[show abstract][hide abstract] ABSTRACT: Within the framework of the HERM33ES key project, we are studying the star forming interstellar medium in the nearby, metal-poor spiral galaxy M33, exploiting the high resolution and sensitivity of Herschel. We use PACS and SPIRE maps at 100, 160, 250, 350, and 500 micron wavelength, to study the variation of the spectral energy distributions (SEDs) with galacto-centric distance. Detailed SED modeling is performed using azimuthally averaged fluxes in elliptical rings of 2 kpc width, out to 8 kpc galacto-centric distance. Simple isothermal and two-component grey body models, with fixed dust emissivity index, are fitted to the SEDs between 24 and 500 micron using also MIPS/Spitzer data, to derive first estimates of the dust physical conditions. The far-infrared and submillimeter maps reveal the branched, knotted spiral structure of M33. An underlying diffuse disk is seen in all SPIRE maps (250-500 micron). Two component fits to the SEDs agree better than isothermal models with the observed, total and radially averaged flux densities. The two component model, with beta fixed at 1.5, best fits the global and the radial SEDs. The cold dust component clearly dominates; the relative mass of the warm component is less than 0.3% for all the fits. The temperature of the warm component is not well constrained and is found to be about 60K plus/minus 10K. The temperature of the cold component drops significantly from about 24K in the inner 2 kpc radius to 13K beyond 6 kpc radial distance, for the best fitting model. The gas-to-dust ratio for beta=1.5, averaged over the galaxy, is higher than the solar value by a factor of 1.5 and is roughly in agreement with the subsolar metallicity of M33.
[show abstract][hide abstract] ABSTRACT: We have studied the molecular hydrogen energetics of the edge-on spiral galaxy NGC 891, using a 34 position map in the lowest three pure rotational H2 lines observed with the Spitzer Infrared Spectrograph. The S(0), S(1), and S(2) lines are bright with an extinction-corrected total luminosity of ~2.8 × 107 L sun, or 0.09% of the total-infrared luminosity of NGC 891. The H2 line ratios are nearly constant along the plane of the galaxy---we do not observe the previously reported strong drop-off in the S(1)/S(0) line intensity ratio in the outer regions of the galaxy, so we find no evidence for the very massive cold CO-free molecular clouds invoked to explain the past observations. The H2 level excitation temperatures increase monotonically indicating that there is more than one component to the emitting gas. More than 99% of the mass is in the lowest excitation (T ex ~ 125 K) "warm" component. In the inner galaxy, the warm H2 emitting gas is ~16% of the CO(1-0)-traced cool molecular gas, while in the outer regions the fraction is twice as high. This large mass of warm gas is heated by a combination of the far-UV photons from stars in photodissociation regions (PDRs) and the dissipation of turbulent kinetic energy. Including the observed far-infrared [O I] and [C II] fine-structure line emission and far-infrared continuum emission in a self-consistent manner to constrain the PDR models, we find essentially all of the S(0) and most (70%) of the S(1) line arise from low excitation PDRs, while most (80%) of the S(2) and the remainder of the S(1) line emission arise from low-velocity microturbulent dissipation.
