Abstract
To extend the molecular gas measurements to typical star-forming galaxies
(SFGs) with SFR < 40 Msun yr^-1 and M* < 2.5x10^10 Msun at z~1.5-3, we have
observed CO emission with the IRAM Plateau de Bure Interferometer and 30m
telescope for five strongly-lensed galaxies selected from the Herschel Lensing
Survey. These observations are combined with a compilation of CO measurements
from the literature. From this, we infer the luminosity correction factors r2,1
= 0.8 and r3,1 = 0.65 of the CO rotational levels J=2 and J=3, respectively,
valid for SFGs and sub-mm galaxies at z>1. The combined sample of CO-detected
SFGs at z>1 shows a large spread in star formation efficiency (SFE), such that
SFE extend beyond the low values of local spirals and overlap the distribution
of z>1 sub-mm galaxies. We find that the spread in SFE (or equivalently in
molecular gas depletion timescale) is due to primarily the specific star
formation rate, but also stellar mass and redshift. Correlations of SFE with
the offset from the main-sequence and the compactness of the starburst are less
clear. The increase of the molecular gas depletion timescale with M* now
revealed by low M* SFGs at z>1 is opposed to the admitted constant molecular
gas depletion timescale and the linear Kennicutt-Schmidt relation. We confirm
an increase of the molecular gas fraction (fgas) from z~0.2 to z~1.2, followed
by a quasi non-evolution toward higher redshifts. At each redshift fgas shows a
large dispersion due to the dependence of fgas on M*, producing a gradient of
increasing fgas with decreasing M*. We provide the first measure of fgas of z>1
SFGs at the low-M* end (10^9.4 < M*/Msun < 10^9.9), reaching a mean fgas =
0.67+/-0.20 which shows a clear fgas upturn. Finally, we find evidence for a
non-universal dust-to-gas ratio among high-redshift SFGs and sub-mm galaxies,
local spirals and ULIRGs with near-solar metallicities.
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