1999ApJ...517L..73M

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^IGM_ν 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^IGM_ν 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.