Evolution of Lyman-Limit Absorption Systems over the Redshift Range 0.40 < Z < 4.69

Abstract

We present the results of a study of 15 z > 4.2 QSOs that extend statistical studies of Lyman-limit absorption line systems [N(H I) >= 1.6 x 10^17^ cm^-2^] to the highest redshifts currently possible. This data set has been combined with homogeneous data sets of low-redshift Hubble Space Telescope observations and intermediate- redshift ground-based observations. Assuming a power law of the form N(z) = N_0_(1 + z)^gamma^ for the number density, we find γ = 1.55 and N_0_ = 0.27, with N = 3.27 per unit redshift at z = 4. The > 99.7% confidence limits for γ are 2.37 and 0.82. For the first time this indicates intrinsic evolution of these absorbers for an {OMEGA} = 1 universe (γ = 1/2 for no evolution). This result is marginally consistent with no evolution for {OMEGA} = 0 (γ = 1 for no evolution). These results differ significantly from those of Sargent, Steidel, & Boksenberg (1989) who found no intrinsic evolution in Lyman-limit systems up to z = 3.5, and Lanzetta (1991) who found much stronger evolution for z > 2.5. stronger evolution for z> 2.5. For z <~ 2 the space density of Lyman limit systems [N(1.5)~ 1.1] and Mg II absorbers with rest equivalent width W_0_ > 0.3 A [N(1.5)~ 1.0] is almost identical. This supports the picture that Lyman-limit and Mg II absorbers are drawn from the same population.