The Bluedisk Survey: molecular gas distribution and scaling relations in
  the context of galaxy evolution
  One of the key goals of the Bluedisk survey is to characterize the impact of
gas accretion in disc galaxies in the context of galaxy evolution. It contains
50 disc galaxies in the stellar mass range 10^10-10^11 Msun, of which half are
bluer and more HI-rich galaxies than their HI-normal (control) counterparts. In
this paper, we investigate how ongoing disc growth affects the molecular gas
distribution and the star-formation efficiency in these galaxies. We present
12CO observations from the IRAM 30-m telescope in 26 galaxies of the Bluedisk
survey. We compare the amount and spatial distribution of the molecular gas to
key quantities such as atomic gas, stellar mass and surface density,
star-formation rate and metallicity. We analyse the star-formation rate per
unit gas (SFR/HI and SFR/H2) and relate all those parameters to general galaxy
properties (HI-rich/control disc, morphology, etc.). We find that the HI-rich
galaxies have similar H2 masses as the control galaxies. In their centres,
HI-rich galaxies have lower H2/HI ratios and marginally shorter molecular gas
depletion times. However, the main differences between the two samples occur in
the outer parts of the discs, with the HI-rich galaxies having slightly smaller
CO discs (relative to the optical radius R25) and steeper CO and metallicity
gradients than the control galaxies. The ongoing accretion of HI at large radii
has thus not led to an appreciable growth of the CO discs in our sample. Based
on depletion times, we estimate that this gas will contribute to star formation
on time-scales of at least 5 Gyr.
