We present an analysis of the evolution of the Ly alpha absorption lines in the redshift interval 0.0 to \sim 1.5 based upon a sample of 987 lines identified in the spectra of 63 QSOs obtained with ...the Faint Object Spectrograph on the Hubble Space Telescope. Fits to the number of absorbers per unit redshift dN/dz = A \times (1+z)^\gamma yield values of \gamma in the range 0.1 to 0.3, decidedly flatter than results from groundbased data pertaining to the redshift range z > 1.7. The combination of the space and groundbased data suggest a marked transition in the rate of evolution of the Ly alpha lines at a redshift of about 1.7. The distribution in redshift is well represented by a power law and the distribution of equivalent widths above a rest equivalent width of 0.1 \AA is fit well by an exponential. The sample of Ly alpha systems which has ions from heavy elements has a much larger value of \gamma than a sample of lines of comparable strength without heavy elements. We argue that this is not likely due to either line misidentification or incomplete spectral coverage and suggest that it is due to rapid evolution of the very highest column density systems, rather than differences in metallicity. We also find evidence that the rate of evolution increases with increasing equivalent width. We compare our results for the variation of line density with redshift to recent numerical simulations of Ly alpha absorbers, and find fairly good agreement. Our results for the stronger Ly alpha lines in our sample are compatible with the recent analysis of the absorber--galaxy correlation by Chen et al. 1998 but for the weaker lines our results suggest that the association between absorbers and galaxies is different from that for the stronger lines.
We present evidence that clumps of \lya lines are physically associated with about half of the extensive metal-line systems (absorption systems with four or more observed metal-line species) found in ...this paper, demonstrate that all four \lylimit systems discussed here correspond to extensive metal-line absorption systems, and present an extraordinary pair of extensive metal-line absorption systems within 2000~km/s of each other at \(z~=~0.95\) that are probably an early manifestation of large scale structure. These results are obtained using ultraviolet spectra, taken with the higher-resolution gratings of the Faint Object Spectrograph of the Hubble Space Telescope, for four quasars with emission-line redshifts between 1.0 and 1.3. We also determine the evolution of \lya absorption lines at redshifts less than 1.3 by combining the results for 13 smaller redshift quasars discussed in Paper I of this series with the 4 moderate redshift quasars analyzed in the present paper. For \(z_{\rm abs}~\leq~1.3\), the density of \lya lines with equivalent widths greater than 0.24~\AA\ is adequately fit by \(\left(dN/dz\right) =\left(dN/dz\right)_0 \cdot(1 + z)^\gamma\) with \((dN/dz)_0 = 24.3 \pm 6.6\) \lya lines per unit redshift, and \(\gamma = 0.58 \pm 0.50\) (1-\(\sigma\) uncertainties). This rate of evolution at low redshifts is less than the evolutionary rate inferred from several different ground-based data samples that pertain to high redshifts. The observed gaseous structures at redshifts of \(0.5\) to \(1.0\) with velocity dispersions of \(6 \times 10^2\) \kms to \(1.4 \times 10^3\) \kms (or velocity spans of \(1.2 \times 10^3\) \kms to \(3 \times 10^3\) \kms) constitute a constraint on cosmological models of structure formation.
Reports of Sections and Societies Roberts, Walter Orr; Warren, Harry V.; Givens, Wallace ...
Science (American Association for the Advancement of Science),
02/1965, Letnik:
147, Številka:
3660
Journal Article
PAPERS IN MECHANICS Pering, Rich; Duckworth, J. T.; Wise, W. F. ...
Transactions of the Society, Instituted at London, for the Encouragement of Arts, Manufactures, and Commerce,
01/1822, Letnik:
40
Journal Article
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