The Astrophysical Journal 01/2010; 721(1):59-73. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: An updated Science Vision for the SOFIA project is presented, including an overview of the characteristics and capabilities of the observatory and first generation instruments. A primary focus is placed on four science themes: 'The Formation of Stars and Planets', 'The Interstellar Medium of the Milky Way', 'Galaxies and the Galactic Center' and 'Planetary Science'. Comment: 128 pages pdf format Version 2 corrects: Attribution for Figure 5-1 Definition of "SIS" Affiliation of one of the contributors to the Science Vision
[show abstract][hide abstract] ABSTRACT: When and how did galaxies form and their metals accumulate? Over the last decade, this has moved from an archeological question to a live investigation: there is now a broad picture of the evolution of galaxies in dark matter halos: their masses, stars, metals and supermassive blackholes. Galaxies have been found and studied in which these formation processes are taking place most vigorously, all the way back in cosmic time to when the intergalactic medium (IGM) was still largely neutral. However, the details of how and why the interstellar medium (ISM) in distant galaxies cools, is processed, recycled and enriched in metals by stars, and fuels active galactic nuclei (AGNs) remain uncertain. In particular, the cooling of gas to fuel star formation, and the chemistry and physics of the most intensely active regions is hidden from view at optical wavelengths, but can be seen and diagnosed at mid- & far-infrared (IR) wavelengths. Rest-frame IR observations are important first to identify the most luminous, interesting and important galaxies, secondly to quantify accurately their total luminosity, and finally to use spectroscopy to trace the conditions in the molecular and atomic gas out of which stars form. In order to map out these processes over the full range of environments and large-scale structures found in the universe - from the densest clusters of galaxies to the emptiest voids - we require tools for deep, large area surveys, of millions of galaxies out to z~5, and for detailed follow-up spectroscopy. The necessary tools can be realized technically. Here, we outline the requirements for gathering the crucial information to build, validate and challenge models of galaxy evolution.
[show abstract][hide abstract] ABSTRACT: The Molecular Hydrogen Explorer, H2 EX, was proposed in response to the ESA 2015 - 2025 Cosmic Vision Call as a medium class space mission with NASA and CSA participations. The mission, conceived to understand the formation of galaxies, stars and planets from molecular hydrogen, is designed to observe the first rotational lines of the H2 molecule (28.2, 17.0, 12.3 and 9.7 mum) over a wide field, and at high spectral resolution. H2 EX can provide an inventory of warm (>= 100 K) molecular gas in a broad variety of objects, including nearby young star clusters, galactic molecular clouds, active galactic nuclei, local and distant galaxies. The rich array of molecular, atomic and ionic lines, as well as solid state features available in the 8 to 29 mum spectral range brings additional science dimensions to H2 EX. We present the optical and mechanical design of the H2 EX payload based on an innovative Imaging Fourier Transform Spectrometer fed by a 1.2 m telescope. The 20'Ã20' field of view is imaged on two 1024Ã1024 Si:As detectors. The maximum resolution of 0.032 cm - 1 (full width at half maximum) means a velocity resolution of 10 km s - 1 for the 0 - 0 S(3) line at 9.7 mum. This instrument offers the large field of view necessary to survey extended emission in the Galaxy and local Universe galaxies as well as to perform unbiased extragalactic and circumstellar disks surveys. The high spectral resolution makes H2 EX uniquely suited to study the dynamics of H2 in all these environments. The mission plan is made of seven wide-field spectro-imaging legacy programs, from the cosmic web to galactic young star clusters, within a nominal two years mission. The payload has been designed to re-use the Planck platform and passive cooling design.
[show abstract][hide abstract] ABSTRACT: We report the first astronomical detection of a far-IR emission line at (69.524 ± 0.008) μm from aboard the Kuiper Airborne Observatory (KAO). We tentatively assign this line to the R-branch transition 43− → 33+ of H3O+. The wavelength of the discovered line lies between -112 and -60 km s-1 from predicted values. The position at which the emission line was detected is located about 20'' north of Orion BN, which is close to the shocked molecular hydrogen peak 1. We exclude hot shocked gas as the possible origin of the H3O+ emission. Hydrogen densities of 5 × 108 cm-3 and temperatures 100 K are required to match the observed integrated intensity of the 70 μm line. A speculative explanation for the 70 μm H3+O emission is that it might originate from very dense clumps.
