1999/04/05 by Daniel H. McIntosh, D. H. McIntosh, H. -W. Rix +4 · 5 citations
Physics and Astronomy · #Astrophysical Phenomena and Observations #Galaxies: Formation, Evolution, Phenomena #Stellar, planetary, and galactic studies #astro-ph
paper · pdf · doi:10.1086/312033
published as Astrophys.J.517:L73,1999 · 12 pages, Latex, 2 figures, 2 tables, Accepted for publication in ApJ Letters
arxiv created 1999/04/05 · openalex publication_date 1999/06/01 · arxiv updated 2009/12/01 · openalex created_date 2016/06/24 · openalex updated_date 2026/07/31
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 z sys 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 J 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 J by factors of ~1.9-2.3. Similarly, corresponding lower limits on the density of baryons Ω b 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.