Rest-Frame Ultraviolet Spectra of z~3 Lyman Break Galaxies

Abstract

We present the results of a systematic study of the rest-frame UV spectroscopic properties of Lyman Break Galaxies (LBGs). The database of almost 1000 LBG spectra proves useful for constructing high S/N composite spectra. The composite spectrum of the entire sample reveals a wealth of features attributable to hot stars, HII regions, dust, and outflowing neutral and ionized gas. By grouping the database according to galaxy parameters such as Lyman-alpha equivalent width, UV spectral slope, and interstellar kinematics, we isolate some of the major trends in LBG spectra which are least compromised by selection effects. We find that LBGs with stronger Lyman-alpha emission have bluer UV continua, weaker low-ionization interstellar absorption lines, smaller kinematic offsets between Lyman-alpha and the interstellar absorption lines, and lower star-formation rates. There is a decoupling between the dependence of low- and high-ionization outflow features on other spectral properties. Most of the above trends can be explained in terms of the properties of the large-scale outflows seen in LBGs. According to this scenario, the appearance of LBG spectra is determined by a combination of the covering fraction of outflowing neutral gas which contains dust, and the range of velocities over which this gas is absorbing. Higher sensitivity and spectral resolution observations are still required for a full understanding of the covering fraction and velocity dispersion of the outflowing neutral gas in LBGs, and its relationship to the escape fraction of Lyman continuum radiation in galaxies at z~3.