LICK OPTICAL-SPECTRA OF QUASAR HS-1946- LYMAN-ALPHA FOREST AND METAL ABSORPTION SYSTEMS(7658 AT 10 KILOMETERS PER SECOND RESOLUTION )

We present optical spectra of the most luminous known QSO HS 1946+7658 (z(em) = 3.051). Our spectra have both full wavelength coverage, 3240 -10570 angstrom, and in selected regions, either high signal-to-noise ratio, SNR congruent-to 40-100, or unusually high approximately 10 km s-1 resolution, and in parts of the Lyalpha forest and to the red of L yalpha emission they are among the best published. We find 113 Lyalpha systems and six metal-line systems, three of which are new. The metal systems at z(abs) = 2.844 and 3.050 have complex velocity structure w ith four and three prominent components, respectively. We find that th e system at z(abs) = 2.844 is a damped Lyalpha absorption (DLA) system , with a neutral hydrogen column density of log N(H I) = 20.2+/-0.4, a nd it is the cause of the Lyman limit break at lambda approximately 35 20 angstrom. We believe that most of the H I column density in this sy stem is in z(abs) = 2.8443 component which shows the strongest low-ion ization absorption lines. The metal abundance in the gas phase of the system is [M/H] congruent-to -2.6 +/- 0.3, with a best estimate of [M/ H] = -2.8, with ionization parameter log GAMMA = -2.75, from a photoio nization model. The ratios of the logarithmic abundances of C, O, Al, and Si are all within a factor of 2 of solar, which is important for t wo reasons. First, we believe that the gas abundances which we measure are close to the total abundances, because the ratio of aluminum to o ther elements is near cosmic, and Al is a refractory element which dep letes very readily, like chromium, in the interstellar medium. Second, we do not see the enhancement of O with respect to C of [O/C] congrue nt-to 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 fo r observed ions and [O/C] congruent-to 0.2 after ionization correction s, which is consistent with solar abundances. We see C II(lambda1335) offset by 15 km s-1 with respect to C II(lambda 1334), presumably bec ause the gas density varies from 2 to 8 cm-3 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 d isk. They also imply that the cloud is relatively highly ionized with more C III than C II, more O III than O I, and log N(H I) = 20.72, whi ch is 3 times the H I column.The system at z(abs) = 1.7382 is also bel ieved to be damped with N(H I) approximately 10(21) cm-2, because we s ee Cr II, but its Lyalpha line will never be seen because it is below the Lyman limit of the other DLA system. We see a 2.6 sigma lack of Ly alpha forest lines well away from the QSO redshift, which may be a cha nce fluctuation. We also see a correlation between column density N(H I) and Doppler parameter b for 96 unsaturated Lyalpha forest absorptio n lines, and although this correlation persists in the 36 Lyalpha line s which lie in regions where the SNR congruent-to 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 le ss-than-or-equal-to 8 we see many Lyalpha lines which appear to have 1 0 less-than-or-equal-to b less-than-or-equal-to 20, but when 8 less-th an-or-equal-to SNR less-than-or-equal-to 16 we see only one line with b less-than-or-equal-to 15 km s-1, and two others which we believe hav e b less-than-or-equal-to 20, with values of 20 and 16 km s-1. Traditi onal Lyalpha line samples which include all lines which have W/sigma(W ) greater-than-or-equal-to 4 are not adequate to explore the distribut ion of the properties of individual clouds, because we need much highe r W/sigma(W) and SNR to avoid the strong biases.