The Origin of a Chemically Enriched lya Absorption System at z=0.167

Abstract

We present the first detailed analysis of the chemical abundances, ionization state, and origin of a partial Lyman limit system (N(\hI) ~ 10^{16} \cmjj) at low redshift (z=0.167 towards PKS0405-1219). Two galaxies at ~ 70 h^{-1} kpc projected distance to the QSO have been identified at the absorber redshift. We analyze an echelle spectrum of the QSO obtained with the Space Telescope Imaging Spectrograph and find that this absorption system exhibits associated lines produced by C^+, N^+, O^0, Si^+, Si^{++}, Si^{+3}, Fe^+, and Fe^{++}, and most interestingly, O^{+5} and N^{+4}. The results of our analysis show that the partial Lyman limit system traced by various ions is likely to be embedded in a collisionally ionized \ovi gas of larger spatial extent. Furthermore, the partial Lyman limit system appears to have a metallicity of {\em at least} 1/10 solar and most likely solar or super solar despite the fact that no luminous galaxies are seen within a projected distance ρ= 60 h^{-1} kpc to the QSO. Finally, adopting the temperature estimated for the hot gas T ~ 2.5\times 10^5 K and assuming a simple isothermal halo, we estimate that the galaxy or galaxy group that supports the extended gas may have a total mass ~ 1.5\times 10^{12} M_\odot and a gas number density \apl 3\times 10^{-5} cm^{-3}.