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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