Lyman Break Galaxies as collision-driven starbursts

Abstract

Explaining the nature of the Lyman Break Galaxies (LBGs) recently discovered [1,2] at redshift z∼3, is an exciting challenge for the paradigm of hierarchical structure formation. These galaxies are forming stars at a rate comparable to locally rare “starburst” galaxies [3], but are as luminous and numerous as local bright galaxies. In addition, the brightest LBGs have small emission line-widths [4], indicating viral masses of ∼1−5×1010 M⊙, however LBGs exhibit strong clustering, similar to the properties expected of the most massive (∼1012M⊙) dark matter halos [5–8] at this redshift. We explore a possible solution to these apparent paradoxes: that LBGs are a population of collision-driven starburst galaxies which are abundant due to an increased collision rate at high redshift [2,9]. We use high-resolution cosmological N-body simulations and a hierarchical halo finder to estimate the galaxy collision rate as a function of time in a popular cosmological model (ΛCDM). We find that appropriate collisions are frequent enough, and the ensuing bursts are plausibly bright enough, to account for most of the LBGs. Although many of the simulated collisions have relatively small masses (∼1010M⊙), they tend to cluster about large-mass halos. They therefore exhibit strong clustering, similar to that observed [1,11,8] and stronger than that of halos. The collision-induced starburst scenario [2,9] thus appears to explain the key observed properties of the high-z galaxies. This picture can be further tested observationally, and distinguished from other scenarios, by more detailed studies of the evolution of the number density of LBGs with redshift and the dependence of their clustering on scale and environment.