The PG X-ray QSO sample: Links between the ultraviolet-X-ray continuum and emission lines

Beverley J. Wills, A. Laor, M. S. Brotherton, D. Wills, B. J. Wilkes, G. J. Ferland, Zhaohui Shang

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81 Scopus citations


Two sets of relationships relate QSO UV to soft X-ray continua with the broad-line region. These are (i) the Baldwin relationships, which are inverse relationships between the broad-line equivalent width and the continuum luminosity, and (ii) Boroson & Green's optical "Principal Component 1" relationships, linking steeper soft X-ray spectra with narrower Hβ emission, stronger Hβ blue wings, stronger optical Fe II emission, and weaker [O III] λ5007 lines. In order to understand these relationships, we extended the spectra into the UV for 22 QSOs with high-quality soft X-ray spectra. These are from the complete sample of QSOs from the Bright Quasar Survey for which Laor et al. demonstrated strong luminosity and X-ray-optical Principal Component 1 relationships. We show that these extend to a whole new set of UV relationships: Principal Component 1 (in the sense of steeper X-ray spectra) is related to narrower C III] λ1909 lines, larger Si III] λ1892/C III] λ1909 ratios (a high-density indicator), stronger low-ionization lines, and weaker C IV λ1549 but stronger N V λ1240 lines. We speculate that high accretion rates are linked to high columns of dense (∼1011 cm-3), nitrogen-enhanced, low-ionization gas from nuclear starbursts. Line width, inverse Fe II-[O III] and inverse Fe II-C IV relationships hint at the geometrical arrangement of this gas. These Principal Component 1 relationships appear to be independent of luminosity and therefore of the Baldwin relationships.

Original languageEnglish
Pages (from-to)L53-L56
JournalAstrophysical Journal
Issue number2 PART 2
StatePublished - Apr 20 1999

Bibliographical note

Funding Information:
We gratefully acknowledge the help of C. D. Keyes and A. Roman of STScI, M. Dahlem (now of ESTEC), and also M. Cornell and R. Wilhelm, who provided computer support in the Department of Astronomy of the University of Texas. This research is supported by NASA through LTSA grant NAG5-3431 (B. J. W.) and grant GO-06781 from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. M. S. B.’s research has been performed under the auspices of the US Department of Energy by the Lawrence Livermore National Laboratory under contract W-7405-ENG-48. We have used the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.


  • Accretion, accretion disks
  • Galaxies: starburst
  • Quasars: emission lines
  • Quasars: general
  • Ultraviolet: galaxies
  • X-rays: galaxies

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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