Molecular and atomic gas along and across the main sequence of star-forming galaxies

TL;DR

This study analyzes how atomic and molecular gas fractions vary across the star-forming galaxy main sequence, revealing that cold gas reservoirs primarily drive galaxy position in the SFR-M* plane.

Contribution

It provides the first comprehensive analysis of atomic and molecular gas variations along and across the galaxy main sequence using spectral stacking techniques.

Findings

Gas fractions decrease with increasing stellar mass.

Variations in star formation efficiency are minimal among massive main sequence galaxies.

The flattening of the main sequence is mainly due to decreasing cold gas reservoirs.

Abstract

We use spectra from the ALFALFA, GASS and COLD GASS surveys to quantify variations in the mean atomic and molecular gas mass fractions throughout the SFR-M* plane and along the main sequence (MS) of star-forming galaxies. Although galaxies well below the MS tend to be undetected in the Arecibo and IRAM observations, reliable mean atomic and molecular gas fractions can be obtained through a spectral stacking technique. We find that the position of galaxies in the SFR-M* plane can be explained mostly by their global cold gas reservoirs as observed in the HI line, with in addition systematic variations in the molecular-to-atomic ratio and star formation efficiency. When looking at galaxies within +/-0.4 dex of the MS, we find that as stellar mass increases, both atomic and molecular gas mass fractions decrease, stellar bulges become more prominent, and the mean stellar ages increase. Both star formation efficiency and molecular-to-atomic ratios vary little for massive main sequence galaxies, indicating that the flattening of the MS is due to the global decrease of the cold gas reservoirs of galaxies rather than to bottlenecks in the process of converting cold atomic gas to stars.