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

Abstract

view Abstract Citations (341) References (37) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Survey of Lyman-Limit Absorption in the Spectra of 59 High-Redshift QSOs Sargent, Wallace L. W. ; Steidel, Charles C. ; Boksenberg, A. Abstract We have obtained spectra of 59 QSOs with redshifts in the range 2.75 <= z_em_ <= 4.11 at a resolution of 4 A in the wavelength range λλ 3150-4700 and 6 A in the range λλ4600-7000. The spectra are suitable for crude spectrophotometry. The signal-to-nolse 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_ is proportional to v^-α^ for the spectrum longward of Lyman-α emission result in a range of slopes 0.18 <= α <= 1.51, with a mean value <α> = 0.78 +/- 0.04 (error in the mean). Known radio sources have a significantly steeper mean slope than the remainder. There is a significant correlation between the rest equivalent width of the C IV emission line and absolute luminosity (the "Baldwin effect"). Absorption lines were measured longward of Lyman-α emission, and redshifts were determined. It was possible to derive an unbiased sample of redshifts based on the Mg II λλ2796, 2803 doublet. We found 62 Mg II absorption redshifts in 58 QSOs. These were used to improve the statistics of Mg II absorption in the range 0.7 <= z_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 τ >= 1.5. It was possible to measure the size of the Lyman discontinuity in 12 cases; these had 1.5 <= τ <= 3.5. In the case of all the other LLSs, τ was indeterminate but greater than 3.0. By taking all single LLSs and the highest redshift one in the three QSOs with more than one we derived an unbiased sample of 33 LLSs with τ >= 1.5 in the rpectra of 54 QSOs. Survival statistics were used to derive a mean density per unit redshift for LLSs of N = 1.91 +/- 0.33 at <z_LLS_> = 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) is proportional to (1 + z)^0.68+/-0.54^ which is consistent with either q_0_ = 0 or q_0_ = 1/2 and no evolution in the properties of the absorbing objects. We estimate that the Lyman-α 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 identifled by the C IV doublet, in these low-resolution spectra. A plot of the distribution of the apparent ejection velocity β 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-α" lines with H I column densities in excess of 1.0 x 10^21^ cm^-2^ were found; the correrponding density per unit redshift is N_DLA_ = 0.09 +/- 0.04 at a mean redshift (< z_abs_> = 2.62, larger than the local density of disks with the same H I column density. Our data from the LLSs and from the damped Lyman-α sample were used to determine the distribution of H I column density at the high end. As found by Tytler, the H I column density distribution over the range 13.25 <= log N(H I) <= 22.0 can be represented as a single power law, although formally the fit is not very good. Publication: The Astrophysical Journal Supplement Series Pub Date: April 1989 DOI: 10.1086/191326 Bibcode: 1989ApJS...69..703S Keywords: Intergalactic Media; Lyman Spectra; Quasars; Red Shift; Signal To Noise Ratios; Emission Spectra; Line Spectra; Lyman Alpha Radiation; Astrophysics; ATOMIC PROCESSES; QUASARS; COSMOLOGY; GALAXIES: INTERGALACTIC MEDIUM; LINE PROFILES full text sources ADS | data products NED (295) SIMBAD (61)