The Astrophysical Journal 01/2009; 463(2):L109. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: We report measurements of the [C II] fine-structure line at 157.714 μm in 30 normal star-forming galaxies with the Long Wavelength Spectrometer (LWS) on the Infrared Space Observatory (ISO). The ratio of the line to total far-infrared (FIR) luminosity, LC II/LFIR, measures the ratio of the cooling of gas to that of dust, and thus the efficiency of the grain photoelectric heating process. This ratio varies by more than a factor of 40 in the current sample. About two-thirds of the galaxies have LC II/LFIR ratios in the narrow range of (2-7) × 10−3. The other one-third show trends of decreasing LC II/LFIR with increasing dust temperature, as measured by the flux ratio of infrared emission at 60 and 100 μm, Fν(60 μm)/Fν(100 μm), and with increasing star formation activity, measured by the ratio of FIR and blue-band luminosity, LFIR/LB. We also find three FIR-bright galaxies that are deficient in the [C II] line, which is undetected with 3 σ upper limits of LC II/LFIR < (0.5-2) × 10−4. The trend in the LC II/LFIR ratio with the temperature of dust and with star formation activity may be due to decreased efficiency of photoelectric heating of gas at high UV radiation intensity as dust grains become positively charged, decreasing the yield and the energy of the photoelectrons. The three galaxies with no observed photodissociation region lines have among the highest LFIR/LB and Fν(60 μm)/Fν(100 μm) ratios. Their lack of [C II] lines may be due to a continuing trend of decreasing LC II/LFIR with increasing star formation activity and dust temperature seen in one-third of the sample with warm IRAS colors. In that case, the upper limits on LC II/LFIR imply a ratio of UV flux to gas density of G0/n>10 cm3 (where G0 is in units of the local average interstellar field). The low LC II/LFIR ratio could also be due to either weak [C II], owing to self-absorption, or a strong FIR continuum from regions weak in [C II], such as dense H II regions or plasma ionized by hard radiation of active galactic nuclei. The mid-infrared and radio images of these galaxies show that most of the emission comes from a compact nucleus. CO and H I are detected in these galaxies, with H I seen in absorption toward the nucleus.
The Astrophysical Journal 01/2009; 491(1):L27. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: We report measurements of the [C II] 157.74 μm fine-structure line in a sample of seven ultraluminous infrared galaxies (ULIGs) (LIR > 1012 L☉) with the Long Wavelength Spectrometer on the Infrared Space Observatory. The [C II] line is an important coolant in galaxies and arises in interstellar gas exposed to far-ultraviolet photons (hν≥11.26 eV); in ULIGs, this radiation stems from the bursts of star formation and/or from the active galactic nuclei that power the tremendous infrared luminosity. The [C II] 158 μm line is detected in four of the seven ULIGs; the absolute line flux (about a few times 10-20 W cm-2) represents some of the faintest extragalactic[C II] emission yet observed. Relative to the far-infrared continuum, the [C II] flux from the observed ULIGs is ~10% of that seen from nearby normal and starburst galaxies. We discuss possible causes for the [C II] deficit, namely (1) self-absorbed or optically thick [C II] emission, (2) saturation of the [C II] emission in photodissociated gas with high gas density n (3 × 103 cm-3) or with a high ratio of incident UV flux G0 to n (G0/n 10 cm3), or (3) the presence of a soft ultraviolet radiation field caused, for example, by a stellar population deficient in massive main-sequence stars. As nearby examples of colliding galaxies, ULIGs may resemble high-redshift protogalaxies in both morphology and spectral behavior. If true, the suggested [C II] deficit in ULIGs poses limitations on the detection rate of high-z sources and on the usefulness of [C II] as an eventual tracer of protogalaxies.
The Astrophysical Journal 01/2009; 504(1):L11. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: We present the first complete far-infrared spectrum (43-197 μm) of M82, the brightest infrared galaxy in the sky, taken with the Long Wavelength Spectrometer of the Infrared Space Observatory (ISO). We detected seven fine structure emission lines, [O I] 63 and 145 μm, [O III] 52 and 88 μm , [N II] 122 μm, [N III] 57 μm, and [C II] 158 μm, and fitted their ratios to a combination starburst and photodissociation region (PDR) model. The best fit is obtained with H II regions with n=250 cm-3, an ionization parameter of 10-3.5, and PDRs with n=103.3 cm-3 and a far-ultraviolet flux of G0=102.8. We applied both continuous and instantaneous starburst models, with our best fit being a 3-5 Myr old instantaneous burst model with a 100 M☉ cutoff. We also detected the ground-state rotational line of OH in absorption at 119.4 μm. No excited level OH transitions are apparent, indicating that the OH is almost entirely in its ground state with a column density ~4×1014 cm-2. The spectral energy distribution over the long-wavelength spectrometer wavelength range is well fitted with a 48 K dust temperature and an optical depth, τDust λ-1.
