Molecular Gas in the Lensed Lyman Break Galaxy cB58

We have used the IRAM Plateau de Bure Interferometer to map CO (3-2) emission from the gravitationally lensed Lyman break galaxy MS 1512-cB58. This is the first detection of a molecular emission line in any Lyman break system; its integrated intensity implies a total molecular gas mass of (6.6) × 109 h M☉, while its width implies a dynamical mass of (1.0) × 1010 i h M☉ (for a flat ΩΛ = 0.7 cosmology). These estimates are in excellent concordance with nearly all parameters of the system measured at other wavelengths and yield a consistent picture of past and future star formation with no obvious discrepancies requiring explanation by differential lensing. In particular, we find that the age and remaining lifetime of the current episode of star formation are likely to be similar, the surface densities of star formation and molecular gas mass are related by a Schmidt law, and the fraction of baryonic mass already converted into stars is sufficient to account for the observed enrichment of the interstellar medium to 0.4 Z☉. Barring substantial gas inflow or a major merger, the stars forming in the current episode will have mass and coevality at z = 0 similar to those of a spiral bulge. Assuming that cB58 is a typical Lyman break galaxy apart from its magnification, its global parameters suggest that the prescriptions for star formation used in some semianalytic models of galaxy evolution require moderate revision, although the general prediction that gas mass fraction should increase with redshift is validated. The length of cB58's star formation episode relative to the time elapsed over the redshift range 2.5 ≤ z ≤ 3.5 strongly argues against scenarios in which observed LBGs cohabit their halos with a large number of similar but "dormant" systems whose starbursts have faded or not yet begun. As a useful empirical result, we find that the observed line/continuum ratio for cB58 is similar to those of high-redshift systems with quite different dust luminosities and nuclear activity levels. Finally, we report the detection of a second source close to the position of the cD elliptical in the z = 0.37 lensing cluster, which may be nonthermal continuum emission from the cD or CO line emission from a hitherto unknown background galaxy at z ~ 1.48 or ~2.73.