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)^γ 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 y are 2.37 and 0.82. For the first time this indicates intrinsic evolution of these absorbers for an Q = 1 universe (γ = 1/2 for no evolution). This result is marginally consistent with no evolution for n = 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. 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 Å [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.

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