A Keck HIRES Investigation of the Metal Abundances and Kinematics of Three Damped Lyα Systems toward Q2206−199

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Abstract
We present high-resolution, high signal-to-noise ratio spectra of the QSO Q2206-199 obtained with HIRES on the 10 m W. M. Keck Telescope. Our analysis focuses on the two previously identified damped Lyα systems found at z = 1.920 and z = 2.076. For the z = 1.920 system, we measure accurate abundances (relative to solar) for Fe, Cr Si, Ni, Ti, and Zn: [Fe/H] = -0.705 ± 0.097, [Cr/H] = -0.580 ± 0.100, [Si/H] = -0.402 ± 0.098, [Ni/H] = -1.012 ± 0.095, [Ti/H] = -0.776 ± 0.081, and [Zn/H] = -0.379 ± 0.097. This system exhibits the highest metallicity we have measured for a damped Lyα system. By contrast the z = 2.076 system is the most metal poor ([Zn/H] < -1.745) we have analyzed, showing absorption features for only the strongest transitions. We determine accurate abundances for Fe, Si, and Al: [Fe/H] = -2.621 ± 0.071, [Si/H] = -2.225 ± 0.075, and [Al/H] = -2.727 ± 0.070. Analyses of the abundance variations of Fe, Ni, Cr, and Si relative to Zn and the abundance trends versus condensation temperature do not offer positive evidence for the presence of dust in the damped system at z = 1.920. In addition, the relative abundance ratio [Ti/Fe] ≈ 0 further suggests the absence of ISM-like dust. Unfortunately, the lack of measurable Zn absorption in the z = 2.076 system does not allow a similar investigation for the presence of dust. In addition to the significant difference in metallicity, the two damped systems have vastly different kinematic characteristics. The z = 1.920 system spans ≈ 150 km s-1 in velocity space (measured from the low-ion transitions), while the z = 2.076 system spans a mere 30 km s-1. Furthermore, the z = 1.920 profiles are significantly asymmetric, while the z = 2.076 profiles are highly symmetric. Even with these differences, we contend the two systems are consistent kinematically with one physical description, that of a thick, rotating disk. In addition to the two previously identified damped Lyα systems, we investigate a very strong Mg II system at z = 0.752, which we believe is a third damped Lyα system along the line of sight. The very weak Mn II λλ2606, 2594, 2576 and Ti II λλ3073, 3342, and 3384 transitions have been positively measured and suggest a very conservative lower limit H I abundance of log N(H I) > 19.0 assuming metallicity relative to solar equal to 0 and no depletion. Together with the damped system at z = 1.920, this marks the first confident (>5 σ) detection of Ti in QSO absorption-line systems. We analyze the abundance ratios [Mn/Fe] and [Ti/Fe] and their values are inconsistent with significant dust depletion, yet consistent with the abundance pattern detected for halo stars in the Galaxy (Lu, Sargent, & Barlow 1996). Finally, we identify a C IV system at z = 2.014 that shows a very narrow feature in Si IV and C IV absorption. The corresponding b-values (5.5 and 8.9 km s-1 for Si IV and C IV, respectively) for this component suggest a temperature of 4.7 × 104 K. Because collisional ionization can explain the observed abundances only for T > 8 × 104 K, we contend these ions must have formed through a different physical process (e.g., photoionization).