We compare molecular gas traced by 12 CO(2-1) maps from the HERACLES survey, with tracers of the recent star formation rate (SFR) across 30 nearby disk galaxies. We demonstrate a first-order linear correspondence between Σ mol and Σ SFR but also find important second-order systematic variations in the apparent molecular gas depletion time, τ mol dep = Σ mol /Σ SFR . At the 1 kpc common resolution of HERACLES, CO emission correlates closely with many tracers of the recent SFR. Weighting each line of sight equally, using a fixed alpha CO equivalent to the Milky Way value, our data yield a molecular gas depletion time, τ mol dep = Σ mol /Σ SFR ≈ 2.2 Gyr with 0.3 dex 1σ scatter, in very good agreement with recent literature data. We apply a forward-modeling approach to constrain the power-law index, N , that relates the SFR surface density and the molecular gas surface density, Σ SFR ∝ Σ N mol . We find N = 1 ± 0.15 for our full data set with some scatter from galaxy to galaxy. This also agrees with recent work, but we caution that a power law treatment oversimplifies the topic given that we observe correlations between τ mol dep and other local and global quantities. The strongest of these are a decreased τ mol dep in low-mass, low-metallicity galaxies and a correlation of the kpc-scale τ mol dep with dust-to-gas ratio, D/G. These correlations can be explained by a CO-to-H 2 conversion factor (α CO ) that depends on dust shielding, and thus D/G, in the theoretically expected way. This is not a unique interpretation, but external evidence of conversion factor variations makes this the most conservative explanation of the strongest observed τ mol dep trends. After applying a D/G-dependent α CO , some weak correlations between τ mol dep and local conditions persist. In particular, we observe lower τ mol dep and enhanced CO excitation associated with nuclear gas concentrations in a subset of our targets. These appear to reflect real enhancements in the rate of star formation per unit gas and although the distribution of τ dep does not appear bimodal in galaxy centers, τ dep does appear multivalued at fixed Σ mol , supporting the the idea of "disk" and "starburst" modes driven by other environmental parameters.

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ABSTRACT. The Spitzer Survey of Stellar Structure in Galaxies (S 4 G) is an Exploration Science Legacy Program approved for the Spitzer post-cryogenic mission. It is a volume-, magnitude-, and size-limited (d < 40 Mpc, jbj > 30°, m Bcorr < 15:5, and D 25 > 1 0 ) survey of 2331 galaxies using the Infrared Array Camera (IRAC) at 3.6 and 4.5 μm. Each galaxy is observed for 240 s and mapped to ≥1:5 × D 25 . The final mosaicked images have a typical 1σ rms noise level of 0.0072 and 0:0093 MJy sr À1 at 3.6 and 4.5 μm, respectively. Our azimuthally averaged surface brightness profile typically traces isophotes at μ 3:6μm ðABÞð1σÞ ∼ 27 mag arcsec À2 , equivalent to a stellar mass surface density of ∼1 M ⊙ pc À2 . S 4 G thus provides an unprecedented data set for the study of the distribution of mass and stellar structures in the local universe. This large, unbiased, and extremely deep sample of all Hubble types from dwarfs to spirals to ellipticals will allow for detailed structural studies, not only as a function of stellar mass, but also as a function of the local environment. The data from this survey will serve as a vital testbed for cosmological simulations predicting the stellar mass properties of present-day galaxies. This article introduces the survey and describes the sample selection, the significance of the 3.6 and 4.5 μm bands for this study, and the data collection and survey strategies. We describe the S 4 G data analysis pipeline and present measurements for a first set of galaxies, observed in both the cryogenic and warm mission phases of Spitzer. For every galaxy we tabulate the galaxy diameter, position angle, axial ratio, inclination at μ 3:6μm ðABÞ ¼ 25:5, and 26:5 mag arcsec À2 (equivalent to ≈μ B ðABÞ ¼ 27:2 and 28:2 mag arcsec À2 , respectively). These measurements will form the initial S 4 G catalog of galaxy properties. We also measure the total magnitude and the azimuthally averaged radial profiles of ellipticity, position angle, surface brightness, and color. Finally, using the galaxy-fitting code GALFIT, we deconstruct each galaxy into its main constituent stellar components: the bulge/spheroid, disk, bar, and nuclear point source, where necessary. Together, these data products will provide a comprehensive and definitive catalog of stellar structures, mass, and properties of galaxies in the nearby universe and will enable a variety of scientific investigations, some of which are highlighted in this introductory S 4 G survey paper.

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 edgeon 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.

Using combinations of Hα, ultraviolet (UV), and infrared (IR) emission, we estimate the star formation rate (SFR) surface density, Σ SFR , at 1 kpc resolution for 30 disk galaxies that are targets of the IRAM HERACLES CO survey. We present a new physically-motivated IR spectral energy distribution-based approach to account for possible contributions to 24µm emission not associated with recent star formation. Considering a variety of "reference" SFRs from the literature, we revisit the calibration of the 24µm term in hybrid (UV+IR or Hα+IR) tracers. We show that the overall calibration of this term remains uncertain at the factor of two level because of the lack of wide-field, robust reference SFR estimates. Within this uncertainty, published calibrations represent a reasonable starting point for 1 kpc-wide areas of star-forming disk galaxies but we re-derive and refine the calibration of the IR term in these tracers to match our resolution and approach to 24µm emission. We compare a large suite of Σ SFR estimates and find that above Σ SFR ∼ 10 −3 M ⊙ yr −1 kpc −2 the systematic differences among tracers are less than a factor of two across two orders of magnitude dynamic range. We caution that methodology and data both become serious issues below this level. We note from simple model considerations that focusing on a part of a galaxy dominated by a single stellar population the intrinsic uncertainty in Hα and FUV-based SFRs are ∼ 0.3 and ∼ 0.5 dex. 2.1. HERACLES CO The HERA CO Line Extragalactic Survey (HERA-CLES, Leroy et al. 2009) used the Heterodyne Receiver Array (HERA, Schuster et al. 2004) on the IRAM 30mtelescope to map CO J = 2 → 1 emission from 48 nearby galaxies. The HERACLES cubes cover out to r 25 with angular resolution 13 ′′ and typical 1σ sensitivity 20 mK per 5 km s −1 channel. Leroy et al. (2012, in prep.) present the full data set and more details 10 .We estimate H 2 mass surface density, Σ H2 , from CO J = 2 → 1 intensity via(1) where R 21 is the CO(2→1)-to-CO(1→0) line ratio and α CO is the CO(1→0)-to-H 2 conversion factor. The formula includes a factor of 1.36 to account for helium. We adopt a line ratio of 0.7 11 , and a Galactic conversion factor, α CO = 4.4 M ⊙ pc −2 K km s −1 −1 equivalent to X CO = 2 × 10 20 cm −2 (K km s −1 ) −1 . This value is intermediate among recent determinations of the Milky Way α CO (e.g., Strong & Mattox 1996;Dame et al. 2001;Heyer et al. 2009;Abdo et al. 2010) and represents a commonly adopted "best single value" for work on nearby disk galaxies (e.g., Wong & Blitz 2002;Leroy et al. 2008). HERACLES does span a range 10 All HERACLES data are publicly available from the IRAM and NRAO web pages.Current URL www.cv.nrao.edu/$\sim$aleroy/HERACLES 11 This line ratio is slightly lower than the 0.8 used by Leroy et al. (2009), reflecting the revised efficiency used in the data reduction. This value, 0.7, is the mean ratio of integrated CO J = 2 → 1 HERACLES flux divided by the CO J = 1 → 0 flux measured by Young et al.