Low-redshift Interlopers in the SDSS DR16 z  >  5 Quasar Catalog

Abstract

High-redshift quasars (z  >  5) are crucial for studying early black hole growth and reionization. Sloan Digital Sky Survey (SDSS) Data Release 16 catalogs 655 such sources with multiple redshift estimators. We systematically examined this subsample and found 480 sources (73.2%) where pipeline redshift (Z_DR16Q) disagrees with the derived estimate (Z_FIT) significantly. Visual inspection confirms that 125 of these (19.1% of the total) are actually low-redshift active galactic nuclei. In 114 cases, broad Mg ii with C iii and C iv was misidentified as Lyα; in 11 cases, Hα was misidentified as Lyα. This error is immediately revealed by the absence of Gunn–Peterson trough blueward of Lyα. Crucially, Z_FIT recovers correct redshift in the affected sources where emission lines are prominent, indicating the algorithm is sound; the issue lies in community preference for Z_DR16Q or Z_SPEC. We urge caution against uncritical use of pipeline redshifts for z  >  5 DR16Q sources and recommend Z_FIT as the preferred estimator.