Young galaxies: What turns them on?

Lyman break galaxies (LBGs) at z similar to 3 exhibit number densities and clustering similar to local L-* galaxies; however, their star formation rat es are much higher. We explore the scenario in which LBGs are starburst gal axies triggered by collisions and thus provide an explanation for these key properties. The relative paucity of starburst galaxies at low redshift can be attributed to a much higher collision rate in the past. We use high-res olution cosmological N-body simulations and a hierarchical halo finder to e stimate the galaxy collision rate as a function of time in the popular Lamb da CDM cosmological model. We find that bright collisional starbursts are f requent enough to account for most of the high-z (2.5-4.5) LBGs. Moreover, many of the objects are of relatively small mass, but they cluster about la rge-mass halos. They therefore exhibit strong clustering, similar to that o bserved and stronger than that of the relevant massive halos.