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