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

Abstract

We present the results of a detailed study of the interstellar medium (ISM) of MS 1512-cB58 (cB58 for short), an ~ L * Lyman break galaxy at z = 2.7276, based on new spectral observations obtained with the Echelle Spectrograph and Imager on the Keck II telescope at 58 km s -1 resolution. We focus in particular on the chemical abundances and kinematics of the interstellar gas deduced from the analysis of 48 ultraviolet absorption lines, at rest wavelengths between 1134 and 2576 Å, due to elements from H to Zn. Our main findings are as follows. Even at this relatively early epoch, the ISM of this galaxy is already highly enriched in elements released by Type II supernovae; the abundances of O, Mg, Si, P, and S are all ~2/5 of their solar values. In contrast, N and the Fe-peak elements Mn, Fe, and Ni are underabundant by a factor of ~3. Based on current ideas of stellar nucleosynthesis, these results can be understood if most of the metal enrichment in cB58 has taken place within the last ~300 Myr, the timescale for the release of N from intermediate-mass stars. Such a young age is consistent with the UV-optical spectral energy distribution. Thus, cB58 seems to be an example of a galaxy in the process of converting its gas into stars on a few dynamical timescales—quite possibly we are witnessing the formation of a galactic bulge or an elliptical galaxy. The energetic star formation activity has stirred the interstellar medium to high velocities; the strongest absorption lines span a velocity interval of ~1000 km s -1 . The net effect is a bulk outflow of the ISM at a speed of ~255 km s -1 and at a rate that exceeds the star formation rate. It is unclear whether this gas will be lost or retained by the galaxy. On the one hand, the outflow probably has sufficient energy to escape the potential well of cB58, for which we derive a baryonic mass of ~10 10 M ☉ . On the other hand, at least some of the elements manufactured by previous generations of stars must have mixed efficiently with the ambient, neutral, ISM to give the high abundances we measure. We point out that the chemical and kinematic properties of cB58 are markedly different from those of most damped Lyα systems at the same redshift.