Aims. The nuclei of active galaxies harbor massive young stars, an accreting central black hole, or both. In order to determine the physical conditions that pertain to molecular gas close to the sources of radiation, numerical models are constructed. Methods. These models iteratively determine the thermal and chemical balance of molecular gas that is exposed to X-rays (1-100 keV) and far-ultraviolet radiation (6-13.6 eV), as a function of depth.Results. We present a grid of XDR and PDR models that span ranges in density (10 2 −10 6.5 cm −3 ), irradiation (10 0.5 −10 5 G 0 and 1-0 ratio becomes larger than one, although the individual HCN 1-0 and HCO + 1-0 line intensities are weaker. For modest densities, n = 10 4 −10 5 cm −3 , and strong radiation fields (>100 erg s −1 cm −2 ), HCN/HCO + ratios can become larger in XDRs than PDRs as well. Also, the HCN/CO 1-0 ratio is typically smaller in XDRs, and the HCN emission in XDRs is boosted with respect to CO only for high (column) density gas, with columns in excess of 10 23 cm −2 and densities larger than 10 4 cm −3 . Furthermore, CO is typically warmer in XDRs than in PDRs, for the same total energy input. This leads to higher CO J = N + 1 − N/CO 1-0, N ≥ 1, line ratios in XDRs. In particular, lines with N ≥ 10, like CO(16-15) and CO(10-9) observable with HIFI/Herschel, discriminate very well between XDRs and PDRs. This is crucial since the XDR/AGN contribution will typically be of a much smaller (possibly beam diluted) angular scale and a 10-25% PDR contribution can already suppress XDR distinguishing features involving HCN/HCO+ and HNC/HCN. For possible future observations, column density ratios indicate that CH, CH + , NO, HOC + and HCO are good PDR/XDR discriminators.

We present new far infrared maps of the Small Magellanic Cloud (SMC) at 24, 70, and 160 µm obtained as part of the Spitzer Survey of the Small Magellanic Cloud (S 3 MC, Bolatto et al. 2006). These maps cover most of the active star formation in the SMC Bar and the more quiescent Wing. We combine our maps with literature data to derive the surface density across the SMC. We find a total dust mass of M dust =3 × 10 5 M ⊙ , implying a dust-to-hydrogen ratio over the region studied of log 10 D/H = −2.86, or 1-to-700, which includes H 2 . Assuming the dust to trace the total gas column, we derive H 2 surface densities across the SMC. We find a total H 2 mass M H2 = 3.2 × 10 7 M ⊙ in a distribution similar to that of the CO, but more extended. We compare profiles of CO and H 2 around six molecular peaks and find that on average H 2 is more extended than CO by a factor of ∼ 1.3. The implied CO-to-H 2 conversion factor over the whole SMC is X CO = 13 ± 1 × 10 21 cm −2 (K km s −1 ) −1 . Over the volume occupied by CO we find a lower conversion factor, X CO = 6 ± 1 × 10 21 cm −2 (K km s −1 ) −1 , which is still a few times larger than that found using virial mass methods. The molecular peaks have H 2 surface densities Σ H2 ≈ 180 ± 30 M ⊙ pc −2 , similar to those in Milky Way GMCs, and correspondingly low extinctions, A V ∼ 1 -2 mag. To reconcile these measurements with predictions by the theory of photoionization-regulated star formation, which requires A V ∼ 6, the GMCs must be ∼ 3 times smaller than our 46 pc resolution element. We find that for a given hydrostatic gas pressure, the SMC has a 2 -3 times lower ratio of molecular to atomic gas than spiral galaxies. Combined with the lower mean densities in the SMC this may explain why this galaxy has only 10% of its gas in the molecular phase.

