2018ApJ...867...92S

We present an analysis of the molecular gas properties, based on CO (2-1) emission, of 12 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 more 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 α_CO (i.e., the ratio of molecular gas mass to L_CO1–0^′^), 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. However, the balance between an increase in star formation efficiency and gas fraction depends on the adopted value of α_CO as discussed.