Do Galaxy Morphologies Really Affect the Efficiency of Star Formation
  during the Phase of Galaxy Transition?
  Recent simulations predict that the presence of stellar bulge suppress the
efficiency of star formation in early-type galaxies, and this `morphological
quenching' scenario is supported by many observations. In this study, we
discuss the net effect of galaxy morphologies on the star formation efficiency
(SFE) during the phase of galaxy transition, on the basis of our CO($J=1-0$)
observations of 28 local `green-valley' galaxies with the Nobeyama 45m Radio
Telescope. We observed 13 `disk-dominated' and 15 `bulge-dominated'
green-valley galaxies at fixed stellar mass ($M_*$) and star formation rate
(SFR), supplemented by 1 disk- and 6 bulge-dominated galaxies satisfying the
same criteria from the xCOLD~GASS survey. By using a total of 35 green-valley
galaxies, we reveal that the distributions of molecular gas mass, molecular gas
fraction, and SFE of green-valley galaxies do not change with their
morphologies, suggesting little impact of galaxy morphologies on their SFE, and
interestingly this result is also valid for normal star-forming galaxies on the
SF main-sequence selected from the xCOLD~GASS galaxies. On the other hand, we
find that $\sim$20 % of bulge-dominated green-valley galaxies do not show
significant CO emission line, showing high SFEs for their M$_*$ and SFR. These
molecular gas deficient sources identified only in the bulge-dominated
green-valley galaxies may represent an important population during the
quenching phase under the influence of stellar bulge, but our results suggest
that the presence of stellar bulge does not decrease the efficiency of on-going
star formation, in contrast to the prediction of the morphological quenching
scenario.
