We use the existing catalog of Damped Lyman–Alpha ( DLA ) systems to place constraints on the amount of evolution in the baryonic content of galaxies and on the value of the cosmological constant .
The density of cold gas at redshifts z = 3 \pm 1 is obtained from the mean HI column density of DLAs per cosmological path length .
This path length per unit redshift is in turn a sensitive function of the vacuum density parameter , \Omega _ { \Lambda } .
We compare the total inferred mass of cold gas at high redshifts to that observed in stars today for cosmologies with \Omega _ { m } + \Omega _ { \Lambda } = 1 , where \Omega _ { m } is the matter density parameter .
We define \eta to be net fraction of the baryonic content of local galaxies which was expelled since z = 3 , and use Bayesian inference to derive confidence regions in the ( \eta, \Omega _ { \Lambda } ) plane .
In all cosmologies we find that \eta < 0.4 with at least 95 \% confidence if < 25 \% of the current stellar population formed before z = 3 .
The most likely value of \eta is negative , implying a net increase by several tens of percent in the baryonic mass of galaxies since z = 3 \pm 1 .
On the other hand , recent observations of high metal abundances in the intracluster medium of rich clusters ( Loewenstein & Mushotzky 1996 ) require that metal–rich gas be expelled from galaxies in an amount approximately equal to the current mass in stars .
Based on our results and the low metallicity observed in DLAs at z \ga 2 , we infer that more than half of the baryonic mass processed through galaxies must have been assembled and partly expelled from galaxies after z = 2 .
We expect our constraints to improve considerably as the size of the DLA sample will increase with the forthcoming Sloan Digital Sky Survey .