A new empirical method to estimate the molecular gas mass in galaxies
  We find a tight correlation between the dust extinction, traced by the Balmer
Decrement (BD$=$H$\alpha$/H$\beta$), the CO(1-0) line luminosity (L$_{CO}$) and
total molecular gas mass (M$_{H2}$) in a sample of $222$ local star-forming
galaxies drawn from the xCOLD GASS survey. As expected, the galaxy disk
inclination affects the correlation by inducing a saturation of the Balmer
decrement on highly inclined galaxies. Once this effect is taken into account,
L$_{CO}$ and M$_{H2}$ can be expressed as a function of BD with a scatter of
$\sim 0.3$ dex. We do not find any dependence on galaxy size, mass, morphology,
star formation activity, and gas metallicity. The correlation disappears if the
atomic gas phase is considered. This is likely due to the fact that the region
traced by the BD, the stellar disk, is much smaller than the HI disk.
