The Fine Line Between Normal and Starburst Galaxies

TL;DR

This study investigates the transition between normal and starburst galaxies by analyzing gas content and star formation efficiency in intermediate galaxies using ALMA data, revealing that deviations from the main sequence are mainly due to increased gas rather than efficiency.

Contribution

It provides new insights into the properties of intermediate galaxies, highlighting the dominant role of gas content over star formation efficiency in their deviation from the main sequence.

Findings

Gas content primarily drives deviation from the main sequence.

Star formation efficiency is not significantly increased in intermediate galaxies.

Gas properties differ between normal and starburst galaxies.

Abstract

Recent literature suggests that there are two modes through which galaxies grow their stellar mass - a normal mode characterized by quasi-steady star formation, and a highly efficient starburst mode possibly triggered by stochastic events such as galaxy mergers. While these differences are established for extreme cases, the population of galaxies in-between these two regimes is poorly studied and it is not clear where the transition between these two modes of star formation occurs. We utilize ALMA observations of the CO J=3-2 line luminosity in a sample of 20 infrared luminous galaxies that lie in the intermediate range between normal and starburst galaxies at z ~ 0.25-0.6 in the COSMOS field to examine the gas content and star formation efficiency of these galaxies. We compare these quantities to the galaxies' deviation from the well-studied "main sequence" correlation between star formation rate and stellar mass (MS) and find that at log($SFR/SFR_{MS}$) < 0.6, a galaxy's distance to the main sequence is mostly driven by increased gas content, and not a more efficient star formation process.