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For refcode 1994ApJ...427L..13S:
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1994ApJ...427L..13S EVOLUTION OF LYMAN-LIMIT ABSORPTION SYSTEMS OVER THE REDSHIFT RANGE 0.40 < z < 4.69 L. J. STORRIE-LOMBARDI AND R. G. MCMAHON Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK M. J. IRWIN Royal Greenwich Observatory, Madingley Road, Cambridge CB3 0EZ UK AND C. HAZARD University of Pittsburgh, Pittsburgh, PA 15260; and Institute of Astronomy Received 1993 October 28; accepted 1994 March 4 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 intermediate- redshift ground-based observations. Assuming a power law of the form N(z) = N_0_(1 + z)^{gamma}^ for the number density, we find {gamma} = 1.55 and N_0_ = 0.27, with N = 3.27 per unit redshift at z = 4. The > 99.7% confidence limits for {gamma} are 2.37 and 0.82. For the first time this indicates intrinsic evolution of these absorbers for an {OMEGA} = 1 universe ({gamma} = 1/2 for no evolution). This result is marginally consistent with no evolution for {OMEGA} = 0 ({gamma} = 1 for no evolution). These results differ significantly from those of Sargent, Steidel, & Boksenberg (1989) who found no intrinsic evolution in Lyman-limit 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(1.5)~ 1.1] and Mg II absorbers with rest equivalent width W_0_ > 0.3 A [N(1.5)~ 1.0] is almost identical. This supports the picture that Lyman-limit and Mg II absorbers are drawn from the same population. Subject headings: galaxies: evolution - quasars: absorption lines
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