The Sloan Digital Sky Survey Reverberation Mapping Project: Biases in z>1.46 Redshifts due to Quasar Diversity

TL;DR

This study reveals systematic biases in SDSS quasar redshift measurements at z>1.46 due to quasar diversity, showing that HeII-based redshifts can differ significantly from pipeline estimates, affecting interpretations of quasar properties.

Contribution

It introduces a method using HeII 1640 to identify biases in SDSS pipeline redshifts and demonstrates the variability of CIV blueshifts, challenging previous assumptions.

Findings

HeII-based redshifts align with [OII] within uncertainties.

Significant biases exist in BOSS pipeline redshifts for some quasars.

CIV blueshifts vary widely, from negligible to thousands of km/s.

Abstract

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 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 HeII 1640. HeII 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 [OII] to within 1-sigma of the quadrature sum of the HeII and [OII] centroid measurement uncertainties. The large redshift differences of ~1000 km/s, on average, between the BOSS-pipeline and HeII-centroid redshifts suggest there are significant biases in a portion of BOSS quasar redshift measurements. Adopting the HeII-based redshifts shows that CIV does not exhibit a ubiquitous blueshift for all quasars, given the precision probed by our measurements. Instead, we find a distribution of CIV centroid blueshifts across our sample, with a dynamic range that (i) is wider than that previously reported for this line, and (ii) spans CIV 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.