New Observations of the Interstellar Medium in the Lyman Break Galaxy MS 1512−cB58

We present the results of a detailed study of the interstellar medium of MS1512-cB58, an L* Lyman break galaxy at z = 2.7276, based on new spectralobservations obtained with the Echelle Spectrograph and Imager on the Keck IItelescope at 58 km/s resolution. We focus in particular on the chemicalabundances and kinematics of the interstellar gas and our main findings are asfollows. Even at this relatively early epoch, the ISM of this galaxy is alreadyhighly enriched in elements released by Type II supernovae; the abundances ofO, Mg, Si, P, and S are all about 2/5 of their solar values. In contrast, N andthe Fe-peak elements Mn, Fe, and Ni are underabundant by a factor of about 3.Based on current ideas of stellar nucleosynthesis, these results can beunderstood if most of the metal enrichment in cB58 has taken place within thelast 300 million years, the timescale for the release of N from intermediatemass stars. cB58 appears to be an example of a galaxy in the process ofconverting its gas into stars on a few dynamical timescales; quite possibly weare witnessing the formation of a galactic bulge or an elliptical galaxy. Theenergetic star formation activity has stirred the interstellar medium to highvelocities of up to 1000 km/s. The net effect is a bulk outflow of the ISM at aspeed of 255 km/s and at a rate which exceeds the star formation rate. It isunclear whether this gas will be lost or retained by the galaxy. We point outthat the chemical and kinematic properties of cB58 are markedly different fromthose of most damped Lyman alpha systems at the same redshift.Comment: 38 pages, LaTeX, 9 Postscript Figures. Accepted for publication in the Astrophysical Journal. Sections 3.3 and 5.3 expanded, and two additional figures included, following referee's repor