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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.