Broad Ne VIII λ774 emission from quasars

Ne VIII lambda 774 is an important tracer of the high-ionization gas in QSOs. We examine the Ne VIII emission-line properties using new Hubble Space Telescope (HST)-FOS spectra of four sources, mean spectra derived 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 larger than O VI, C IV lambda 1549 and other broad lines in the same spectra. 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 fits in these cases show that the unusually large widths might be caused by blending with emission from N Iv lambda 765 and O Iv lambda 789. However, standard photoionization calculations indicate that N Iv, O Iv, and all other lines near this wavelength should be too weak, leaving (very broad) Ne VIII as the only viable identification for the similar to 774 Angstrom feature. (This conclusion might be avoided if there are large radial velocity dispersions [greater than or similar to 1000 km s(-)1] in the emitting region, and the resonant absorption of continuum photons enhances the flux in weaker lines.) The calculations also 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 factor of greater than or similar to 30% (for solar abundances and a nominal QSO continuum shape). The Ne vm-emitting region is therefore more extensive, more highly ionized, and has much higher velocities than the rest of the broad emission line region (BELR). This highly ionized BELR component would be a strong X-ray "warm" absorber if it lies along our line of sight to the X-ray continuum source.