NEW HIGH-REDSHIFT DAMPED LYMAN-ALPHA ABSORPTION SYSTEMS AND THE REDSHIFT EVOLUTION OF DAMPED ABSORBERS

We have discovered two high-redshift (z(abs) = 3.388 and 2.908) damped Lyalpha absorption systems and a third possible damped system (z(abs) = 3.080) while obtaining spectra of optical counterparts from a sample of flat spectrum radio sources. These three systems all have higher redshifts than any previously known damped Lyalpha absorption systems in the spectra of radio-loud quasars, and they are among the highest redshift damped Lyalpha systems known. It is unlikely that we would have found as many as two or three damped Lyalpha absorption systems among our sample unless there is a significant increase in the area-weighted comoving density of damped systems for z greater than or similar to 3. Using data from this paper combined with previous surveys for damped Lyalpha absorbers, we derive the area-weighted comoving density of damped Lyalpha absorbers and find that it increases sharply at redshifts above z = 3. A constant comoving density can be ruled out at the 95% confidence level if q0 = 0 and the 97% confidence level if q0 = 0.5. This is the first good statistical evidence for evolution in the number (or size) of damped Lyalpha absorbers at high redshift. We argue that the most likely explanation for the rapid disappearance of damped Lyalpha absorbers between z = 3.5 and z = 3 is that we are observing the epoch of galaxy formation.