The ALMaQUEST Survey: The molecular gas main sequence and the origin of the star forming main sequence

TL;DR

This study uses the ALMaQUEST survey to reveal that local star formation and gas properties in galaxies are interconnected through three scaling relations, with the resolved star-forming main sequence being a secondary consequence.

Contribution

It identifies a new 'molecular gas main sequence' relation and demonstrates that the resolved star-forming main sequence arises from the combination of two more fundamental relations.

Findings

The three local quantities ( SFR surface density, stellar mass surface density, H2 mass surface density) are strongly correlated.

The discovery of the 'molecular gas main sequence' (MGMS) relation between S2H and \sigsm.

The resolved SFMS is less fundamental and results from the combination of the SK and MGMS relations.

Abstract

The origin of the star forming main sequence ( i.e., the relation between star formation rate and stellar mass, globally or on kpc-scales; hereafter SFMS) remains a hotly debated topic in galaxy evolution. Using the ALMA-MaNGA QUEnching and STar formation (ALMaQUEST) survey, we show that for star forming spaxels in the main sequence galaxies, the three local quantities, star-formation rate surface density (\sigsfr), stellar mass surface density (\sigsm), and the \h2~mass surface density (\sigh2), are strongly correlated with one another and form a 3D linear (in log) relation with dispersion. In addition to the two well known scaling relations, the resolved SFMS (\sigsfr~ vs. \sigsm) and the Schmidt-Kennicutt relation (\sigsfr~ vs. \sigh2; SK relation), there is a third scaling relation between \sigh2~ and \sigsm, which we refer to as the `molecular gas main sequence' (MGMS). The latter indicates that either the local gas mass traces the gravitational potential set by the local stellar mass or both quantities follow the underlying total mass distributions. The scatter of the resolved SFMS ($\sigma \sim 0.25$ dex) is the largest compared to those of the SK and MGMS relations ($\sigma \sim$ 0.2 dex). A Pearson correlation test also indicates that the SK and MGMS relations are more strongly correlated than the resolved SFMS. Our result suggests a scenario in which the resolved SFMS is the least physically fundamental and is the consequence of the combination of the SK and the MGMS relations.