Low-redshift Interlopers in the SDSS DR16 z > 5 Quasar Catalog
Year: 2026
Type: article
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 Ly α ; in 11 cases, H α was misidentified as Ly α . This error is immediately revealed by the absence of Gunn–Peterson trough blueward of Ly α . 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.
Source: Research Notes of the AAS
Authors: Vivek Kumar Jha, Harikumar N, Yogesh Wadadekar
Institution: Tata Institute of Fundamental Research
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: hybrid
OpenAlex ID: W7212153301