Redshifted and blueshifted broad lines in luminous quasars

D. H. McIntosh, H. W. Rix, M. J. Rieke, C. B. Foltz

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64 Scopus citations


We have observed a sample of 22 luminous quasars, in the range 2.0 ≲ z ≲ 2.5, at 1.6 μm with the near-infrared (NIR) spectrograph FSPEC on the Multiple Mirror Telescope. Our sample contains 13 radio-loud and nine radio-quiet objects. We have measured the systemic redshifts zsys directly from the strong [O III] λ5007 line emitted from the narrow-line region. From the same spectra, we have found that the nonresonance broad Hβ lines have a systematic mean redward shift of 520 ± 80 km s-1 with respect to systemic. Such a shift was not found in our identical analysis of the low-redshift sample of Boroson & Green. The amplitude of this redshift is comparable to half the expected gravitational redshift and transverse Doppler effects and is consistent with a correlation between redshift differences and quasar luminosity. From data in the literature, we confirm that the high-ionization rest-frame ultraviolet broad lines are blueshifted ∼550-1050 km s-1 from systemic and that these velocity shifts systematically increase with ionization potential. Our results allow us to quantify the known bias in estimating the ionizing flux from the intergalactic medium J1GMv via the proximity effect. Using redshift measurements commonly determined from strong broad-line species, like Lyα or C IV λ1549, results in an overestimation of J1GMv by factors of ∼1.9-2.3. Similarly, corresponding lower limits on the density of baryons Ωb, will be overestimated by factors of ∼1.4-1.5. However, the low-ionization Mg II λ2798 broad line is within ∼50 km s-1 of systemic and thus would be the line of choice for determining the true redshift of 1.0 < z < 2.2 quasars without NIR spectroscopy and z > 3.1 objects using NIR spectroscopy.

Original languageEnglish (US)
Pages (from-to)L73-L76
JournalAstrophysical Journal
Issue number2 PART 2
StatePublished - Jun 1 1999


  • Cosmology: miscellaneous
  • Infrared: general
  • Quasars: emission lines
  • Quasars: general
  • Relativity

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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