EVOLUTION OF NEUTRAL GAS AT HIGH-REDSHIFT - IMPLICATIONS FOR THE EPOCH OF GALAXY FORMATION

Although observationally fare, damped Ly alpha absorption systems domi nate the mass density of neutral gas in the Universe. 11 high-redshift damped Ly alpha systems covering 2.8 less than or equal to z less tha n or equal to 4.4 were discovered in 26 QSOs from the APMz>4 SO survey , extending these absorption system surveys to the highest redshifts c urrently possible. Combining our new data set with previous surveys, w e find that the cosmological mass density in neutral gas, Omega(g) doe s not rise as steeply prior to z similar to 2 as indicated by previous studies. There is evidence in the observed Omega(g), for a flattening at z similar to 2 and a possible turnover at z similar to 3. When com bined with the decline at z>3.5 in number density per unit redshift of damped systems with column densities log N(Ht)greater than or equal t o 21 atom cm(-2), these results point to an epoch at 2 greater than or similar to 3 prior to which the highest column density damped systems are still forming. We find that, over the redshift range 2<z<4, the t otal mass in neutral gas is marginally comparable to the total visible mass in stars in present-day galaxies. However, if one considers the total mass visible in stellar discs alone, i.e. excluding galactic bul ges, the two values are comparable. We are observing a mass of neutral gas that is comparable to the mass of visible disc stars. Lanzetta, W olfe & Turnshek found that Omega(2 approximate to 3.5) was twice Omega (z approximate to 2), implying that a much larger amount of star forma tion must have taken place between z=3.5 and 2 than is indicated by me tallicity studies. This created a 'cosmic G-dwarf problem'. The more g radual evolution of Omega(g), that we find alleviates this. These resu lts have profound implications for theories of galaxy formation.