Dust continuum emission as a tracer of gas mass in galaxies
  We use a sample of 36 galaxies from the KINGFISH (Herschel IR), HERACLES
(IRAM CO), and THINGS (VLA HI) surveys to study empirical relations between
Herschel infrared (IR) luminosities and the total mass of the interstellar gas
(H2+HI). Such a comparison provides a simple empirical relationship without
introducing the uncertainty of dust model fitting. We find tight correlations,
and provide fits to these relations, between Herschel luminosities and the
total gas mass integrated over entire galaxies, with the tightest, almost
linear, correlation found for the longest wavelength data (SPIRE500). However,
we find that accounting for the gas-phase metallicity (affecting the
dust-to-gas ratio) is crucial when applying these relations to low-mass, and
presumably high-redshift, galaxies. The molecular (H2) gas mass is found to be
better correlated with the peak of the IR emission (e.g. PACS160), driven
mostly by the correlation of stellar mass and mean dust temperature. When
examining these relations as a function of galactocentric radius we find the
same correlations, albeit with a larger scatter, up to a radius of 0.7 r_25
(within which most of the galaxy's baryonic mass resides). However, beyond this
radius the same correlations no longer hold, with the gas mass (predominantly
HI) increasing relative to the infrared emission. The tight relations found for
the bulk of the galaxy's baryonic content suggest that the total gas masses of
disk-like (non-merging) galaxies can be inferred from far-infrared continuum
measurements in situations where only the latter are available, e.g. in ALMA
continuum observations of high-redshift galaxies.
