Total Molecular Gas Masses of z~3 Lyman-Break Galaxies: CO(1-0) Emission
  in MS1512-cB58 and the Cosmic Eye
  We report the detection of CO(1-0) emission toward the lensed L*_UV
Lyman-break galaxies (LBGs) MS1512-cB58 (z=2.73) and the Cosmic Eye (z=3.07),
using the Expanded Very Large Array. The strength of the CO line emission
reveals molecular gas reservoirs with masses of (4.6+/-1.1) x 10^8 (mu_L/32)^-1
(alpha_CO/0.8) Msun and (9.3+/-1.6) x 10^8 (mu_L/28)^-1 (alpha_CO/0.8) Msun,
respectively. These observations suggest by ~30%-40% larger gas reservoirs than
estimated previously based on CO(3-2) observations due to subthermal excitation
of the J=3 line. These observations also suggest gas mass fractions of
0.46+/-0.17 and 0.16+/-0.06. The CO(1-0) emission in the Cosmic Eye is slightly
resolved on scales of 4.5"+/-1.5", consistent with previous studies of nebular
emission lines. This suggests that the molecular gas is associated with the
most intensely star-forming regions seen in the ultraviolet (UV). We do not
resolve the CO(1-0) emission in cB58 at ~2" resolution, but find that the
CO(1-0) emission is also consistent with the position of the UV-brightest
emission peak. The gas masses, gas fractions, moderate CO line excitation, and
star formation efficiencies in these galaxies are consistent with what is found
in nearby luminous infrared galaxies. These observations thus currently
represent the best constraints on the molecular gas content of `ordinary'
(i.e., ~L*_UV) z~3 star-forming galaxies. Despite comparable star formation
rates, the gas properties of these young LBGs seem to be different from the
recently identified optical/infrared-selected high-z massive, gas-rich
star-forming galaxies, which are more gas-rich and massive, but have lower star
formation efficiencies, and presumably trace a different galaxy population.
