The molecular gas content and fuel efficiency of starbursts at z ~ 1.6 with ALMA

TL;DR

This study analyzes molecular gas in starburst galaxies at z~1.6 using ALMA, revealing increased star formation efficiency in starbursts compared to main sequence galaxies, with implications for galaxy evolution.

Contribution

It provides the first high-z measurement of lower L'_CO/L_IR ratios in starbursts and links star formation efficiency to galaxy position relative to the main sequence.

Findings

Starbursts have higher star formation efficiency than main sequence galaxies.

Gas fractions are similar between starbursts and main sequence galaxies.

Star formation efficiency increases continuously from main sequence to starburst regimes.

Abstract

We present an analysis of the molecular gas properties, based on CO(2 - 1) emission, of twelve starburst galaxies at z~1.6 selected by having a boost (>~4x) in their star formation rate (SFR) above the average star-forming galaxy at an equivalent stellar mass. ALMA observations are acquired of six additional galaxies than previously reported through our effort. As a result of the larger statistical sample, we significantly detect, for the first time at high-z, a systematically lower L'_CO/L_IR ratio in galaxies lying above the star-forming `main sequence' (MS). Based on an estimate of alpha_CO (i.e., the ratio of molecular gas mass to L'_CO(1-0)), we convert the observational quantities (e.g., L'_CO/L_IR) to physical units (M_gas/SFR) that represent the gas depletion time or its inverse, the star formation efficiency. We interpret the results as indicative of the star formation efficiency increasing in a continuous fashion from the MS to the starburst regime, whereas the gas fractions remain comparable to those of MS galaxies. Although, the balance between an increase in star-formation efficiency or gas fraction depends on the adopted value of alpha_CO as discussed.