The role of molecular gas in galaxy transition in compact groups
  Compact groups (CGs) provide an environment in which interactions between
galaxies and with the intra-group medium enable and accelerate galaxy
transitions from actively star forming to quiescent. Galaxies in transition
from active to quiescent can be selected, by their infrared (IR) colors, as
canyon or infrared transition zone (IRTZ) galaxies. We used a sample of CG
galaxies with IR data from the Wide Field Infrared Survey Explorer (WISE)
allowing us to calculate the stellar mass and star formation rate (SFR) for
each galaxy. Furthermore, we present new CO(1-0) data for 27 galaxies and
collect data from the literature to calculate the molecular gas mass for a
total sample of 130 galaxies. This data set allows us to study the difference
in the molecular gas fraction (Mmol/Mstar) and star formation efficiency
(SFE=SFR/Mmol) between active, quiescent, and transitioning (i.e., canyon and
IRTZ) galaxies. We find that transitioning galaxies have a mean molecular gas
fraction and a mean SFE that are significantly lower than those of actively
star-forming galaxies. The molecular gas fraction is higher than that of
quiescent galaxies, whereas the SFE is similar. These results indicate that the
transition from actively star-forming to quiescent in CG galaxies goes along
with a loss of molecular gas, possibly due to tidal forces exerted from the
neighboring galaxies or a decrease in the gas density. In addition, the
remaining molecular gas loses its ability to form stars efficiently, possibly
owing to turbulence perturbing the gas, as seen in other, well-studied examples
such as Stephan's Quintet and HCG~57. Thus, the amount and properties of
molecular gas play a crucial role in the environmentally driven transition of
galaxies from actively star forming to quiescent.
