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

We present an intermediate resolution, high S/N spectrum of the z = 2.7268galaxy MS 1512-cB58, obtained with LRIS on the Keck I telescope and coveringthe rest frame far-UV from 1150 to 1930 A. Gravitational lensing by aforeground cluster boosts the flux from cB58 by a factor of about 30 andprovides the opportunity for a first quantitative study of the physicalproperties of star forming galaxies at high redshift. Our main results are asfollows. The ultraviolet spectral properties of cB58 are remarkably similar tothose of nearby star forming galaxies. The P-Cygni profiles of CIV and NV arebest matched by continuous star formation with a Salpeter IMF extending beyond50 solar masses---we find no evidence for either a flatter IMF (at the highmass end), or an IMF deficient in the most massive stars. There are clues inour data that the metallicity of both the stars and the gas is a few timesbelow solar. Our best estimate, approximately equal to 1/4 solar, is 3 timeshigher than the typical metallicity of damped Lyman alpha systems at the sameredshift, consistent with the proposal that the galaxies which dominate the H Iabsorption 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 oflarge-scale outflows in the interstellar medium of cB58, with a bulk outwardmotion of 200 km/s and a mass loss rate of approximately 60 solar masses peryear, roughly comparable to the star formation rate. Such galactic winds maywell be the mechanism which self-regulates star formation, distributes metalsover large volumes and allows the escape of ionizing photons into the IGM.Comment: 26 pages, LaTex, 4 Postscript Figures. Accepted for Publication in The Astrophysical Journa