Dust emissivity in resolved spiral galaxies

Context. The far-infrared (FIR) and sub-millimeter (submm) emissivity, ϵ_ν, 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.
Aims: We seek to derive the FIR/submm ϵ_ν in nine nearby spiral galaxies to check its compatibility with MW cirrus measurements.
Methods: We obtained values of ϵ_ν 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_ν with the surface brightness ratio I_ν(250 μm)/I_ν(500 μm), a proxy for the intensity of the interstellar radiation field heating the dust.
Results: We find that the average value of ϵ_ν agrees with MW estimates for pixels sharing the same color as the cirrus, namely, for I_ν(250 μm)/I_ν(500 μm)=4.5. For I_ν(250 μm)/I_ν(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_ν(250 μm)/I_ν(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.
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_ν at high I_ν(250 μm)/I_ν(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.

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 http://dustpedia.astro.noa.gr.