Dust and Stellar Emission of Nearby Galaxies in the KINGFISH Herschel Survey Ramin A. Skibba Charles W. Engelbracht, et al. I. Introduction We exploit data from the UV to submillimeter wavelengths of a heterogeneous sample of 62 galaxies from the KINGFISH project (Key Insights on Nearby Galaxies: a Far-Infrared Survey with Herschel), to empirically study the emission from stars and dust in these galaxies. We use the spectral energy distributions computed by Dale et al., using data from GALEX, SDSS (and other optical measurements), 2MASS, Spitzer, SCUBA (when available), as well as some new data from Herschel. We estimate the stellar and dust emission of these galaxies, in a way that is as empirical and model-independent as possible, and we use these to estimate the ratio of stellar-to-dust emission. The stellar-to-dust ratios can be compared to gas-to-dust ratios determined from SED models, and will yield important information about the properties of the ISM in these galaxies. This is an ongoing project. We expect that Herschel observations will allow us to trace cold dust components invisible to Spitzer and will have greatly reduced systematic uncertainties relative to ground-based submillimeter measurements. We examine how our estimated stellar-to-dust ratios correlate with various galaxy properties: total infrared luminosity, morphology, stellar mass, and metallicity. Finally, for a few well-resolved galaxies, such as M101, we plan to use Herschel observations to study the spatial variations of the stellar-to-dust ratio within the galaxies. II. Data For the 58 SINGS galaxies included in KINGFISH, we use the global flux densities estimated in Dale et al. (2007). For the 17 LVL galaxies included in KINGFISH, two of which are not in SINGS (M101, NGC 3077), we use the global flux densities estimated in Dale et al. (2009). Data for the remaining two galaxies, IC 342 and NGC 2146, were obtained separately. UV data from GALEX (1528, 2271 Å); optical data from either SDSS (ugriz) or Kitt Peak (BVRI); near-infrared data from 2MASS (JHK); mid-infrared data from Spitzer (3.6, 4.5, 5.8, 8, 24, 70, 160  m; submm data for 1/3 of the galaxies from SCUBA (450, 850  m). III. Stellar-to-Dust Ratio In order to demarcate “stellar” and “dust” emission in the SEDs, ( f ) stars and ( f ) dust, we choose a strict wavelength cut at 5  m and simply integrate over the SED at 5  m for the latter. The area under the SED is computed directly, without applying any model, using Jy and log(  m) as our units. We choose not to extrapolate the SEDs for the galaxies missing UV data (4/60 galaxies) or submm data (38/60). Finally, we take the ratio ( f ) stars / ( f ) dust for the stellar/dust emission. To estimate the uncertainties, we simply assume that the errors of the flux densities have a Gaussian distribution; we sample from these distributions 100 times, and compute the variance around the mean stellar/dust ratio. The figure shows two example SEDs: IV. Current Results Distribution of stellar-to- dust ratios for 60 nearby galaxies: Many of the galaxies have stellar/dust emission near unity, with very few extremely dusty passive galaxies. NGC 1404 and DDO 165 have the largest ratios, and NGC 1482 and 1266 have the lowest. Correlation between total infrared (TIR) luminosity (estimated from 8, 24, 70, 160µm; Draine & Li 2007) vs. stellar/dust emission: There appear to be two distinct populations: some of the galaxies with large stellar/dust emission and large L TIR are earlier types, while some of them with large stellar/dust and small L TIR are irregulars. Many of the galaxies with stellar/dust~1 and high L TIR are metal-rich late-type spirals, suggesting an evolutionary sequence (such that they’re between the two populations with higher stellar/dust). Specific SFR vs. stellar-to-dust ratio of the 32 KINGFISH galaxies studied by Noll et al. (2009; their sample excludes the dwarf galaxies). This dramatically strong correlation could simply be explained by mostly warm dust heated by stars in galaxies with higher formation rates. V. Plans for Herschel This is a work in progress. By adding new data from Herschel (especially SPIRE), we will detect more cold dust in these galaxies, reducing systematic uncertainties in our estimates of the amount of dust (vis-à-vis stellar) emission. We plan to estimate the mass in dust relative to mass in stars (with few assumptions), rather than just the luminosity ratio. The mass ratio M stellar /M dust is a more physical quantity that could distinguish between two possibilities: the trends may be due to galaxies with different dust populations, or to some galaxies with stronger interstellar radiation fields. We also plan to analyze the spatial dependence of stellar/dust emission for a couple galaxies, such as M101, IC 342, or M33.