Investigation of the Metal Content of Damped Lyman Alpha Systems with the Keck 10 m Telescope

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Session 25 -- QSO Absorption-Line Studies with HST and Keck
Oral presentation, Tuesday, June 13, 1995, 8:30am - 12:30pm

[25.01] Investigation of the Metal Content of Damped Lyman Alpha Systems with the Keck 10 m Telescope

J. X. Prochaska \& A. M. Wolfe (UCSD)

\newcommand{\lya}{Ly$\alpha$} \newcommand{\etal}{et al.}

We use the HIRES echelle spectrograph on the Keck 10 m telescope to obtain accurate velocity profiles of low-ion transitions in damped {\lya} systems. The principal goals are to (a) record the emergence of metals in galaxies, (b) trace metal abundances in galaxies from $z$ $\approx$ 4.5 to the present, and (c) determine the kinematic state of galaxies from $z$ $\approx$ 4.5 to the present.

We have determined element abundances for 3 damped systems. We concentrate on weak, unsaturated resonance lines arising in low-ionization species such as Zn$^{+}$, and Cr$^{+}$, because these are the dominant ionization states in gas with $N$(H I) $>$ 2$\times$10$^{20}$ cm$^{-2}$, the threshold column density for damped {\lya} surveys. We assume that element abundances of Zn are given by $N$(Zn$^{+}$)/$N$(H$^{0}$). The gas phase abundance of Zn is especially important, since Zn is relatively undepleted by grains in the ISM. We find metal abundances ranging from [Fe/H] = $-$ 1.55 to [Fe/H] $>$ $-$ 0.25. We present abundance vs. condensation temperature diagrams to show that while evidence for depletion is present in one case, the evidence is problematic in the other two. In one system the ratio Cr:Si:Ni:Fe is the same in all velocity components. This suggests that dust is absent in this system. Also, in two other systems the lack of OI 1355.6 absorption implies upper limits on [O/Fe] that are consistent with element production by either type II or type Ia supernovae.

We find the velocity profiles of the 5 damped systems we have investigated to be asymmetric. The velocity components with the highest column densities are either at the red edge or the blue edge of the line profiles. This can be explained by passage of the line of sight through a rotating thick disk of gas in which the density of clouds falls off with cylindrical radius and $z$ height from midplane. Rotation speeds of $\approx$ 250 km s$^{-1}$ and disk scale heights of $\approx$ 2 - 3 kpc are required to explain the data. The implication is that well formed rotating disk galaxies are already in place by $z$ $>$ 2.

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