Lyman break galaxies as collisional starbursts

The nature of the galaxies recently discovered at redshift [1,2] z greater than or similar to 3, when the Universe was only about two billion years old, is an intriguing puzzle. They are forming stars at a high rate, comparable to local "starburst" galaxies [3], but while the latter are relatively rare, the high-z galaxies are as numerous as local normal galaxies. Since many local starbursts are triggered by galaxy collisions, a much higher collision rate in the past could provide a simple explanation for the high abundance of high-z galaxies [2,4]. Here 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 (Lambda CDM), We find that bright collisional starbursts are indeed frequent enough to account for most of the high-z galaxies. Moreover, many of the objects are of relatively small mass, but they cluster about large-mass halos. They therefore exhibit strong clustering, similar to that observed [5-7] and stronger than that of the relevant massive halos. The collisional starburst scenario [2,4] thus appears consistent with several key properties of high-z galaxies.