No 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 intermediateredshift ground-based observations. Assuming a power law of the form N(z) = N 0(1 + z) 1 for the number density, we find y = 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 (y = t for no evolution). This result is marginally consistent with no evolution for n = 0 (y = 1 for no evolution). These results differ significantly from those of Sargent, Steidel, & Boksenberg (1989) who found no intrinsic evolution in Lymanlimit 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(l.5) ~ 1.1] and Mg II absorbers with rest equivalent width W 0 > 0.3 A [N(l.5) ~ 1.0] is almost identical. This supports the picture that Lyman-limit and Mg II absorbers are drawn from the same population.

The APM multicolor survey for bright z > 4 objects, covering 2500 deg 2 of sky to m r ∼ 19, resulted in the discovery of thirty-one quasars with z > ∼ 4. High signalto-noise optical spectrophotometry at 5Å resolution has been obtained for the twentyeight quasars easily accessible from the northern hemisphere. These spectra have been surveyed to create new samples of high redshift Lyman-limit systems, damped Lyman-α absorbers, and metal absorption systems (e.g. CIV and MgII). In this paper we present the spectra, together with line lists of the detected absorption systems. The QSOs display a wide variety of emission and absorption line characteristics, with 5 exhibiting broad absorption lines and one with extremely strong emission lines (BR2248−1242). Eleven candidate damped Lyα absorption systems have been identified covering the redshift range 2.8 ≤ z ≤ 4.4 (8 with z > 3.5). An analysis of the measured redshifts of the high ionization emission lines with the low ionization lines shows them to be blueshifted by 430 ± 60 km s −1 . In a previous paper we discussed the redshift evolution of the Lyman limit systems catalogued here. In subsequent papers we will discuss the properties of the Lyα forest absorbers and the redshift and column density evolution of the damped Lyα absorbers.

No abstract