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