LICK OPTICAL SPECTRA OF QUASAR HS 1946+7658 AT 10 km/sec RESOLUTION: LYMAN-ALPHA FOREST AND METAL ABSORPTION SYSTEMS

We present optical spectra of the most luminous known QSO HS 1946+7658 (zem = 3.051). Our spectra have both full wavelength coverage, 3240 – 10570 Å , and in selected regions, either high signal to noise ratio, SNR ≃ 40 – 100, or unusually high ∼ 10 km s resolution, and in parts of the Lyα forest and to the red of Lyα emission they are among the best published. We find 113 Lyα systems and six metal line systems, three of which are new. The metal systems at zabs = 2.844 and 3.050 have complex velocity structure with four and three prominent components respectively. We find that the system at zabs = 2.844 is a damped Lyα absorption (DLA) system, with a neutral hydrogen column density of log N(H I) = 20.2±0.4, and it is the cause of the Lyman limit break at λ∼ 3520 Å. We believe that most of the H I column density in this system is in zabs = 2.8443 component which shows the strongest low ionization absorption lines. The metal abundance in the gas phase of the system is [M/H]≃ −2.6 ± 0.3, with a best estimate of [M/H]=−2.8, with ionization parameter Γ = −2.75, from a photoionization model. The ratios of the abundances of C, O, Al, and Si are all within a factor of two of solar, which is important for two reasons. First, we believe that the gas abundances which we measure are close to the total abundances, because the ratio of Aluminum to other elements is near cosmic, and Al is a refractory element which depletes very readily, like Chromium, in the interstellar medium. Second, we do not see the enhancement of O with respect to C of [O/C] ≃ 0.5− 0.9 reported in three partial Lyman limit systems by Reimers et al. 1992 and Vogel & Reimers 1993; we measure [O/C] = −0.06 for observed ions and [O/C] ≃ 0.2 after ionization corrections, which is consistent with solar abundances. We see C II(λ1335) offset by 15 km s −1 with respect to C II(λ1334), presumably because the gas density varies from 2 – 8 cm with changing velocity in the DLA system. These densities imply that the damped component is 6 – 25 pc thick, which is reasonable for a single cloud in a cold spiral disk. The system at zabs = 1.7382 is also believed to be damped with N(H I) ∼ 10 cm, because we see Cr II, but its Lyα line will never be seen because it is below the Lyman limit system (LLS) in the other DLA system. We see a 2.6σ lack of Lyα forest lines well away from the QSO redshift, which may be a chance fluctuation. We also see a correlation between column density N(H I) and Doppler parameter b for 96 unsaturated Lyα forest absorption lines, and although this correlation persists in the 36 Lyα lines which lie in regions where the SNR ≃ 8 – 16, we agree with Rauch et al. (1993) that it is probably a bogus effect of low SNR. The same applies to lines with very low b values: in regions where SNR ≤ 8 we see many Lyα lines which appear to have 10 ≤ b ≤ 20, but when 8 ≤ SNR ≤ 16 we see only one line with b ≤ 15 km s −1 , and two others which we believe have b ≤ 20, with values of 20 and 16 km s −1 . Traditional Lyα line samples which include all lines which have W/σ(W ) ≥ 4 are not adequate to explore the