The Astrophysical Journal 01/2009; 511(2):721. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: We report the first detection of the 205 μm 3P1 P0 [N II] line from a ground-based observatory using a direct detection spectrometer. The line was detected from the Carina star formation region using the South Pole Imaging Fabry-Perot Interferometer (SPIFI) on the Antarctic Submillimeter Telescope and Remote Observatory (AST/RO) at the South Pole. The [N II] 205 μm line strength indicates a low-density (n ~ 32 cm-3) ionized medium, similar to the low-density ionized halo previously reported in its [O III] 52 and 88 μm line emission. When compared with the Infrared Space Observatory [C II] observations of this region, we find that 27% of the [C II] line emission arises from this low-density ionized gas, but the large majority (~73%) of the observed [C II] line emission arises from the neutral interstellar medium. This result supports and underpins prior conclusions that most of the observed [C II] 158 μm line emission from Galactic and extragalactic sources arises from the warm, dense photodissociated surfaces of molecular clouds. The detection of the [N II] line demonstrates the utility of Antarctic sites for THz spectroscopy.
The Astrophysical Journal 12/2008; 652(2):L125. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: Using the large multiwavelength data set in the Chandra/SWIRE Survey (0.6 deg2 in the Lockman Hole), we show evidence for the existence of highly obscured (Compton-thick) AGNs, estimate a lower limit to their surface density, and characterize their multiwavelength properties. Two independent selection methods based on the X-ray and infrared spectral properties are presented. The two selected samples contain (1) five X-ray sources with hard X-ray spectra and column densities 1024 cm-2 and (2) 120 infrared sources with red and AGN-dominated infrared SEDs. We estimate a surface density of at least 25 Compton-thick AGNs deg-2 detected in the infrared in the Chandra/SWIRE field, of which ~40% show distinct AGN signatures in their optical/near-infrared SEDs, the remaining being dominated by the host galaxy emission. Only ~33% of all Compton-thick AGNs are detected in the X-rays at our depth [F(0.3-8 keV) > 10-15 ergs cm-2 s-1]. We report the discovery of two sources in our sample of Compton-thick AGNs, SWIRE J104409.95+585224.8 (z = 2.54) and SWIRE J104406.30+583954.1 (z = 2.43), which are the most luminous Compton-thick AGNs at high z currently known. The properties of these two sources are discussed in detail with an analysis of their spectra, SEDs, luminosities, and black hole masses.
The Astrophysical Journal 12/2008; 642(2):673. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: We present new 37.7 μm far-infrared imaging of the infrared luminous (LIR ~ 5.16 × 1011 L☉) interacting galaxy Arp 299 (= IC 694 + NGC 3690). We show that the 38 μm flux, like the 60 and 100 μm emission, traces the luminosity of star forming galaxies but at considerably higher spatial resolution. Our data establish that the major star formation activity of the galaxy originates from a point source in its eastern component, IC 694, which is inconspicuous in the optical, becoming visible only at the near- and mid-infrared. We find that IC 694 is 2 times more luminous than NGC 3690, contributing to more than 46% of the total energy output of the system at this wavelength. The spectral energy distribution of the different components of the system clearly shows that IC 694 has 6 times the infrared luminosity of M82, and it is the primary source responsible for the bolometric luminosity of Arp 299.
The Astrophysical Journal 12/2008; 571(1):282. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: We present a trade study for a submillimeter direct-detection spectrometer operating at the background limit for the Cornell Caltech Atacama Telescope (CCAT). In this study we compare the classical echelle spectrometer ZEUS with the waveguide grating spectrometer Z-Spec. The science driver for this instrument is spectroscopic investigation of high redshift galaxies as their far-IR fine structure line emission is redshifted into the telluric submillimeter windows. The baseline detector consists of SQUID multiplexed TES bolometers and the ideal spectrometer to detect weak lines from distant extragalactic sources is a grating with a resolution of ~103 and a large bandwidth, covering an entire telluric submillimeter window instantaneously. Since the density of high-z sources on the sky is ~100 within a 10'×10' field of view and a redshift range of Deltaz~0.2 we also explore multi-object (~50 objects) capability, including articulated mirrors and flexible waveguide fibers.
