Quasars probing quasars. I. Optically thick absorbers near luminous quasars

Joseph F. Hennawi, Jason X. Prochaska, Scott Burles, Michael A. Strauss, Gordon T. Richards, David J. Schlegel, Xiaohui Fan, Donald P. Schneider, Nadia L. Zakamska, Masamune Oguri, James E. Gunn, Robert H. Lupton, Jon Brinkmann

Research output: Contribution to journalArticlepeer-review

135 Scopus citations


With close pairs of quasars at different redshifts, a background quasar sight line can be used to study a foreground quasar's environment in absorption. We search 149 moderate-resolution background quasar spectra from Gemini, Keck, the MMT, and the SDSS to survey Lyman limit systems (LLSs) and damped Lyα systems (DLAs) in the vicinity of 1.8 < z < 4.0 luminous foreground quasars. A sample of 27 new quasar-absorber pairs is uncovered with column densities 1017.2 cm-2 < NH I < 10 20.9 cm-2 and transverse (proper) distances of 22 h -1 kpc < R < 1.7 h-1 Mpc from the foreground quasars. If they emit isotropically, the implied ionizing photon fluxes are a factor of ∼5-8000 times larger than the ambient extragalactic UV background over this range of distances. The observed probability of intercepting an absorber is very high for small separations: six out of eight projected sight lines with transverse separations R < 150 h-1 kpc have an absorber coincident with the foreground quasar, of which four have NH I > 1019 cm-2. The covering factor of NH I > 1019 cm-2 absorbers is thus ∼50% (4/8) on these small scales, whereas ≲2% would have been expected at random. There are many cosmological applications of these new sight lines: they provide laboratories for studying fluorescent Lyα recombination radiation from LLSs; they constrain the environments, emission geometry, and radiative histories of quasars; and they shed light on the physical nature of LLSs and DLAs.

Original languageEnglish (US)
Pages (from-to)61-83
Number of pages23
JournalAstrophysical Journal
Issue number1 I
StatePublished - Nov 1 2006


  • Cosmology: observations
  • Cosmology: theory
  • Intergalactic medium
  • Quasars: absorption lines
  • Quasars: general
  • Surveys

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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