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Session 28 -- QSO Absorption-Line Studies with HST and Keck

Oral presentation, Tuesday, June 13, 1995, 2:00pm - 5:30pm

[28.05] Redshift Evolution of Lyman Limit Absorption in the Spectra of a Large Sample of Quasars

Using a sample of 119 QSOs, containing objects we have selected having previously available high quality ground-based and IUE spectral observations, together with Hubble Space Telescope (HST) observations of 26 QSOs from Bahcall et al. (1993, 1995) and Impey et al. (1995) and new optical observations of 41 objects by Steidel and Sargent (1994), we study the redshift evolution of Lyman limit absorption systems (LLSs; $\gamma > 1.0$) over the reshift range $0.32 \le z_{LLS} \le 4.11$. The HST observations significantly improve the determination of the low redshift ($0.4 \le z_{LLS} \le 1.4$) distribution. After removing objects which may bias our sample, leaving a total of 169 QSOs, we find the distribution is well described by a single power law, and obtain for the number density as a function of redshift the form $N(z) = N_o(1+z)^{\gamma}$ with $\gamma=1.50\pm0.39$ and $N_o=0.25_{+0.27}^{-0.10}$, consistent with a constant comoving density of absorbers in a Friedmann universe with $q_o=0$ but indicating evolution if $q_o=1/2$.

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