The Origin of a Chemically Enriched Lyα 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(H I)~10¹⁶ cm^-2] at low redshift (z=0.167 toward PKS 0405-1219). Two galaxies at an ~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⁺, O⁰, 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 O VI gas of larger spatial extent. Furthermore, the partial Lyman limit system appears to have a metallicity of at least 1/10 solar and is most likely solar or supersolar 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~2.5×10⁵ 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 ~1.5×10¹² M_solar and a gas number density <~3×10^-5 cm^-3. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.