The Ultraviolet Spectrum of MS 1512−cB58: An Insight into Lyman‐Break Galaxies

Abstract

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 Imaging Spectrograph (LRIS) on the Keck I telescope and covering the rest frame far-UV from 1150 to 1930 Å. Gravitational lensing by a foreground cluster boosts the flux from MS 1512-cB58 by a factor of ~30 and provides the opportunity for a first quantitative study of the physical properties of star-forming galaxies at high redshift. The spectrum we have recorded is very rich in stellar and interstellar features; from our analysis of them, we deduce the following main results. The ultraviolet spectral properties of MS 1512-cB58 are remarkably similar to those of nearby star-forming galaxies and spectral synthesis models based on libraries of O and B stars can reproduce accurately the fine detail of the integrated stellar spectrum. The P Cygni profiles of C IV and N V are best matched by continuous star 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 data that the metallicity of both the stars and the gas is a few times below solar. Our best estimate, Z cB58 ≈ Z ☉ , is ≈3 times higher than the typical metallicity of damped Lyα systems at the same redshift, which is consistent with the proposal that the galaxies which dominate the H I absorption cross section are generally forming stars at a slower rate than L * Lyman-break galaxies like MS 1512-cB58. The relative velocities of the stellar lines, interstellar absorption, and H II emission indicate the existence of large-scale outflows in the interstellar medium of MS 1512-cB58, with a bulk outward motion of 200 km s -1 and a mass-loss rate of ≈60 M ☉ yr -1 , which is roughly comparable to the star formation rate. Such galactic winds seem to be a common feature of starburst galaxies at all redshifts and may well be the mechanism that self-regulates star formation, distributes metals over large volumes, and allows the escape of ionizing photons into the intergalactic medium. We suggest further observations of MS 1512-cB58 that would provide more precise measurements 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.