A New Spectroscopic Survey for Damped LY alpha Absorption Lines from HighRedshift Galaxies
Abstract
The results of a new Survey for distant galaxies identified on the basis of the damped Lyα absorption lines they produce in the spectra of highredshift background QSOs are presented. These results are based on new, lowresolution (FWHM~46 A) spectroscopy of a sample of 57 QSOs, which are combined with similar lowresolution spectroscopy of 53 QSOs recently acquired by Sargent, Steidel, & Boksenberg to yield a total of 101 spectra of highredshift QSOs (after allowing for duplication between the data sets) which are suitable for identifying damped Lyα absorption lines. The spectra are analyzed to generate a complete sample of absorption features which, if the features are assumed to be Lyα lines, have restframe equivalent widths satisfying W>= 5 A. A total of 84 such damped Lyα absorptionline candidates are detected. Spectroscopy of 39 of these candidates has so far revealed that 22 are in fact damped Lyα absorption lines [18 of which indicate H I column densities of N( H I)>= 2 x 10^20^ cm^2^], whereas 17 are either blends of weaker components or single Lyα lines. The candidate damped Lyα absorption lines identified here are combined with a similar sample obtained by Wolfe et al. to yield a total of 125 damped Lyα absorptionline candidates from the spectra of 156 background QSOs. This larger sample is used to investigate the statistical properties of the damped Lyα absorption systems and the evolution of these properties with redshift. The general features of the damped Lyα Systems found previously by Wolfe et al. are confirmed, but the larger data set now available allows the properties of the absorbers to be determined in more detail. The main results of the survey are as follows: 1. The mean number density of the damped Lyα absorbers with N(H I)>=2x10^20^ cm^2^ is 0.16 +/ 0.03 <=<n(z)<= 0.25 +/ 0.04 at an average redshift of <Z_abs_>~2.5, where the bounds are derived from two distinct subsamples of the data. The value of <n(z)> found here is consistent with (although slightly smaller than) the value inferred from the data of Wolfe et al. The rate of incidence of the damped Lyα absorbers is greater than that expected by assuming that the absorption arises in the H I disks of spiral galaxies and that the size and comoving number density of these galaxies remain constant with time by a factor of at least 2.1 if q_0_=0 or by a factor of at least 3.9 if q_0_ = 0.5, adopting standard values of the properties of galaxies found locally. It is argued that it is unlikely that the discrepancy between the observed and predicted rates of incidence may be reconciled by appealing to uncertainties in the properties of local dwarf galaxies. 2. The H I column density distribution of the damped Lyα absorbers is well fitted by a power law of the form f(N)=BN^β^ over the column density range 20.3 <=log N(H I)/cm^2^<=21.8, with log B= 12.33+/0.O7 and β= 1.67 +/ 0.19. There is apparently an excess of absorption systems with N(H I)>= 2x10^20^ cm^2^ compared with what is expected based on an extrapolation of the column density distribution for systems with N(H I) <2 x 10^20^ cm^2^. 3. The mean value of the cosmological mass density contributed by the damped Lyα absorbers is {{OMEGA}_D_> = 0.79+/0.14x10^3^h^1^ for q_0_= 0 and <{OMEGA}_D_> = 1.45 +/ 0.25 x 10^3^h^1^ for q_0_=0.5. This value is roughly comparable to the mass density of luminous matter in presentday spiral disks. 4. The distribution of the number of damped Lyα absorbers observed per background QSO spectrum is consistent with what is expected for randomly distributed, intervening objects. This result apparently rules out the possibility that the large rate of incidence observed for the damped Lyα absorbers arises because the sample of background QSOs that exhibit damped Lyα absorption lines has been artificially enhanced due to amplification by gravitational lensing. 5. No significant evidence for clustering of the damped Lyα absorbers on velocity scales v > 5000 km s^1^ is found, although given the large uncertainty in the determination of the correlation function it is not possible to rule out a signal of the strength recently found for C C IVselected absorbers. 6. The number density distribution of the damped Lyα absorbers is satisfactorily fitted by a power law of the form n(Z) = n_0_(1+Z)^{gamma}^, with n_0_=0.16+/0.03 and γ=0.3+/1.4. The rate of incidence of the absorbers is thus consistent with a lack of evolution over the redshift interval Z~1.64.1, although due to the large uncertainty in γ, Intrinsic evolution of the absorbers over this redshift range cannot be ruled out. 7. No evidence for a correlation of N( H I) with the absorption redshift Z_abs_ is found, thus suggesting that there is no evolution of the average H I column density of the damped Lyα absorbers over the redshift interval Z~1.64.1. 8. The cosmological mass density distribution of the damped Lyα absorbers as a function of the "absorption distance" X is satisfactorily fitted by a power law of the form {OMEGA}_D_= {OMEGA}_D0_X^{zeta}, with ζ= 0.2 +/ 0.9 for q_0_=0 and ζ = 0.5 +/ 1.1 for q_0_=0.5. There is thus no significant evidence for evolution of the cosmological mass density of the damped Lyα absorbers over the redshift interval Z~1.64.1, although due to the large uncertainty in ζ evolution of the total gas content of the absorbers over this redshift range cannot be ruled out.
 Publication:

The Astrophysical Journal Supplement Series
 Pub Date:
 September 1991
 DOI:
 10.1086/191596
 Bibcode:
 1991ApJS...77....1L
 Keywords:

 Astronomical Spectroscopy;
 Galaxies;
 Lyman Alpha Radiation;
 Red Shift;
 Absorption Spectra;
 Line Spectra;
 Sky Surveys (Astronomy);
 Astrophysics;
 COSMOLOGY;
 GALAXIES: EVOLUTION;
 GALAXIES: FORMATION;
 QUASARS