The Sloan Digital Sky Survey Reverberation Mapping Project: biases in z > 1.46 redshifts due to quasar diversity

K. D. Denney, Keith Horne, W. N. Brandt, C. J. Grier, Luis C. Ho, B. M. Peterson, J. R. Trump, J. Ge

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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 SDSS- and Baryon Oscillation Spectroscopic Survey (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 ii λ1640. 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 which, 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 the He ii and [O ii] centroid measurement uncertainties. The large redshift differences of ∼1000 km s-1, 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.

Original languageEnglish
Article number33
Number of pages16
JournalAstrophysical Journal
Issue number1
Publication statusPublished - 6 Dec 2016


  • Galaxies: active
  • Galaxies: distances and redshifts
  • Galaxies: nuclei
  • Quasars: emission lines
  • Quasars: general
  • Quasars: supermassive black holes


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