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 ^ { +5.8 } _ { -4.3 } \times 10 ^ { 9 } h _ { 0.7 } ^ { -2 } M _ { \odot } , while its width implies a dynamical mass of 1.0 ^ { +0.6 } _ { -0.4 } \times 10 ^ { 10 } { csc } ^ { 2 } i h _ { 0.7 } ^ { -1 } M _ { \odot } ( for a flat \Omega _ { \Lambda } = 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 _ { \odot } .
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 cB58 is a typical Lyman break galaxy apart from its magnification , its global parameters suggest that the prescriptions for star formation used in some semi–analytic 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 \leq z \leq 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 \sim 1.48 or z \sim 2.73 .