Molecular gas masses of gamma-ray burst host galaxies
  We obtained CO(2-1) observations of seven GRB hosts with the APEX and IRAM
30m telescopes. We analysed these data together with all other hosts with
previous CO observations. We obtained detections for 3 GRB hosts (980425,
080207, and 111005A) and upper limits for the remaining 4 (031203, 060505,
060814, and 100316D). In our entire sample of 12 CO-observed GRB hosts, 3 are
clearly deficient in molecular gas, even taking into account their metallicity
(980425, 060814, and 080517). Four others are close to the best-fit line for
other star-forming galaxies on the SFR-MH2 plot (051022, 060505, 080207, and
100316D). One host is clearly molecule rich (111005A). Finally, the data for 4
GRB hosts are not deep enough to judge whether they are molecule deficient
(000418, 030329, 031203, and 090423). The median value of the molecular gas
depletion time, MH2/SFR, of GRB hosts is ~0.3 dex below that of other
star-forming galaxies, but this result has low statistical significance. A
Kolmogorov-Smirnov test performed on MH2/SFR shows an only ~2sigma difference
between GRB hosts and other galaxies. This difference can partly be explained
by metallicity effects, since the significance decreases to ~1sigma for MH2/SFR
versus~metallicity. We found that any molecular gas deficiency of GRB hosts has
low statistical significance and that it can be attributed to their lower
metallicities; and thus the sample of GRB hosts has molecular properties that
are consistent with those of other galaxies, and they can be treated as
representative star-forming galaxies. Given the concentration of atomic gas
recently found close to GRB and supernova sites, indicating recent gas inflow,
our results about the weak molecular deficiency imply that such an inflow does
not enhance the SFRs significantly, or that atomic gas converts efficiently
into the molecular phase, which fuels star formation.
