Redshifts due to quasar diversity

We use the coadded spectra of 32 epochs of Sloan Digital Sky Survey (SDSS) Reverberation Mapping Project observations of 482 quasars with z > 1.46 to highlight systematic biases in the SDSSand BOSS-pipeline redshifts due to the natural diversity of quasar properties. We investigate the characteristics of this bias by comparing the BOSS-pipeline redshifts to an estimate from the centroid of He iiλ1640. He ii has a low equivalent width but is often well-defined in high-S/N spectra, does not suffer from self-absorption, and has a narrow component that, when present (the case for about half of our sources), produces a redshift estimate that, on average, is consistent with that determined from [O ii] to within 1σ of the quadrature sum of the He ii and [O ii] centroid measurement uncertainties. The large redshift differences of ∼1000 km s, on average, between the BOSS-pipeline and He ii-centroid redshifts suggest there are significant biases in a portion of BOSS quasar redshift measurements. Adopting the He ii-based redshifts shows that C iv does not exhibit a ubiquitous blueshift for all quasars, given the precision probed by our measurements. Instead, we find a distribution of C iv centroid blueshifts across our sample, with a dynamic range that (i) is wider than that previously reported for this line, and (ii) spans C iv centroids from those consistent with the systemic redshift to those with significant blueshifts of thousands of kilometers per second. These results have significant implications for measurement and use of high-redshift quasar properties and redshifts and studies based thereon. Subject headings: galaxies: active — galaxies: nuclei — quasars: emission lines — quasars: general — quasars: supermassive black holes — galaxies: distances and redshifts