HIGH SIGNAL-TO-NOISE ECHELLE SPECTROSCOPY OF QUASAR ABSORPTION-LINE SYSTEMS WITH METALS IN THE DIRECTION OF HS-1700+6416

We have obtained a high signal-to-noise (30 less than or equal to S/N less than or equal to 70), high-resolution (FWHM = 20 km s(-1)) spectr um of the radio-quiet QSO HS 1700 + 6416 (z(em) = 2.72) with the echel le spectrograph on the Kitt Peak National Observatory 4 m telescope. W e detect 13 metal systems in the optical spectrum of this QSO, includi ng six systems with associated optically thin Lyman limit absorption i n the HST spectrum obtained by Reimers et al. We use the apparent colu mn density technique and profile fitting to measure the heavy-element column densities and to evaluate the impact of unresolved absorption s aturation. Profile fitting indicates that four of the C IV systems are narrow, with b < 8 km s(-1), which implies that these absorbers are r elatively cool and are probably photoionized. The dense cluster of C I V doublets at 2.432 < z(abs) < 2.441 shows the weak line of one C IV a bsorber apparently aligned with the strong line of a different C rv do ublet, i.e., line locked, for two pairs of C Iv absorbers. Line lockin g has been detected previously in z(abs) approximate to z(em) absorber s, where radiation pressure is likely to play a role, but is surprisin g in this case since this C IV complex is displaced by similar to 24,0 00 km s(-1) from the QSO emission redshift. This may be the remnant (o r precursor) of a broad absorption line (BAL) outflow. However, it is possible that these alignments are chance alignments rather than true line locking. The high-ion column density ratios in the multicomponent Lyman limit absorber at z(abs) = 2.3150 suggest that the ionization c onditions in this absorber differ significantly from the conditions in the gaseous halo of the Milky Way. From photoionization models we der ive [Si/H] greater than or equal to -0.95 and [A1/H] greater than or e qual to -0.96 for the strongest component of this absorber. These are conservative lower limits derived from lower ionization stages only; p hotoionization models in agreement with the observed low and high ioni zation stages require [M/H] approximate to -0.45. In contrast, Vogel & Reimers derived [N/ H] < -1.04 and [O/H] = -1.52 for this absorber. W e suggest that the discrepancy comes from the low resolution of the Vo gel & Reimers data (FWHM approximate to 300 km s(-1)), which introduce s serious blending and saturation problems. The photoionized model wit h [M/H] = -0.45 has a particle density n(H) approximate to 0.02 cm(-3) a size of a few hundred parsecs, and a mass of roughly 1 x 10(5) M., assuming the absorber is spherical. We detect unsaturated C IV and rat her strong N V ''associated'' absorption at z(abs) = 2.7125. The appar ent column density of the weak N V 1242.8 Angstrom line is greater tha n the apparent column density of the stronger N V 1238.8 Angstrom line in this absorber, which indicates that the N v profile is affected by unresolved saturation or that the N V absorbing gas does not complete ly cover the QSO emission source. If the latter interpretation is corr ect, then the associated absorbing gas must be close to the QSO. We ha ve used the observed spectral energy distribution of the QSO, correcte d for intervening Lyman limit absorption, for photoionization modeling of the associated absorber, and we derive [N/H] greater than or equal to -0.65 and [C/H] greater than or equal to -0.82. Other corrections (e.g., to account for dust in intervening absorbers or ''Lyman valley' ' attenuation) will make the spectral energy distribution harder and i ncrease the metallicity estimates. However, the absorption profiles su ggest that the constant-density single-slab model is too simplistic: w e obtain b(N V) = 25.2 +/- 1.3 km s(-1) and b(C IV)= 11.4 +/- 1.1 km s (-1) from profile fitting, and therefore the gas that produces the N V absorption does not have the same temperature and nonthermal motions as the C Iv gas. Finally, we briefly examine the number of Mg II syste ms detected per unit redshift, and we tentatively conclude that dN/dz is dominated by weak Mg II lines with W-r < 0.3 Angstrom.