High-ionization emission-line ratios from quasar broad-line regions: Metallicity or density?

Matthew J. Temple, Gary J. Ferland, Amy L. Rankine, Marios Chatzikos, Paul C. Hewett

Research output: Contribution to journalArticlepeer-review

4 Scopus citations


The flux ratios of high-ionization lines are commonly assumed to indicate the metallicity of the broad emission-line region in luminous quasars. When accounting for the variation in their kinematic profiles, we show that the N v/C iv, (Si iv + O iv])/C iv, and N v/Ly α line ratios do not vary as a function of the quasar continuum luminosity, black hole mass, or accretion rate. Using photoionization models from cloudy, we further show that the observed changes in these line ratios can be explained by emission from gas with solar abundances, if the physical conditions of the emitting gas are allowed to vary over a broad range of densities and ionizing fluxes. The diversity of broad-line emission in quasar spectra can be explained by a model with emission from two kinematically distinct regions, where the line ratios suggest that these regions have either very different metallicity or density. Both simplicity and current galaxy evolution models suggest that near-solar abundances, with parts of the spectrum forming in high-density clouds, are more likely. Within this paradigm, objects with stronger outflow signatures show stronger emission from gas that is denser and located closer to the ionizing source, at radii consistent with simulations of line-driven disc-winds. Studies using broad-line ratios to infer chemical enrichment histories should consider changes in density and ionizing flux before estimating metallicities.

Original languageEnglish
Pages (from-to)3247-3259
Number of pages13
JournalMonthly Notices of the Royal Astronomical Society
Issue number3
StatePublished - Aug 1 2021

Bibliographical note

Funding Information:
MJT acknowledges support from Fondo ALMA 31190036, and thanks the IoA, Cambridge, for the award of visitor status while this work was completed. GJF acknowledges support by NSF (1816537), NASA (ATP 17-ATP17-0141), and STScI (HST-AR-15018). ALR and PCH acknowledge funding from the Science and Technology Facilities Council. MC acknowledges support from STScI (HSTAR- 14556.001-A), NSF (1910687), and NASA (19-ATP19-0188). This work made use of ASTROPY (Astropy Collaboration et al. 2013, 2018), CORNER.PY (Foreman-Mackey 2016), MATPLOTLIB (Hunter 2007), NUMPY (Harris et al. 2020), and SCIPY (Virtanen et al. 2020). We thank Bob Carswell, Fred Hamann and James Matthews for useful discussions and encouraging feedback, and an anonymous reviewer for their thoughtful comments. Funding for the Sloan Digital Sky Survey IV has been provided by the Alfred P. Sloan Foundation, the US Department of Energy Office of Science, and the Participating Institutions. SDSS-IV acknowledges support and resources from the Center for High- Performance Computing at the University of Utah. The SDSS website is www.sdss.org.

Publisher Copyright:
© 2021 The Author(s).


  • line: formation
  • quasars: emission lines
  • quasars: general

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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