The ultraviolet spectrum of MS 1512-cB58: An insight into Lyman-Break galaxies

We present an intermediate-resolution, high signal-to-noise ratio spectrum of the z = 2.7268 galaxy MS 1512-cB58 obtained with the Low Resolution Imag ing Spectrograph (LRIS) on the Keck I telescope and covering the rest frame far-UV from 1150 to 1930 Angstrom. Gravitational lensing by a foreground c luster boosts the flux from MS 1512-cB58 by a factor of similar to 30 and p rovides the opportunity for a first quantitative study of the physical prop erties of star-forming galaxies at high redshift. The spectrum we have reco rded is very rich in stellar and interstellar features; from our analysis o f them, we deduce the following main results. The ultraviolet spectral prop erties of MS 1512-cB58 are remarkably similar to those of nearby star-formi ng galaxies and spectral synthesis models based on libraries of O and B sta rs can reproduce accurately the fine detail of the integrated stellar spect rum. The P Cygni profiles of C IV and N V are best matched by continuous st ar formation with a Salpeter initial mass function (IMF) extending beyond M = 50 M.-we find no evidence for either a flatter IMF (at the high-mass end ) or an IMF deficient in the most massive stars. There are clues in our dat a that the metallicity of both the stars and the gas is a few times below s olar. Our best estimate, Z(cB58) approximate to 1/4 Z., is approximate to 3 times higher than the typical metallicity of damped Ly alpha systems at th e same redshift, which is consistent with the proposal that the galaxies wh ich dominate the H I absorption cross section are generally forming stars a t a slower rate than L* Lyman-break galaxies like MS 1512-cB58. The relativ e velocities of the stellar lines, interstellar absorption, and H II emissi on indicate the existence of large-scale outflows in the interstellar mediu m of MS 1512-cB58, with a bulk outward motion of 200 km s(-1) and a mass-lo ss rate of approximate to 60 M. yr(-1), which is roughly comparable to the star formation rate. Such galactic winds seem to be a common feature of sta rburst galaxies at all redshifts and may well be the mechanism that self-re gulates star formation, distributes metals over large volumes, and allows t he escape of ionizing photons into the intergalactic medium. We suggest fur ther observations of MS 1512-cB58 that would provide more precise measureme nts of element abundances and of detailed physical parameters and highlight the need to identify other examples of gravitationally lensed galaxies for a comprehensive study of star formation at early times.