  What type of objects are being detected as $z\sim 3$ "Lyman break galaxies"?
Are they predominantly the most massive galaxies at that epoch, or are many of
them smaller galaxies undergoing a short-lived burst of merger-induced star
formation? We attempt to address this question using high-resolution
cosmological hydrodynamic simulations including star formation and feedback.
Our $\Lambda$CDM simulation, together with Bruzual-Charlot population synthesis
models, reproduces the observed number density and luminosity function of Lyman
break galaxies when dust is incorporated. The inclusion of dust is crucial for
this agreement. In our simulation, these galaxies are predominantly the most
massive objects at this epoch, and have a significant population of older
stars. Nevertheless, it is possible that our simulations lack the resolution
and requisite physics to produce starbursts, despite having a physical
resolution of $\la 700$ pc at z=3. Thus we cannot rule out merger-induced
starburst galaxies also contributing to the observed population of
high-redshift objects.
