Do Galaxy Morphologies Really Affect the Efficiency of Star Formation During the Phase of Galaxy Transition?

Abstract

Recent simulations predict that the presence of the stellar bulge suppress the efficiency of star formation (SF) 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 45 m 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 ∼20% of the 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 that are identified only in the bulge-dominated green valley galaxies may represent an important population during the quenching phase under the influence of the stellar bulge, but our results suggest that the presence of the stellar bulge does not decrease the efficiency of ongoing SF, in contrast to the prediction of the morphological quenching scenario.