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

## TL;DR

This study investigates whether galaxy morphology influences star formation efficiency during galaxy transition and finds little overall impact, challenging the morphological quenching theory, with some bulge-dominated galaxies showing high efficiencies despite low molecular gas.

## Contribution

The paper provides observational evidence that galaxy morphology has minimal effect on star formation efficiency during the transition phase, contradicting the morphological quenching hypothesis.

## Key findings

- Molecular gas mass and SFE do not significantly vary with morphology.
- Approximately 20% of bulge-dominated green-valley galaxies lack CO emission.
- Some bulge-dominated galaxies exhibit high star formation efficiencies despite low molecular gas.

## Abstract

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.