THE EMISSION BY DUST AND STARS OF NEARBY GALAXIES IN THEHERSCHELKINGFISH SURVEY

Abstract

Using new far-infrared imaging from the Herschel Space Observatory with ancillary data from ultraviolet (UV) to submillimeter (submm) wavelengths, we estimate the total emission from dust and stars of 62 nearby galaxies in the KINGFISH survey in a way that is as empirical and modelindependent as possible. We collect and exploit these data in order to measure from the spectral energy distributions (SEDs) precisely how much stellar radiation is intercepted and re-radiated by dust, and how this quantity varies with galaxy properties. By including SPIRE data, we are more sensitive to emission from cold dust grains than previous analyses at shorter wavelengths, allowing for more accurate estimates of dust temperatures and masses.The dust/stellar flux ratio, which we measure by integrating the SEDs, has a range of nearly three decades (from 10 −2.2 to 10 0.5 ). The inclusion of SPIRE data shows that estimates based on data not reaching these far-IR wavelengths are biased low by 17% on average. We find that the dust/stellar flux ratio varies with morphology and total infrared (IR) luminosity, with dwarf galaxies having faint luminosities, spirals having relatively high dust/stellar ratios and IR luminosities, and some earlytypes having low dust/stellar ratios. We also find that dust/stellar flux ratios are related to gas-phase metallicity (log(f dust /f * ) = −0.66 ± 0.08 and −0.22 ± 0.12 for metal-poor and intermediate-metallicity galaxies, respectively), while the dust/stellar mass ratios are less so (differing by ≈ 0.2 dex); the more metal-rich galaxies span a much wider range of the flux ratios. In addition, the substantial scatter between dust/stellar flux and dust/stellar mass indicates that the former is a poor proxy of the latter. Comparing the dust/stellar flux ratios and dust temperatures, we also show that early-types tend to have slightly warmer temperatures (by up to 5 K) than spiral galaxies, which may be due to more intense interstellar radiation fields, or possibly to different dust grain compositions. Finally, we show that early-types and early-type spirals have a strong correlation between the dust/stellar flux ratio and specific star formation rate, which suggests that the relatively bright far-IR emission of some of these galaxies is due to ongoing (if limited) star formation as well as to the radiation field from older stars, which is heating the dust grains.

Vindicating single-Tmodified blackbody fits toHerschelSEDs

“…The difference comes simply from (500/250) (β−1.5) , the fitted temperature being independent of the choice for the normalization. A similar remark is made by Skibba et al (2011), who fitted KINGFISH SEDs from Spitzer and SPIRE using β = 1.5 and κ abs (500 μm) from D03. However, they inexplicably conclude that their normalization results in a mass that is a factor three higher than when κ abs (250 μm) is used.…”

Does the stellar disc flattening depend on the galaxy type?

“…Masters et al 2010;Skibba et al 2011), and we see that the bright region is populated mainly by the galaxies with dust traces.…”

Encoding of the infrared excess in theNUVrKcolor diagram for star-forming galaxies

“…It is worth noting that the reference sSFR for quiescent galaxies might also suffer from systematic error. In fact, recent analysis with Herschel data have reported evidences for a warmer dust temperature in early-than in late-type galaxies (Skibba et al 2011;Smith et al 2012). This will result in an over-estimation of L IR as derived from the 24 µm luminosity (by adopting a too high L IR /L 24 µ ratio).…”