We present an atlas of ultraviolet and infrared images of ∼ 15, 750 local (d 50 Mpc) galaxies, as observed by NASA's WISE and GALEX missions. These maps have matched resolution (FWHM 7.5 and 15 ), matched astrometry, and a common procedure for background removal. We demonstrate that they agree well with resolved intensity measurements and integrated photometry from previous surveys. This atlas represents the first part of a program (the z = 0 Multi-wavelength Galaxy Synthesis) to create a large, uniform database of resolved measurements of gas and dust in nearby galaxies. The images and associated catalogs will be publicly available at the NASA/IPAC Infrared Science Archive. This atlas allows us estimate local and integrated star formation rates (SFRs) and stellar masses (M ) across the local galaxy population in a uniform way. In the appendix, we use the population synthesis fits of Salim et al. (2016Salim et al. ( , 2018 to calibrate integrated M and SFR estimators based on GALEX and WISE. Because they leverage an SDSS-base training set of > 100, 000 galaxies, these calibrations have high precision and allow us to rigorously compare local galaxies to Sloan Digital Sky Survey results. We provide these SFR and M estimates for all galaxies in our sample and show that our results yield a "main sequence" of star forming galaxies comparable to previous work. We also show the distribution of intensities from resolved galaxies in NUV-to-WISE1 vs. WISE1-to-WISE3 space, which captures much of the key physics accessed by these bands.

We compare the structure of molecular gas at 40 pc resolution to the ability of gas to form stars across the disk of the spiral galaxy M51. We break the PAWS survey into 370 pc and 1.1 kpc resolution elements, and within each we estimate the molecular gas depletion time (τ mol Dep ), the star formation efficiency per free fall time (ǫ ff ), and the mass-weighted cloud-scale (40 pc) properties of the molecular gas: surface density, Σ, line width, σ, andvir , a parameter that traces the boundedness of the gas. We show that the cloud-scale surface density appears to be a reasonable proxy for mean volume density. Applying this, we find a typical star formation efficiency per free-fall time, ǫ ff ( Σ 40pc ) ∼ 0.3−0.36%, lower than adopted in many models and found for local clouds. More, the efficiency per free fall time anti-correlates with both Σ and σ, in some tension with turbulent star formation models. The best predictor of the rate of star formation per unit gas mass in our analysis is b ≡ Σ/σ 2 , tracing the strength of self gravity, with τ mol Dep ∝ b −0.9 . The sense of the correlation is that gas with stronger self-gravity (higher b) forms stars at a higher rate (low τ

We use new ALMA observations to investigate the connection between dense gas fraction, star formation rate, and local environment across the inner region of four local galaxies showing a wide range of molecular gas depletion times. We map HCN (1-0), HCO + (1-0), CS (2-1), 13 CO (1-0), and C 18 O (1-0) across the inner few kpc of each target. We combine these data with short spacing information from the IRAM large program EMPIRE, archival CO maps, tracers of stellar structure and recent star formation, and recent HCN surveys by Bigiel et al. and Usero et al. We test the degree to which changes in the dense gas fraction drive changes in the SFR. I HCN /I CO (tracing the dense gas fraction) correlates strongly with I CO (tracing molecular gas surface density), stellar surface density, and dynamical equilibrium pressure, P DE . Therefore, I HCN /I CO becomes very low and HCN becomes very faint at large galactocentric radii, where ratios as low as I HCN /I CO ∼ 0.01 become common. The apparent ability of dense gas to form stars, Σ SF R /Σ dense (where Σ dense is traced by the HCN intensity and the star formation rate is traced by a combination of Hα and 24µm emission), also depends on environment. Σ SF R /Σ dense decreases in regions of high gas surface density, high stellar surface density, and high P DE . Statistically, these correlations between environment and both Σ SF R /Σ dense and I HCN /I CO are stronger than that between apparent dense gas fraction (I HCN /I CO ) and the apparent molecular gas star formation efficiency Σ SF R /Σ mol . We show that these results are not specific to HCN.

We present EMPIRE, an IRAM 30 m large program that mapped λ = 3–4 mm dense gas tracers at ∼1–2 kpc resolution across the whole star-forming disk of nine nearby massive spiral galaxies. We describe the EMPIRE observing and reduction strategies and show new whole-galaxy maps of HCN(1−0), HCO+(1−0), HNC(1−0), and CO(1−0). We explore how the HCN-to-CO and IR-to-HCN ratios, observational proxies for the dense gas fraction and dense gas star formation efficiency, depend on host galaxy and local environment. We find that the fraction of dense gas correlates with stellar surface density, gas surface density, molecular-to-atomic gas ratio, and dynamical equilibrium pressure. In EMPIRE, the star formation rate per unit dense gas is anticorrelated with these same environmental parameters. Thus, although dense gas appears abundant in the central regions of many spiral galaxies, this gas appears relatively inefficient at forming stars. These results qualitatively agree with previous work on nearby galaxies and the Milky Way’s Central Molecular Zone. To first order, EMPIRE demonstrates that the conditions in a galaxy disk set the gas density distribution and that the dense gas traced by HCN shows an environment-dependent relation to star formation. However, our results also show significant (±0.2 dex) galaxy-to-galaxy variations. We suggest that gas structure below the scale of our observations and dynamical effects likely also play an important role.

We explore the use of mm-wave emission line ratios to trace molecular gas density when observations integrate over a wide range of volume densities within a single telescope beam. For observations targeting external galaxies, this case is unavoidable. Using a framework similar to that of Krumholz & Thompson (2007), we model emission for a set of common extragalactic lines from lognormal and power law density distributions. We consider the median density of gas producing emission and the ability to predict density variations from observed line ratios. We emphasize line ratio variations, because these do not require knowing the absolute abundance of our tracers. Patterns of line ratio variations have the prospect to illuminate the high-end shape of the density distribution, and to capture changes in the dense gas fraction and median volume density. Our results with and without a high density power law tail differ appreciably; we highlight better knowledge of the PDF shape as an important area. We also show the implications of sub-beam density distributions for isotopologue studies targeting dense gas tracers. Differential excitation often implies a significant correction to the naive case. We provide tabulated versions of many of our results, which can be used to interpret changes in mm-wave line ratios in terms of changes in the underlying density distributions.