[show abstract][hide abstract] ABSTRACT: We report the detection of the 158 mum [CII] line from the LIR ˜ 1014 L&sun; galaxy FSC 10026+4949 at z = 1.12. This is the first detection of [CII] from the z = 1-3 epoch of enhanced starformation. The L[CII]/LFIR ratio is 1.3 × 10-3, larger than the ratio typically seen in local ULIRGs, but similar to that measured in normal and starburst galaxies. This high ratio implies that the UV fields powering the far-infrared emission are of only modest intensity, and that FSC 10026+4949 may be undergoing a galaxy-wide starburst.
[show abstract][hide abstract] ABSTRACT: We have begun a survey of the [CI] 370 mum fine structure line and of mid-J CO line emission from ULIRGs and starburst galaxies using our grating spectrometer ZEUS on the CSO. Here we present observations from six ULIRGs: Mrk 231, Zw 049.057, NGC 6240, Arp 299, Arp 220, and IRAS 17208. The CO (6-5)/FIR ratio in ULIRGs appears to be lower for ULIRGs than for LIRGs. Comparing the CO SEDs of the ULIRGs shows the necessity of obtaining CO rotational transitions up to at least J=7 to properly constrain LVG models.
[show abstract][hide abstract] ABSTRACT: We report the first detection of a mid-J isotopic CO line from an
external galaxy. We detected the 13CO (6-5) line from the
starburst nucleus of NGC 253. The line is suprisingly bright with an
integrated intensity 7% of the 12CO (6-5) line, indicating
optical depth in the 12CO line. Our LVG modeling shows that a
single warm (T ~ 120 K), dense (n ~ 104 cm-3)
component emits most of the 12CO and 13CO line
emission from J = 2-1 through J = 7-6. The CO(1-0) line comes from an
additional lower excitation envelope. About 60% of the total molecular
gas mass within 70 pc of the nucleus is in the warm, dense component. We
show that stellar far-UV photons or X-ray photons from a nuclear source
are unlikely to be the primary sources of the gas heating. The most
likely sources of heat are cosmic rays from the nuclear starburst or
microturbulence within molecular clouds.
[show abstract][hide abstract] ABSTRACT: We report the detection of 13CO J=6-->5 emission from the nucleus of the starburst galaxy NGC 253 with the redshift (z) and Early Universe Spectrometer (ZEUS), a new submillimeter grating spectrometer. This is the first extragalactic detection of the 13CO J=6-->5 transition, which traces warm, dense molecular gas. We employ a multiline LVG analysis and find ~35%-60% of the molecular interstellar medium is both warm (T~110 K) and dense (nH2~104 cm-3). We analyze the potential heat sources and conclude that ultraviolet and X-ray photons are unlikely to be energetically important. Instead, the molecular gas is most likely heated by an elevated density of cosmic rays or by the decay of supersonic turbulence through shocks. If the cosmic rays and turbulence are created by stellar feedback within the starburst, then our analysis suggests the starburst may be self-limiting.
The Astrophysical Journal 01/2008; 689. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: The fine structure line of [CI] at 370 μm, and the CO (7→6) rotational line at 371 μm are important coolants which can be used to probe the temperature and density of the warm gas component in regions of enhanced
star formation. These lines can be used to provide key constrains on the physical properties of the warm molecular gas component
and to deduce the excitation mechanism that heats the molecular gas. We have used the Cornell submillimeter spectrometer,
SPIFI, on the JCMT to map the [CI] and CO (7→6) line emission from the nuclear regions of the nearby galaxies NGC 253 and M82, and from the overlap region of NGC4038/9.
