Evolution of Lyman-limit absorption systems over the redshift range 0.40 < Z < 4.69
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.
© 1994 American Astronomical Society. Received 1993 October 28; accepted 1994 March 4. We would like to thank Max Pettini and the referee for helpful suggestions. L. S. L. acknowledges support from an Isaac Newton Studentship and the Cambridge Overseas Trust. R. G. M. acknowledges the support of the Royal Society.
Published - 1994ApJ___427L__13S.pdf