The structure and clustering of Lyman-break galaxies

The number density and clustering properties of Lyman-break galaxies (LBGs) are consistent with them being the central galaxies of the most massive dark haloes present at z similar to 3. This conclusion holds in all currently popular hierarchical models for structure formation, and is almost independent of the global cosmological parameters. We examine whether the sizes, luminosities, kinematics and star formation rates of LBGs are also consistent with this identification. Simple formation models tuned to give good fits to low-redshift galaxies can predict the distribution of these quantities in the LEG population. The LBGs should be small (with typical half-light radii of 0.6-2 h(-1) kpc), should inhabit haloes of moderately high circular velocity (180-290 km s(-1)) but have low stellar velocity dispersions (70-120 km s(-1)) and should have substantial star formation rates (15-100 M-. yr(-1)). The numbers here refer to the predicted median values in the LEG sample of Adelberger et al.; the first number assumes an Omega(0) = 1 universe and the second number a flat universe with Omega(0) = 0.3. For either cosmology these predictions are consistent with the current (rather limited) observational data. Following the work of Kennicutt, we assume stars to form more rapidly in gas of higher surface density. This predicts that LEG samples should preferentially contain objects with low angular momentum, and therefore small size, for their mass. In contrast, samples of damped Ly alpha systems (DLSs) should be biased towards objects with large angular momentum. Bright LBGs and DLSs may therefore form distinct populations, with very different sizes and star formation rates, LBGs being smaller and more metal-rich than DLSs of similar mass and redshift.