COLD GASS, an IRAM Legacy Survey of Molecular Gas in Massive Galaxies:
  II. The non-universality of the Molecular Gas Depletion Timescale
  We study the relation between molecular gas and star formation in a
volume-limited sample of 222 galaxies from the COLD GASS survey, with
measurements of the CO(1-0) line from the IRAM 30m telescope. The galaxies are
at redshifts 0.025<z<0.05 and have stellar masses in the range
10.0<log(M*/Msun)<11.5. The IRAM measurements are complemented by deep Arecibo
HI observations and homogeneous SDSS and GALEX photometry. A reference sample
that includes both UV and far-IR data is used to calibrate our estimates of
star formation rates from the seven optical/UV bands. The mean molecular gas
depletion timescale, tdep(H2), for all the galaxies in our sample is 1 Gyr,
however tdep(H2) increases by a factor of 6 from a value of ~0.5 Gyr for
galaxies with stellar masses of 10^10 Msun to ~3 Gyr for galaxies with masses
of a few times 10^11 Msun. In contrast, the atomic gas depletion timescale
remains contant at a value of around 3 Gyr. This implies that in high mass
galaxies, molecular and atomic gas depletion timescales are comparable, but in
low mass galaxies, molecular gas is being consumed much more quickly than
atomic gas. The strongest dependences of tdep(H2) are on the stellar mass of
the galaxy (parameterized as log tdep(H2)= (0.36+/-0.07)(log M* -
10.70)+(9.03+/-0.99)), and on the specific star formation rate. A single
tdep(H2) versus sSFR relation is able to fit both "normal" star-forming
galaxies in our COLD GASS sample, as well as more extreme starburst galaxies
(LIRGs and ULIRGs), which have tdep(H2) < 10^8 yr. Normal galaxies at z=1-2 are
displaced with respect to the local galaxy population in the tdep(H2) versus
sSFR plane and have molecular gas depletion times that are a factor of 3-5
times longer at a given value of sSFR due to their significantly larger gas
fractions.
