A survey of Lyman-limit absorption in the spectra of 59 high-redshift QSOs

Abstract

We have obtained spectra of 59 QSOs with redshifts in the range 2.75 < z em < 4.11 at a resolution of 4 À in the wavelength range ÀÀ3150-4700 and 6 A in the range AX4600-7000. The spectra are suitable for crude spectrophotometry. The signal-to-noise ratio is typically 30-50 in the blue and 50-70 in the red. The objects constitute an unbiased sample as far as Lyman-limit detection is concerned. We list emission-line equivalent widths and fluxes and use the new data to derive new emission redshifts. Power-law fits to the overall continuum of the form f v (Xv~ a for the spectrum longward of Lyman-a emission result in a range of slopes 0.18 < ¿ abs < 1.45. A total of 37 Lyman-limit systems (LLSs) were identified in the 59 QSOs; three QSOs each had two LLSs. All the LLSs have r >1.5. It was possible to measure the size of the Lyman discontinuity in 12 cases; these had 1.5 1.5 in the spectra of 54 QSOs. Survival statistics were used to derive a mean density per unit redshift for LLSs of V = 1.91 + 0.33 at (^ LL s) = 2.950. A hazard plot and the Kaplan-Meier estimator were used to show that the distribution of LLSs is Poissonian. Our data were combined with published samples to produce an extended sample of 54 LLSs in 90 QSOs covering the range 0.67 < z LLS < 3.58. The resulting number density as a function of redshift has the form N(z) a (1 + z)° 68± 054 , which is consistent with either = 0 or ^r 0 = 1/2 and no evolution in the properties of the absorbing objects. We estimate that the Lyman-a forest clouds only contribute N(z)-0.3 to the total at z-3.5, and less at lower redshifts. Thus, we believe that most of the LLSs are produced by galaxies. About two-thirds of the LLSs have associated heavy-element absorption systems, usually identified by the C iv doublet, in these low-resolution spectra. A plot of the distribution of the apparent ejection velocity ß is flat over the whole observed range; thus, there is no significant tendency for LLSs to congregate around z em. The selection of QSOs was not biased against BAL objects. Three unambiguous and five possible BAL QSOs were found; they constitute 5.1%-13.6% of the total sample. Six "damped Lyman-a" lines with H i column densities in excess of 1.0XlO 21 cm-2 were found; the corresponding density per unit redshift is Y DLA = 0.09 + 0.04 at a mean redshift < log Y(H i) < 22.0 can be represented as a single power law, although formally the fit is not very good.