THE ULTRAVIOLET SPECTRUM OF MS l5l2--cB58: AN INSIGHT INTO LYMAN-BREAK GALAXIES

摘要

We present an intermediate-resolution, high signal - t o -noise ratio spectrum of the z = 2. 7268 galaxy MS 15l2--cB58 obtained with the Low Resolution Imaging Spectrograph (LRIS) on the Keck I tele- scope and covering the rest frame far-UV from 1150 to 1930 A. 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, ZcB58 ≈1/4 Z⊙ ≈ 3 times higher than the typical metallicity of damped Lya 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, inter- stellar 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 earlv times.