Dust emissivity and absorption cross section in DustPedia late-type galaxies

TL;DR

This study compares dust emission properties in DustPedia galaxies with Milky Way cirrus to assess the validity of using Galactic dust properties for external galaxies, revealing similarities and variations linked to galaxy type and metallicity.

Contribution

It provides a comparative analysis of dust emissivity and absorption cross sections in external galaxies versus the Milky Way, highlighting potential underestimations of dust mass when using Galactic standards.

Findings

Dust emissivity at 250 micrometres is similar in galaxies and the Milky Way for certain color ratios.

Dust emission properties vary with galaxy type, metallicity, and molecular gas fraction.

Using Milky Way dust properties may underestimate dust masses in external galaxies by up to a factor of two.

Abstract

Aims: We compare the far-infrared to sub-millimetre dust emission properties measured in high Galactic latitude cirrus with those determined in a sample of 204 late-type DustPedia galaxies. The aim is to verify if it is appropriate to use Milky Way dust properties to derive dust masses in external galaxies. Methods: We used Herschel observations and atomic and molecular gas masses to estimate the disc-averaged dust emissivity at 250 micrometres, and from this, the absorption cross section per H atom and per dust mass. The emissivity requires one assumption, which is the CO-to-H_2 conversion factor, and the dust temperature is additionally required for the absorption cross section per H atom; yet another constraint on the dust-to-hydrogen ratio D/H, depending on metallicity, is required for the absorption cross section dust mass. Results: We find epsilon(250) = 0.82 +/- 0.07 MJy sr^-1 (1E20 H cm^-2)^-1 for galaxies with 4 < F(250)/F(500) < 5. This depends only weakly on the adopted CO-to-H_2 conversion factor. The value is almost the same as that for the Milky Way at the same colour ratio. Instead, for F(250)/F(500) > 6, epsilon(250) is lower than predicted by its dependence on the heating conditions. The reduction suggests a variation in dust emission properties for spirals of earlier type, higher metallicity, and with a higher fraction of molecular gas. When the standard emission properties of Galactic cirrus are used for these galaxies, their dust masses might be underestimated by up to a factor of two. Values for the absorption cross sections at the Milky Way metallicity are also close to those of the cirrus. Mild trends of the absorption cross sections with metallicity are found, although the results depend on the assumptions made.