Broad Ne VIII λ774 Emission from Quasars

Ne VIII λ774 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 VIII lines are common in QSOs, with an average flux of ~42% of O VI λ1034 and velocity widths that are ~2 to 5 times larger than O VI, C IV λ1549 and other broad lines in the same spectra. The strongest and most reliably measured Ne VIII λ774 lines (in two sources) have FWHM ~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 λ765 and O IV λ789. 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 ~774 Å feature. (This conclusion might be avoided if there are large radial velocity dispersions [ ≳ 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 VIII-emitting regions have ionization parameters in the range 5 ≲ U ≲ 30, total hydrogen column densities of 10²² ≲ N_H ≲ 3 × 10²³ cm^-2, and an average covering factor of ≳30% (for solar abundances and a nominal QSO continuum shape). The Ne VIII-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.