BROAD NE-VIII-LAMBDA-774 EMISSION FROM QUASARS

Ne VIII lambda 774 is an important tracer of the high-ionization gas i n QSOs. We examine the Ne VIII emission-line properties using new Hubb le Space Telescope (HST)-FOS spectra of four sources, mean spectra der ived from two QSO samples in the HST archives, and new photoionization calculations. The results support our previous claim that broad Ne vm lines are common in QSOs, with an average flux of similar to 42% of O VI lambda 1034 and velocity widths that are similar to 2 to 5 times l arger than O VI, C IV lambda 1549 and other broad lines in the same sp ectra. The strongest and most reliably measured Ne vm lambda 774 lines (in two sources) have FWHM similar to 14,500 km s(-1). Line profile f its in these cases show that the unusually large widths might be cause d by blending with emission from N Iv lambda 765 and O Iv lambda 789. However, standard photoionization calculations indicate that N Iv, O I v, and all other lines near this wavelength should be too weak, leavin g (very broad) Ne VIII as the only viable identification for the simil ar to 774 Angstrom feature. (This conclusion might be avoided if there are large radial velocity dispersions [greater than or similar to 100 0 km s(-)1] in the emitting region, and the resonant absorption of con tinuum photons enhances the flux in weaker lines.) The calculations al so indicate that the Ne vm-emitting regions have ionization parameters in the range 5 less than or similar to U less than or similar to 30, total hydrogen column densities of 10(22) less than or similar to N-H less than or similar to 3 x 10(23) cm(-2), and an average covering fac tor of greater than or similar to 30% (for solar abundances and a nomi nal QSO continuum shape). The Ne vm-emitting region is therefore more extensive, more highly ionized, and has much higher velocities than th e rest of the broad emission line region (BELR). This highly ionized B ELR component would be a strong X-ray ''warm'' absorber if it lies alo ng our line of sight to the X-ray continuum source.