Redshifted and Blueshifted Broad Lines in Luminous Quasars

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Abstract
We have observed a sample of 22 luminous quasars, in the range 2.0z2.5, 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 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 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<z3.1 objects using NIR spectroscopy.
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