/~draine/dust/dustmix.html)
We present a method for calculating the infrared emission from a population of dust grains heated by starlight, including very small grains for which stochastic heating by starlight photons results in high temperature transients. Because state-to-state transition rates are generally unavailable for complex molecules, we consider model PAH, graphitic, and silicate grains with realistic vibrational mode spectra and realistic radiative properties. The vibrational density of states is used in a statistical-mechanical description of the emission process. Unlike previous treatments, our approach fully incorporates multiphoton heating effects, important for large grains or strong radiation fields. We discuss how the "temperature" of the grain is related to its vibrational energy. By comparing with an "exact" statistical calculation of the emission process, we determine the conditions under which the "thermal" and the "continuous cooling" approximations can be used to calculate the emission spectrum. We present results for the infrared emission spectra of PAH grains of various sizes heated by starlight. We show how the relative strengths of the 6.2, 7.7, and 11.3um features depend on grain size, starlight spectrum and intensity, and grain charging conditions. We show results for grains in the "cold neutral medium", "warm ionized medium", and representative conditions in photodissociation regions. Our model results are compared to observed ratios of emission features for reflection nebulae and photodissociation regions, the Milky Way, normal spiral galaxies, and starburst galaxies.
We use CO and H alpha velocity fields to study the gas kinematics in the spiral arms and interarms of M51 (NGC 5194), and fit the 2D velocity field to estimate the radial and tangential velocity components as a function of spiral phase (arm distance). We find large radial and tangential streaming velocities, which are qualitatively consistent with the predictions of density wave theory and support the existence of shocks. The streaming motions are complex, varying significantly across the galaxy as well as along and between arms. Aberrations in the velocity field indicate that the disk is not coplanar, perhaps as far in as 20\arcsec\ (800 pc) from the center. Velocity profile fits from CO and H alpha are typically similar, suggesting that most of the H alpha emission originates from regions of recent star formation. We also explore vortensity and mass conservation conditions. Vortensity conservation, which does not require a steady state, is empirically verified. The velocity and density profiles show large and varying mass fluxes, which are inconsistent with a steady flow for a single dominant global spiral mode. We thus conclude that the spiral arms cannot be in a quasi-steady state in any rotating frame, and/or that out of plane motions may be significant. Comment: 50 pages, including 20 figures; Accepted for publication in ApJ. PDF version with high resolution figures available at http://www.astro.umd.edu/~shetty/Research/
We analyze the new mid-infrared maps of NGC 6946 for variations in the color ratio of the 7-to-15 micron emission. Our preliminary findings are that this mid-infrared color is remarkably constant between arms and inter-arm regions, and as a function of radius in the disk, excluding the nuclear region. As surface brightness ranges by more than an order of magnitude and the radius runs from about 0.5 to 3kpc, the color ratio remains constant to about +/-20%. Our interpretation is that (1) hard UV radiation from OB stars does not dominate the heating of the grains radiating in the mid-infrared; and (2) that surface brightness variations are driven primarily by surface-filling fraction in the disk, and by radiation intensity increases in starburst environments, such as the nucleus of NGC 6946.
Infrared (IR) emission spectra are calculated for dust composed of mixtures of amorphous silicate and graphitic grains, including varying amounts of polycyclic aromatic hydrocarbon (PAH) particles. The models are constrained to reproduce the average Milky Way extinction curve. The calculations include the effects of single-photon heating. Updated IR absorption properties for the PAHs are presented, that are consistent with observed emission spectra, including those newly obtained by Spitzer Space Telescope. We find a size distribution for the PAHs that results in emission band ratios consistent with observed spectra of the Milky Way and other galaxies. Emission spectra are presented for various intensities of the illuminating starlight. We calculate how the efficiency of emission into different IR bands depends on PAH size; the strong 7.7um emission feature is produced mainly by PAH particles containing <1000 C atoms. We show how the emission spectrum depends on U, the starlight intensity relative to the local interstellar radiation field. The submm and far-infrared emission is compared to the observed emission from the local interstellar medium. Using a simple distribution function, we calculate the emission spectrum for dust heated by a distribution of starlight intensities, such as occurs within galaxies. The models are parameterized by the PAH mass fraction qpah, the lower cutoff Umin, and the fraction gamma of the dust heated by starlight with U>Umin. We present graphical procedures using IRAC and MIPS photometry to estimate qpah, Umin, and gamma, the fraction f_PDR of the dust luminosity coming from photodissociation regions with U>100, and the total dust mass Mdust.
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