Broad NeVIII 774 Emission From Quasars

• Main Authors: Hamann, Fred, Cohen, Ross D, Shields, Joseph, Burbidge, E. M, Junkkarinen, Vesa, Crenshaw, D. M
• Format: Journal Article
• Language: English
• Published: 06-11-1997
• Subjects: Physics - Astrophysics of Galaxies; Physics - Cosmology and Nongalactic Astrophysics; Physics - Earth and Planetary Astrophysics; Physics - High Energy Astrophysical Phenomena; Physics - Instrumentation and Methods for Astrophysics; Physics - Solar and Stellar Astrophysics
• DOI: 10.48550/arxiv.astro-ph/9711061

NeVIII 774 is an important tracer of the high-ionization gas in QSOs. We
examine the NeVIII emission-line properties using new 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
NeVIIIlines are common in QSOs, with an average flux of ~42% of OVI 1034 and
velocity widths that are ~2 to 5 times larger than OVI, CIV 1549 and other
broad lines in the same spectra. The strongest and most reliably measured
NeVIII 774 lines (in two sources) have FWHM ~ 14,500 km/s. Line profile fits in
these cases show that the unusually large widths might be caused by blending
with emission from NIV 765 and OIV~\lam 789. However, standard photoionization
calculations indicate that NIV, OIV and all other lines near this wavelength
should be too weak, leaving (very broad) NeVIII as the only viable
identification for the ~774 A feature. (This conclusion might be avoided if
there are large radial velocity dispersions [>~1000 km/s] in the emitting
region and the resonant absorption of continuum photons enhances the flux in
weaker lines.) The calculations also indicate that the NeVIII emitting regions
have ionization parameters in the range 5 > U > 30, total hydrogen column
densities of 10^22 < N_H < 3 x 10^23 cm-2, and an average covering factor of
>30% (for solar abundances and a nominal QSO continuum shape). The NeVIII
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.