Young Galaxies: What Turns Them On?

Lyman break galaxies (LBGs) at $z \sim 3$ exhibit number densities andclustering similar to local $L_*$ galaxies; however, their star formation rates(SFRs) are much higher. We explore the scenario in which LBGs are starburstgalaxies triggered by collisions, and thus provide an explanation for these keyproperties. The relative paucity of starburst galaxies at low redshift can beattributed to a much higher collision rate in the past. We use high-resolutioncosmological $N$-body simulations and a hierarchical halo finder to estimatethe galaxy collision rate as a function of time in the popular $\Lambda$CDMcosmological model. We find that bright collisional starbursts are frequentenough to account for most of the high-$z$ ($2.5-4.5$) LBGs. Moreover, many ofthe objects are of relatively small mass, but they cluster about large-masshalos. They therefore exhibit strong clustering, similar to that observed andstronger than that of the relevant massive halos.Comment: 7 pages, three figures (one jpg), submitted to the Astrophysical Journal Letter