In both NGC 253 and NGC 4038/9 the [CI] emission appears fairly constant, while the CO (7→6) emission peaks at the nucleus and the most active star-forming region, respectively. The observations of NGC 253 suggest
that the majority of the nuclear molecular gas is most likely heated by cosmic rays. The spatial distribution of the line
emission in the overlap region of NGC 4038/9 is suggestive of a PDR, with a decrease in density from the most active region
to the outside. In contrast, in M82 both the CO (7→6) and [CI] line intensities vary over the mapped region, with the line ratios remaining fairly constant. This suggests a
change in the beam filling factor of PDRs with a higher PDR number density in the most active regions.
[show abstract][hide abstract] ABSTRACT: We present the analysis of Infrared Space Observatory long-wavelength spectrometer (LWS) observations of the two nearby late-type galaxies NGC 1313 and NGC 6946. Both galaxies have been fully mapped in the [C II] far-infrared fine-structure line at 158 μm, and some regions have been observed also in the [O I] 63 μm and [N II] 122 μm lines. We use these observations to derive the physical properties of the atomic interstellar medium, to establish how they relate to other interstellar medium components (gas and dust), and to establish how they vary with different galaxy components such as nucleus, spiral arms, and disk. The [C II] line is the main cooling line of the atomic medium. In NGC 6946 and NGC 1313, its emission represents 0.8% of the infrared emission. Moreover, the [C II] emission can be spatially associated with three components: the nucleus, the star-forming regions in spiral arms, and the diffuse galaxy disk. This last component contributes 40% in NGC 6946 and ~30% in NGC 1313 to the total emission. We apply the photodissociation region (PDR) model by Kaufman et al. to derive PDR physical parameters responsible for the neutral atomic gas emission (G0, n, and Ts). The results do not significantly differ from what Malhotra et al. recently found by modeling the integrated emission of a sample of 60 normal galaxies. This suggests that the emission in each region under the LWS beam in NGC 6946 and NGC 1313 (corresponding to a linear size of ~1.5 kpc) is likely to arise from a mixture of components similar to the mixture producing the integrated emission of normal galaxies. However, some regions in NGC 6946 have a G0/n ratio ~2–3 times smaller than the mean value found for the normal galaxy sample (1.3), suggesting that the beam-averaged contribution of a less active component in these regions is higher than its contribution in the integrated emission of normal galaxies or, conversely, that the bulk of the integrated emission of the normal galaxies is dominated by a few active regions probably located in their nuclei. CO(1–0) and [C II] in NGC 6946 are well correlated, and the mean [C II]/CO ratio agrees with the mean integrated ratios of the normal galaxies sample. This value (~500) is a factor of ~2 less than the mean ratio found for a sample of normal galaxies observed with the Kupier Airborne Observatory (KAO) by Stacey et al. This difference is probably due to the fact that the KAO beam (55'') is smaller than the LWS beam (75''), such that the Stacey et al. KAO observations are likely to be more biased toward the nucleus of the galaxies and therefore toward more active regions. In NGC 1313 only four LWS regions have been observed in CO(1–0), and three of them were detected. The [C II]/CO(1–0) seems to systematically increase from the northeast to the south, along the S-shaped spiral arm, indicating that the interstellar medium conditions in NGC 1313 are much more inhomogeneous than the conditions in NGC 6946. H I and [C II] in NGC 6946 are completely uncorrelated, probably because they arise from different gas components: [C II] arises principally in dense and warm PDR and H I from diffuse (n 3 × 103 cm-3) gas. On the other hand, in NGC 1313 we successfully detect two distinct gas components: a cirrus-like component, for which H I and [C II] are weakly correlated as observed in our Galaxy, and a component associated with dense PDRs completely uncorrelated from H I as observed in NGC 6946. Finally, we find that the H I residing in dense gas surrounding the star-forming regions and presumably recently photodissociated constitutes a few percent of the total H I. In turn, this dense gas component produces most of the [C II] emission emitted by the atomic neutral medium, even if its contribution is lower in NGC 1313 than in NGC 6946. On the other hand, the [C II] emission arising from ionized gas is higher in NGC 1313 than in NGC 6946.
The Astronomical Journal 12/2007; 124(2):751. · 4.97 Impact Factor