The ALMaQUEST Survey: III. Scatter in the resolved star forming main sequence is primarily due to variations in star formation efficiency

TL;DR

This study reveals that on kiloparsec scales, the variation in star formation rates around the main sequence is mainly caused by changes in star formation efficiency, while the absolute rate depends on molecular gas availability.

Contribution

It demonstrates that star formation efficiency variations are the primary driver of scatter in the resolved star forming main sequence, using a large ALMaQUEST galaxy sample.

Findings

Star formation rate surface density is mainly regulated by molecular gas surface density.

Scatter in star formation rates is primarily driven by variations in star formation efficiency.

Gas fraction plays a secondary role in star formation rate variations.

Abstract

Using a sample of 11,478 spaxels in 34 galaxies with molecular gas, star formation and stellar maps taken from the ALMA-MaNGA QUEnching and STar formation (ALMaQUEST) survey, we investigate the parameters that correlate with variations in star formation rates on kpc scales. We use a combination of correlation statistics and an artificial neural network to quantify the parameters that drive both the absolute star formation rate surface density (Sigma_SFR), as well as its scatter around the resolved star forming main sequence (Delta Sigma_SFR). We find that Sigma_SFR is primarily regulated by molecular gas surface density (Sigma_H2) with a secondary dependence on stellar mass surface density (Sigma_*), as expected from an `extended Kennicutt-Schmidt relation'. However, Delta Sigma_SFR is driven primarily by changes in star formation efficiency (SFE), with variations in gas fraction playing a secondary role. Taken together, our results demonstrate that whilst the absolute rate of star formation is primarily set by the amount of molecular gas, the variation of star formation rate above and below the resolved star forming main sequence (on kpc scales) is primarily due to changes in SFE.