apj496_761 - THE ASTROPHYSICAL JOURNAL 496 761774 1998...

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T HE A STROPHYSICAL J OURNAL , 496:761 È 774, 1998 April 1 1998. The American Astronomical Society. All rights reserved. Printed in U.S.A. ( BROAD Ne VIII j 774 EMISSION FROM QUASARS F RED AND R OSS D. H AMANN 1 C OHEN The Center for Astrophysics and Space Sciences, University of California San Diego, La Jolla, CA 92093-0424 J OSEPH C. S HIELDS Department of Physics and Astronomy, Ohio University, Athens, OH 45701-2979 E. M. AND V ESA B URBIDGE J UNKKARINEN The Center for Astrophysics and Space Sciences, University of California San Diego, La Jolla, CA 92093-0424 AND D. M. C RENSHAW Goddard Space Flight Center, Code 681, Greenbelt, MD 20771 Received 1997 July 2; accepted 1997 November 5 ABSTRACT Ne VIII j 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 VIII lines are common in QSOs, with an average Ñux of D 42% of O VI j 1034 and velocity widths that are D 2 to 5 times larger than O VI ,C IV j 1549 and other broad lines in the same spectra. The strongest and most reliably measured Ne VIII j 774 lines (in two sources) have FWHM D 14,500 km s ~1 . Line proÐle Ðts in these cases show that the unusually large widths might be caused by blending with emission from N IV j 765 and O IV j 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 identiÐcation for the D 774 feature. (This A ł conclusion might be avoided if there are large radial velocity dispersions km s ~1 ] in the emit- [ Z 1000 ting region, and the resonant absorption of continuum photons enhances the Ñux in weaker lines.) The calculations also indicate that the Ne VIII È emitting regions have ionization parameters in the range 5 [ total hydrogen column densities of cm ~2 , and an average covering factor U [ 30, 10 22 [ N H [ 3 ] 10 23 of (for solar abundances and a nominal QSO continuum shape). The Ne VIII È emitting region is Z 30% 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 ÏÏ absorber if it lies along our line of sight to the X-ray continuum source. Subject headings: galaxies: kinematics and dynamics È quasars: emission lines È quasars: general È ultraviolet: galaxies 1. INTRODUCTION The rest-frame UV spectra of QSOs and active galactic nuclei (AGNs) typically contain strong and broad emission lines (BELs) from the resonance doublets C IV j 1549, N V j 1240, and O VI j 1034 in the lithium-like isoelectronic sequence. Ne VIII j 774 is another resonant doublet in that sequence. Its short wavelength makes j 774 difficult to measure, but this line deserves special attention because Ne VIII requires ionization energies (207 eV) nearly twice as large as O VI (114 eV) and more than 4 times larger than C IV (48 eV). Broad Ne
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apj496_761 - THE ASTROPHYSICAL JOURNAL 496 761774 1998...

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