THE ULTRAVIOLET SPECTRUM OF MS 1512−cB58: AN INSIGHT INTO

Abstract

We present an intermediate resolution, high S/N spectrum of the z = 2.7268 galaxy MS 1512−cB58, obtained with 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 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 their analysis we deduce the following main results. The ultraviolet spectral properties of 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 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, ZcB58 ≈ 1/4 Z , is ≈ 3 times higher than the typical metallicity of damped Lyman α systems at the same redshift, 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 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 cB58, with a bulk outward motion of 200 km s−1 and a mass loss rate of ≈ 60 M yr−1, 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 which self-regulates star formation, distributes metals over large volumes and allows the escape of ionizing photons into the IGM. We suggest further observations of cB58 which should 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. Subject headings: cosmology:observations — galaxies:evolution — galaxies:starburst — galaxies:individual (MS 1512−cB58)

10 Figures and Tables

Cite this paper

@inproceedings{GALAXIES1999THEUS, title={THE ULTRAVIOLET SPECTRUM OF MS 1512−cB58: AN INSIGHT INTO}, author={LYMAN BREAK GALAXIES and Max Pettini and C Steidel and Kurt L. Adelberger and Mark E. Dickinson and Mauro Giavalisco}, year={1999} }