Galaxy pairs in the SDSS - XIII. The connection between enhanced star
  formation and molecular gas properties in galaxy mergers
  We investigate the connection between star formation and molecular gas
properties in galaxy mergers at low redshift (z$\leq$0.06). The study we
present is based on IRAM 30-m CO(1-0) observations of 11 galaxies with a close
companion selected from the Sloan Digital Sky Survey (SDSS). The pairs have
mass ratios $\leq$4, projected separations r$_{\mathrm{p}} \leq$30 kpc and
velocity separations $\Delta$V$\leq$300 km s$^{-1}$, and have been selected to
exhibit enhanced specific star formation rates (sSFR). We calculate molecular
gas (H$_{2}$) masses, assigning to each galaxy a physically motivated
conversion factor $\alpha_{\mathrm{CO}}$, and we derive molecular gas fractions
and depletion times. We compare these quantities with those of isolated
galaxies from the extended CO Legacy Data base for the GALEX Arecibo SDSS
Survey sample (xCOLDGASS, Saintonge et al. 2017) with gas quantities computed
in an identical way. Ours is the first study which directly compares the gas
properties of galaxy pairs and those of a control sample of normal galaxies
with rigorous control procedures and for which SFR and H$_{2}$ masses have been
estimated using the same method. We find that the galaxy pairs have shorter
depletion times and an average molecular gas fraction enhancement of 0.4 dex
compared to the mass matched control sample drawn from xCOLDGASS. However, the
gas masses (and fractions) in galaxy pairs and their depletion times are
consistent with those of non-mergers whose SFRs are similarly elevated. We
conclude that both external interactions and internal processes may lead to
molecular gas enhancement and decreased depletion times.
