2022A&A...664A.187B

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.