The Hubble Space Telescope Quasar Absorption Line Key Project. I. First Observational Results, Including Lyman-Alpha and Lyman-Limit Systems

We present spectra for a total of 37 quasars with small and moderate redshifts; the quasars were observed with the Faint Object Spectrograph of the Hubble Space Telescope. The spectra are obtained either at a resolution of R = 1300 (hereafter "higher resolution") or a resolution of r = 180 (hereafter "lower resolution"). We report new higher resolution measurements of the absorption lines in the ultraviolet spectra of 11 quasars with emission-line redshifts that lie between 0.3 and 1.0. Improved reductions are provided for higher resolution observations of two additional small-redshift quasars observed as part of the Guaranteed Time observational program. These observations cover all or part of the wavelength range 1180-3270 A in the observed frame. In addition, we present lower dispersion spectra (R = 180, wavelength range 1150-2430 A) obtained for 24 objects with redshifts between 0.5 and 1.4. Calibrated spectra and continuum fits are shown for each object. A well-defined sample of absorption lines was constructed algorithmically from all of the higher resolution data; the detection limit is expressed in terms of a limiting equivalent width as a function of wavelength. A total of 358 lines are selected by the software in the higher resolution spectra. Time-dependent flat-field features represent the most serious and difficult-to-quantify source of systematic errors. The identifications of interstellar and of extragalactic lines were made in the higher resolution spectra with the aid of software that utilizes the detection limit as a function of wavelength and the known atomic properties of cosmologically abundant ions. A total of 104 extragalactic Lyα systems are identified, of which nine are found at the same redshifts as metal-line systems. The absorption lines that are not identified Galactic interstellar features or extragalactic metal lines are not uniformly distributed; their line density is more than 10 times larger within (rather than outside) the region accessible to extragalactic hydrogen absorption, providing statistical support for the identifications of Lyα systems. The local number density of Lyα systems with rest equivalent widths larger than 0.32 A and without detected metal lines is (dN/dz)_0_~15.1 +/- 2 Lyα systems per unit redshift, if the local Lyα systems are members of the same population as the previously observed large-redshift Lyα lines (i.e., the evolutionary parameter is fixed at γ = 0.75, and the assumed exponential distribution of equivalent widths is characterized by W^*^ = 0.32 A). If γ and W^*^ are allowed to vary, then (dN/dz)_0_~ 17.7 +/- 4.3 Lyα systems per unit redshift with γ = 0.30 +/- 0.62 and W^*^ = 0.22 +/- 0.02 A. The two-point correlation function of the velocity splittings does not show statistically significant evidence of clumping. A sample at least 3 times larger than the one analyzed in this paper would be required to test the hypothesis that Lyα absorption systems observed at small redshifts are clustered like galaxies. The spectra of 29 quasars have been searched algorithmically for the existence of Lyman-limit systems. A total of 10 Lyman-limit systems with an optical depth greater than 0.4 were identified. The number density of such systems at redshifts between 0.4 and 1.4 in our sample is (dN/dz)_LLS_ = 0.8(1+z)^0.5^ for a nonevolving population in a q_0_ = 0.5 universe and (dN/dz)_LLS_ = 0.3(1 + z)^2.4^ if they evolve as rapidly as Lyα clouds observed at large redshifts. A search for damped Lyα lines revealed only one candidate line, in the spectrum of PKS 2251+11, among the spectra of the 36 quasars that were studied. The paucity of damped Lyα lines at small and moderate redshifts shows that the number density of damped absorption systems decreases with decreasing redshift. Of the observed lines, 44% are identified with Galactic interstellar features, 47% with extragalactic absorption systems, and 9% are unidentified. Eleven extragalactic C IV doublets are detected. The incidence of C IV absorption per unit redshift at <z> ~ 0.3 and with W_rest_(λ1548) >= 0.3 A is dN/ dz = 0.9 +/- 0.4, significantly lower than the value found at higher redshifts. No obvious correlation is found between radio power of the quasars and the seven occurrences of associated absorption. Three individual absorption systems are of special interest: a broad absorption line system in the spectrum of PG 0043+039; a strong, apparently broad high-excitation associated absorption system in 3C 351; and a rich, high-excitation intervening metal-line system in PKS 2145_06 which has 16 identified lines.