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

This paper discusses the detection of P V 1118,1128 and other broadabsorption lines (BALs) in archival HST spectra of the low-redshift BALQSO, PG1254+047. The P V identification is secured by excellent redshift and profilecoincidences with the other BALs, such as C IV 1548,1550 and Si IV 1393,1403,and by photoionization calculations showing that other lines near thiswavelength, e.g. Fe III 1123, should be much weaker than P V. The observed BALstrengths 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 aremuch more optically thick than they appear. I argue that the significantpresence of P V absorption indicates severe line saturation, which is disguisedin the observed (moderate-strength) BALs because the absorber does not fullycover the continuum source(s) along our line(s) of sight. Computed opticaldepths for all UV resonance lines show that the observed BALs are consistentwith solar abundances if 1) the ionization parameter is at least moderatelyhigh, log U >~ -0.6, 2) the total hydrogen column density is log N_H(cm-2) >~22.0, and 3) the optical depths in strong lines like C IV and O VI 1032,1038are >~25 and >~80, respectively. These optical depths and column densities areat least an order of magnitude larger than expected from the residualintensities in the BAL troughs, but they are consistent with the largeabsorbing columns derived from X-ray observations of BALQSOs. The outflowingBALR, at velocities from -15,000 to -27,000 km/s in PG 1254+047, is therefore astrong candidate for the X-ray absorber in BALQSOs.Comment: 16 pages (LaTeX) plus 8 pages of figures in one file (pg1254_figs.ps.gz), in press with Ap