Dust emissivity in resolved spiral galaxies

Abstract

Context. The far-infrared (FIR) and sub-millimeter (submm) emissivity, ϵ<SUB>ν</SUB>, of the Milky Way (MW) cirrus is an important benchmark for dust grain models. Dust masses in other galaxies are generally derived from the FIR/submm using the emission properties of these MW-calibrated models. <BR /> Aims: We seek to derive the FIR/submm ϵ<SUB>ν</SUB> in nine nearby spiral galaxies to check its compatibility with MW cirrus measurements. <BR /> Methods: We obtained values of ϵ<SUB>ν</SUB> at 70-500 μm, using maps of dust emission from the Herschel satellite and of gas surface density from the THINGS and HERACLES surveys on a scale generally corresponding to 440 pc. We studied the variation of ϵ<SUB>ν</SUB> with the surface brightness ratio I<SUB>ν</SUB>(250 μm)/I<SUB>ν</SUB>(500 μm), a proxy for the intensity of the interstellar radiation field heating the dust. <BR /> Results: We find that the average value of ϵ<SUB>ν</SUB> agrees with MW estimates for pixels sharing the same color as the cirrus, namely, for I<SUB>ν</SUB>(250 μm)/I<SUB>ν</SUB>(500 μm)=4.5. For I<SUB>ν</SUB>(250 μm)/I<SUB>ν</SUB>(500 μm)> 5, the measured emissivity is instead up to a factor ∼2 lower than predicted from MW dust models heated by stronger radiation fields. Regions with higher I<SUB>ν</SUB>(250 μm)/I<SUB>ν</SUB>(500 μm) are preferentially closer to the galactic center and have a higher overall (stellar+gas) surface density and molecular fraction. The results do not depend strongly on the adopted CO-to-molecular conversion factor and do not appear to be affected by the mixing of heating conditions. <BR /> Conclusions: Our results confirm the validity of MW dust models at low density, but are at odds with predictions for grain evolution in higher density environments. If the lower-than-expected ϵ<SUB>ν</SUB> at high I<SUB>ν</SUB>(250 μm)/I<SUB>ν</SUB>(500 μm) is the result of intrinsic variations in the dust properties, it would imply an underestimation of the dust mass surface density of up to a factor ∼2 when using current dust models. <P />This work makes use of the DustPedia database. DustPedia is a collaborative focused research project supported by the European Union under the Seventh Framework Programme (2007-2013) call (proposal no. 606824, P.I. J. I. Davies). The database is publicly available at <A href="http://dustpedia.astro.noa.gr">http://dustpedia.astro.noa.gr</A>.