Broad PvAbsorption in the QSO PG 1254+047: Column Densities, Ionizations, and Metal Abundances in Broad Absorption Line Winds

Abstract

This paper discusses the detection of P V λλ1118, 1128 and other broad absorption lines (BALs) in archival Hubble Space Telescope spectra of the low-redshift BAL QSO PG 1254+047. The P V identification is secured by excellent redshift and profile coincidences with the other BALs, such as C IV λλ1548, 1550 and Si IV λλ1393, 1403, and by photoionization calculations showing that other lines near this wavelength, e.g., Fe III λ1123, should be much weaker than P V. The observed BAL strengths imply that either (1) there are extreme abundance ratios such as [C/H] ≳ +1.0, [Si/H] ≳ +1.8, and [P/C] ≳ +2.2, or (2) at least some of the lines are much more optically thick than they appear. I argue that the significant presence of P V absorption indicates severe line saturation, which is disguised in the observed (moderate-strength) BALs because the absorber does not fully cover the continuum source(s) along our line(s) of sight. The variety of observed BAL strengths and profiles results from a complex mixture of ionization, optical depth, and coverage fraction effects, making useful determinations of the abundance ratios impossible without a specific physical model. Computed optical depths for all UV resonance lines show that the observed BALs are consistent with solar abundances if (1) the ionization parameter is at least moderately high, log U ≳ -0.6, (2) the total hydrogen column density is log NH (cm-2) ≳ 22.0, and (3) the optical depths in strong lines like C IV and O VI λλ1032, 1038 are ≳25 and ≳80, respectively. These optical depths and column densities are at least an order of magnitude larger than expected from the residual intensities in the BAL troughs, but they are consistent with the large absorbing columns derived from X-ray observations of BAL QSOs. In particular, a nominal X-ray column density of log NH (cm-2) ~ 23 could produce the observed BAL spectrum if +0.4 ≲ log U ≲ +0.7 in a simple one-zone medium. The outflowing BAL region, at velocities from -15,000 to -27,000 km s-1 in PG 1254+047, is therefore a strong candidate for the X-ray absorber in BAL QSOs.