Cross-calibration of CO- vs dust-based gas masses and assessment of the
  dynamical mass budget in Herschel-SDSS Stripe82 galaxies
  We present a cross-calibration of CO- and dust-based molecular gas masses at
$z \leqslant 0.2$. Our results are based on a survey with the IRAM 30-m
telescope collecting CO(1-0) measurements of 78 massive ($\log M_{\star} /
M_{\odot} > 10$) galaxies with known gas-phase metallicities, and with IR
photometric coverage from WISE (22 $\mu$m ) and Herschel SPIRE (250, 350, 500
$\mu$m). We find a tight relation ($\sim 0.17$ dex scatter) between the gas
masses inferred from CO and dust continuum emission, with a minor systematic
offset of 0.05 dex. The two methods can be brought into agreement by applying a
metallicity-dependent adjustment factor ($\sim 0.13$ dex scatter). We
illustrate that the observed offset is consistent with a scenario in which dust
traces not only molecular gas, but also part of the ${\rm H \small I}$
reservoir, residing in the ${\rm H_2}$-dominated region of the galaxy.
Observations of the CO(2-1) to CO(1-0) line ratio for two thirds of the sample
indicate a narrow range in excitation properties, with a median ratio of
luminosities $ \left\langle R_{21} \right\rangle \sim 0.64 $. Finally, we find
dynamical mass constraints from spectral line profile fitting to agree well
with the anticipated mass budget enclosed within an effective radius, once all
mass components (stars, gas and dark matter) are accounted for.