We compare atomic gas, molecular gas, and the recent star formation rate (SFR) inferred from Hα in the Small Magellanic Cloud (SMC). By using infrared dust emission and local dust-to-gas ratios, we construct a map of molecular gas that is independent of CO emission. This allows us to disentangle conversion factor effects from the impact of metallicity on the formation and star formation efficiency of molecular gas. On scales of 200 pc to 1 kpc (where the distributions of H 2 and star formation match well) we find a characteristic molecular gas depletion time of τ mol dep ∼ 1.6 Gyr, similar to that observed in the molecule-rich parts of large spiral galaxies on similar spatial scales. This depletion time shortens on much larger scales to ∼0.6 Gyr because of the presence of a diffuse Hα component, and lengthens on much smaller scales to ∼7.5 Gyr because the Hα and H 2 distributions differ in detail. We estimate the systematic uncertainties in our dust-based τ mol dep measurement to be a factor of ∼2-3. We suggest that the impact of metallicity on the physics of star formation in molecular gas has at most this magnitude, rather than the factor of ∼40 suggested by the ratio of SFR to CO emission. The relation between SFR and neutral (H 2 + H i) gas surface density is steep, with a power-law index ≈2.2 ± 0.1, similar to that observed in the outer disks of large spiral galaxies. At a fixed total gas surface density the SMC has a 5-10 times lower molecular gas fraction (and star formation rate) than large spiral galaxies. We explore the ability of the recent models by Krumholz et al. and Ostriker et al. to reproduce our observations. We find that to explain our data at all spatial scales requires a low fraction of cold, gravitationally bound gas in the SMC. We explore a combined model that incorporates both large-scale thermal and dynamical equilibrium and cloud-scale photodissociation region structure and find that it reproduces our data well, as well as predicting a fraction of cold atomic gas very similar to that observed in the SMC.

We present the initial results from the Spitzer Survey of the Small Magellanic Cloud (S 3 MC), which imaged the star-forming body of the Small Magellanic Cloud (SMC) in all seven MIPS and IRAC wavebands. We find that the F 8 /F 24 ratio (an estimate of PAH abundance) has large spatial variations and takes a wide range of values that are unrelated to metallicity but anticorrelated with 24 µm brightness and F 24 /F 70 ratio. This suggests that photodestruction is primarily responsible for the low abundance of PAHs observed in star-forming low-metallicity galaxies. We use the S 3 MC images to compile a photometric catalog of ∼ 400, 000 mid-and far-infrared point sources in the SMC. The sources detected at the longest wavelengths fall into four main categories: 1) bright 5.8 µm sources with very faint optical counterparts and very red mid-infrared colors ([5.8] − [8.0] > 1.2), which we identify as YSOs. 2) Bright mid-infrared sources with mildly red colors (0.16 [5.8] − [8.0] < 0.6), identified as carbon stars. 3) Bright mid-infrared sources with neutral colors and bright optical counterparts, corresponding to oxygen-rich evolved stars. And, 4) unreddened early B stars (B3 to O9) with a large 24 µm excess. This excess is reminiscent of debris disks, and is detected in only a small fraction of these stars ( 5%). The majority of the brightest infrared point sources in the SMC fall into groups one to three. We use this photometric information to produce a catalog of 282 bright YSOs in the SMC with a very low level of contamination (∼ 7%).

