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The Nature and Origin of za ≈ ze Absorption Lines in the Redshift 0.20 Quasar, PKS 2135–147

We use new UV and optical spectra and an archival Hubble Space Telescope Wide Field Planetary Camera 2 (HST-WFPC2) image to study the za ≈ ze absorber in the ze ≈ 0.20 QSO PKS 2135-147. The UV spectra, obtained with the Faint Object Spectrograph on HST, show strong za ≈ ze absorption lines of C IV, N V, O VI, Lyα, and Lyβ. The za ≈ ze line profiles are resolved, with deconvolved FWHM of 270-450 km s-1. The Lyman decrement and the O VI and N V doublet ratios indicate that there are also narrower, optically thick line components, and there is evidence in the C IV and Lyα profiles for two blended components. Lower limits on the total column densities are of order 1015 cm-2 for all ions. The ~2:1 ratio of the C IV doublets suggests that the total C IV column density is near the lower limit. If the absorber is photoionized by the QSO and the derived relative columns in C IV and H I are roughly correct, then the metallicity must be at least solar. The location of the za ≈ ze absorber remains uncertain. The line redshifts indicate that the clouds have little radial motion (less than ±200 km s-1) with respect to the QSO. This small velocity shift could mean that the absorber is outside of the deep gravitational potential of the QSO and the host-galaxy nucleus. Two ~L* galaxies in a small cluster centered on PKS 2135-147 lie within 36 h-1 kpc projected distance and have redshifts consistent with causing or contributing to the za ≈ ze lines. The extensive halo of the QSO's host galaxy could also contribute. Calculations show that the QSO is bright enough to photoionize gas up to O VI in the low-density halos of the host and nearby cluster galaxies. Nonetheless, there is indirect evidence for absorption much nearer the QSO, namely (1) the derived high (albeit uncertain) metallicity; (2) the relatively strong N V absorption lines, which might be caused by a higher nitrogen abundance in the metal-rich gas; and (3) strong, lobe-dominated, steep-spectrum radio emission, which is known to correlate with a much higher incidence of (probably intrinsic) za ≈ ze lines. We propose that the C IV/N V/O VI line ratios can be used as a general diagnostic of intrinsic versus intervening absorption, as long as the line saturation effects are understood.