In this paper, we have examined the star formation and chemical enrichment in Lyman break galaxies, assuming them to be the central galaxies of massive haloes at and using simple chemical evolution models. We found that gas cooling in dark haloes provides a natural process which regulates the amount of star forming gas. The predicted star formation rates and effective radii are consistent with observations. The metallicity of the gas associated with an LBG is roughly equal to the chemical yield, or the order of for a Salpeter IMF. Because of the relatively long star-formation time, the colours of these galaxies should be redder than that of short starbursts. It is not clear whether this prediction is consistent with current (rather) limited observations, because the interpretation of the observational data depends strongly on the adopted dust reddening. Stringent constraint can be obtained when full spectral information of the LBG population is carefully analyzed.
The model predicts a marked radial metallicity gradient in an LBG, with the gas in the outer region having lower metallicity. As a result, the metallicities for the damped Lyman-alpha absorption systems expected from the LBG population are lower than those for the LBGs themselves, although high metallicity is expected for a small number of sightlines going through the central regions of an LBG. At the same time, our modeled contribution to the total metal is roughly consistent with that obtained from the observed cosmic star formation history, i.e., there might not exist so-called "missing metal" problem although there could be more than half of the metals in the hot phase. Finally, a prediction of our model is that LBG haloes are filled with hot gas. As a result, these galaxies may have a non-negligible contribution to the soft X-ray background. The contribution of LBGs to the ionizing UV background is found to be small.
There are two basic assumptions in our work. One is that the LBG population is one-to-one associated with the most massive haloes which are generated from the PS formalism, as done by MMWb; another is that the timescale of star formation for LBG population is assumed to be of the order of 1Gyr, which is suggested by Steidel et al. (1999a,b, 1995). However, Baugh et al. (1999) recently argue that the prediction of the clustering properties of LBGs based on this first simple assumption will be a discrepancy with the results of more detailed semi-analytic models. Still, the second will lead to difficulty in reproducing the redshift evolution of bright galaxies (Kolatt et al. 1999). More detailed modelling done by Somerville (1997) suggest the collisional starbursts could be expected to be an important effect in understanding the LBGs. So, further observations are required to investigate the intrinsic properties of LBGs.
© European Southern Observatory (ESO) 2000
Online publication: February 25, 2000