The line-of-sight proximity effect and the mass of quasar host halos

Claude André Faucher-Giguère*, Adam Lidz, Matias Zaldarriaga, Lars Hernquist

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

33 Scopus citations


We show that the Lyα optical depth statistics in the proximity regions of quasar spectra depend on the mass of the dark matter halos hosting the quasars. This is owing to both the overdensity around the quasars and the associated infall of gas toward them. For a fiducial quasar host halo mass of (3.0 ± 1.6) × 1012 h-1 M, as inferred by Croom and coworkers from clustering in the 2dF QSO Redshift Survey, we show that estimates of the ionizing background (Γbkg) from proximity effect measurements could be biased high by a factor of ≈2.5 at z = 3 owing to neglecting these effects alone. The clustering of galaxies and other AGNs around the proximity effect quasars enhances the local background but is not expected to skew measurements by more than a few percent. Assuming the measurements of Γbkg based on the mean flux decrement in the Lyα forest to be free of bias, we demonstrate how the proximity effect analysis can be inverted to measure the mass of the dark matter halos hosting quasars. In ideal conditions, such a measurement could be made with a precision comparable to the best clustering constraints to date from a modest sample of only about 100 spectra. We discuss observational difficulties, including continuum flux estimation, quasar systematic redshift determination, and quasar variability, which make accurate proximity effect measurements challenging in practice. These are also likely to contribute to the discrepancies between existing proximity effect and flux decrement measurements of Γbkg.

Original languageEnglish (US)
Pages (from-to)39-61
Number of pages23
JournalAstrophysical Journal
Issue number1
StatePublished - Jan 20 2008


  • Cosmology: theory
  • Diffuse radiation
  • Galaxies: halos
  • Methods: numerical
  • Methods: statistical
  • Quasars: absorption lines
  • Quasars: general

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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