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Do molecular clouds collapse to form stars at the same rate in all environments? In large spiral galaxies, the rate of transformation of H 2 into stars varies little. However, the SFE in distant objects (z ∼ 1) is much higher than in the large spiral disks that dominate the local universe. Some small Local Group galaxies share at least some of the characteristics of intermediate-redshift objects, such as size or color. Recent work has suggested that the star formation efficiency (SFE, defined as the star formation rate per unit H 2 ) in local Dwarf galaxies may be as high as in the distant objects. A fundamental difficulty in these studies is the independent measure of the H 2 mass in metal-deficient environments. At 490 kpc, NGC 6822 is an excellent choice for this study; it has been mapped in the CO(2-1) line using the multibeam receiver HERA on the 30 m IRAM telescope, yielding the largest sample of giant molecular clouds (GMCs) in this galaxy. Despite the much lower metallicity, we find no clear difference in the properties of the GMCs in NGC 6822 and those in the Milky Way except lower CO luminosities for a given mass. Several independent methods indicate that the total H 2 mass in NGC 6822 is about 5 × 10 6 M in the area we mapped and less than 10 7 M in the whole galaxy. This corresponds to a N(H 2 )/I CO ≈ 4 × 10 21 cm −2 /(K km s −1 ) over large scales, such as would be observed in distant objects, and half that in individual GMCs. No evidence was found for H 2 without CO emission. Our simulations of the radiative transfer in clouds are entirely compatible with these N(H 2 )/I CO values. The SFE implied is a factor 5-10 higher than what is observed in large local universe spirals. The CO observations presented here also provide a high-resolution datacube (1500 a.u. for the assumed 100 pc distance, 0.41 km s −1 velocity resolution) of a local molecular cloud along the line of sight.

To understand the impact of low metallicities on giant molecular cloud (GMC) structure, we compare far-infrared dust emission, CO emission, and dynamics in the star-forming complex N83 in the Wing of the Small Magellanic Cloud (SMC). Dust emission (measured by Spitzer as part of the Spitzer Survey of the SMC and Surveying the Agents of a Galaxy's Evolution in the SMC surveys) probes the total gas column independent of molecular line emission and traces shielding from photodissociating radiation. We calibrate a method to estimate the dust column using only the high-resolution Spitzer data and verify that dust traces the interstellar medium in the H i-dominated region around N83. This allows us to resolve the relative structures of H 2 , dust, and CO within a GMC complex, one of the first times such a measurement has been made in a low-metallicity galaxy. Our results support the hypothesis that CO is photodissociated while H 2 self-shields in the outer parts of low-metallicity GMCs, so that dust/self-shielding is the primary factor determining the distribution of CO emission. Four pieces of evidence support this view. First, the CO-to-H 2 conversion factor averaged over the whole cloud is very high 4-11 × 10 21 cm −2 (K km s −1 ) −1 , or 20-55 times the Galactic value. Second, the CO-to-H 2 conversion factor varies across the complex, with its lowest (most nearly Galactic) values near the CO peaks. Third, bright CO emission is largely confined to regions of relatively high line-of-sight extinction, A V 2 mag, in agreement with photodissociation region models and Galactic observations. Fourth, a simple model in which CO emerges from a smaller sphere nested inside a larger cloud can roughly relate the H 2 masses measured from CO kinematics and dust.

The dust properties in the Large and Small Magellanic clouds (LMC/SMC) are studied using the HERITAGE Herschel Key Project photometric data in five bands from 100 to 500 μm. Three simple models of dust emission were fit to the observations: a single temperature blackbody modified by a power-law emissivity (SMBB), a single temperature blackbody modified by a broken power-law emissivity (BEMBB), and two blackbodies with different temperatures, both modified by the same power-law emissivity (TTMBB). Using these models, we investigate the origin of the submillimeter excess, defined as the submillimeter emission above that expected from SMBB models fit to observations <200 μm. We find that the BEMBB model produces the lowest fit residuals with pixelaveraged 500 μm submillimeter excesses of 27% and 43% for the LMC and SMC, respectively. Adopting gas masses from previous works, the gas-to-dust ratios calculated from our fitting results show that the TTMBB fits require significantly more dust than are available even if all the metals present in the interstellar medium (ISM) were condensed into dust. This indicates that the submillimeter excess is more likely to be due to emissivity variations than a second population of colder dust. We derive integrated dust masses of (7.3 ± 1.7) × 10 5 and (8.3 ± 2.1) × 10 4 M for the LMC and SMC, respectively. We find significant correlations between the submillimeter excess and other dust properties; further work is needed to determine the relative contributions of fitting noise and ISM physics to the correlations.