Dust emissivity in resolved spiral galaxies^⋆

¹ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Florence, Italy
e-mail: simone.bianchi@inaf.it
² INAF – Istituto di Radioastronomia, Via P. Gobetti 101, 40129 Bologna, Italy
³ AIM, CEA, CNRS, Université Paris-Saclay, Université Paris Diderot, Sorbonne Paris Cité, 91191 Gif-sur-Yvette, France
⁴ National Observatory of Athens, Institute for Astronomy, Astrophysics, Space Applications and Remote Sensing, Ioannou Metaxa and Vasileos Pavlou, 15236 Athens, Greece
⁵ Sterrenkundig Observatorium, Universiteit Gent, Krijgslaan 281 S9, 9000 Gent, Belgium
⁶ INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica, Via Alfonso Corti 12, 20133 Milan, Italy
⁷ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218-2463, USA
⁸ Université Paris-Saclay, CNRS, Institut d’Astrophysique Spatiale, 91405 Orsay, France
⁹ Department of Physics and Astronomy, N283 ESC, Brigham Young University, Provo, UT 84602, USA

Received: 3 May 2022
Accepted: 10 June 2022

Abstract

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.