HIGH SIGNAL-TO-NOISE ECHELLE SPECTROSCOPY OF QUASAR ABSORPTION-LINE SYSTEMS IN THE DIRECTION OF HS-1946+7658

We have obtained a high signal-to-noise (40 less than or equal to S/N less than or equal to 80) high-resolution (FWHM = 20 km s(-1)) spectru m of the radio-quiet QSO HS 1946+7658 (z(em) = 3.051) with the echelle spectrograph on the KPNO 4 m telescope. We detect 11 metal systems in the direction of this QSO, including two Mg 11 systems, five C IV sys tems, two damped Ly alpha systems, and two associated systems. We use the apparent column density technique and profile fitting to measure t he heavy element column densities and to assess the effects of absorpt ion-line saturation. Profile fitting indicates that three of the C IV systems are narrow with b < 8 km s(-1). This implies that T < 50,000 K , and therefore these systems are probably photoionized. The abundance patterns in the damped Ly alpha systems are strikingly similar to tho se observed in low-metallicity Milky Way stars and suggest that these absorption lines are due to galaxies in early stages of chemical enric hment (see Lu et al.). The prominent associated system at z(abs) = 3.0 496, 3.0504 is detected in H I, C II, C IV, Si II, Si III, Si IV, Al I I, Al III, and N V. The high ion column density ratios in this associa ted system imply that the gas is more highly ionized than the Galactic halo. The high degree of ionization is not surprising given the extra ordinary luminosity of the QSO (Hagen et al.). To study ionization and abundances in this associated system, we compare the observed column densities to a series of CLOUDY models in which photoionization by the QSO is the dominant ionization mechanism. For the input radiation hel d, we have used the various spectral energy distributions of HS 1946+7 658 observed by Kuhn et al. The model that best fits the observed colu mn densities of singly and doubly ionized species has solar relative a nd absolute abundances, but models with absolute metallicities a few t imes greater than solar fit comparably well. None of the models produc e enough Si IV and C IV, but the QSO flux near the ionization potentia ls of these ions is uncertain. This photoionization modeling raises a dilemma: the high metallicity implied by the best models suggests that the associated absorption occurs near the nucleus of the QSO where th e star formation rate is likely to be enhanced, but the ionization par ameter from the best model combined with a density upper limit from C II implies that the distance between the QSO and the associated absor ber is greater than 300 kpc. We briefly discuss possible explanations of these discordant conclusions, including the possibility that the ga s was enriched near the QSO nucleus and then ejected. By combining our HS 1946+7658 data with similar quality observations from the literatu re, we find that the number density of C IV systems with rest equivale nt width W-r > 0.03 Angstrom is dN/dz = 7.1 + 1.7. This is similar to 3 times larger than the number density implied by a sample of C IV sys tems with W-r > 0.15 Angstrom, which indicates that the number of C IV systems per unit redshift is dominated by very weak lines. An attempt to detect a turndown in the C IV equivalent width distribution, which could indicate that some Ly alpha clouds contain no metals, resulted in ambiguous conclusions due to the small sample size. We briefly disc uss recent oscillator strength revisions which are likely to have a si gnificant impact on QSO absorption studies.