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 ≤ S/N ≤ 70), high-resolution (FWHM = 20 km s-1) spectrum of the radio-quiet QSO HS 1700+6416 (zem = 2.72) with the echelle spectrograph on the Kitt Peak National Observatory 4 m telescope. We detect 13 metal systems in the optical spectrum of this QSO, including six systems with associated optically thin Lyman limit absorption in the HST spectrum obtained by Reimers et al. We use the apparent column density technique and profile fitting to measure the heavy-element column densities and to evaluate the impact of unresolved absorption saturation. Profile fitting indicates that four of the C IV systems are narrow, with b < 8 km s-1, which implies that these absorbers are relatively cool and are probably photoionized. The dense cluster of C IV doublets at 2.432 < zabs < 2.441 shows the weak line of one C IV absorber apparently aligned with the strong line of a different C IV doublet, i.e., line locked, for two pairs of C IV absorbers. Line locking has been detected previously in zabs ≈ zem absorbers, where radiation pressure is likely to play a role, but is surprising in this case since this C IV complex is displaced by ~24,000 km s-1 from the QSO emission redshift. This may be the remnant (or 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 zabs = 2.3150 suggest that the ionization conditions in this absorber differ significantly from the conditions in the gaseous halo of the Milky Way. From photoionization models we derive [Si/H] ≥ -0.95 and [Al/H] ≥ -0.96 for the strongest component of this absorber. These are conservative lower limits derived from lower ionization stages only; photoionization models in agreement with the observed low and high ionization stages require [M/H] ≈ -0.45. In contrast, Vogel & Reimers derived [N/H] < -1.04 and [O/H] = -1.52 for this absorber. We suggest that the discrepancy comes from the low resolution of the Vogel & Reimers data (FWHM ≈ 300 km s-1), which introduces serious blending and saturation problems. The photoionized model with [M/H] = -0.45 has a particle density nH ≈ 0.02 cm-3, a size of a few hundred parsecs, and a mass of roughly 1 × 105 M☉, assuming the absorber is spherical. We detect unsaturated C IV and rather strong N V “associated” absorption at zabs = 2.7125. The apparent column density of the weak N V 1242.8 Å line is greater than the apparent column density of the stronger N V 1238.8 Å 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 completely cover the QSO emission source. If the latter interpretation is correct, then the associated absorbing gas must be close to the QSO. We have used the observed spectral energy distribution of the QSO, corrected for intervening Lyman limit absorption, for photoionization modeling of the associated absorber, and we derive [N/H] ≥ -0.65 and [C/H] ≥ -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 increase the metallicity estimates. However, the absorption profiles suggest that the constant-density single-slab model is too simplistic: we 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 systems detected per unit redshift, and we tentatively conclude that dN/dz is dominated by weak Mg II lines with Wr < 0.3 Å.