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Identifying Missing Quasars from the DESI Bright Galaxy Survey

Lookup NU author(s): Dr Vicky FawcettORCiD

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Abstract

© 2025. The Author(s). Published by the American Astronomical Society.The Dark Energy Spectroscopic Instrument (DESI) cosmology survey includes a Bright Galaxy Survey (BGS), which will yield spectra for over 10 million bright galaxies (r < 20.2 AB mag). The resulting sample will be valuable for both cosmological and astrophysical studies. However, the star/galaxy separation criterion implemented in the nominal BGS target selection algorithm excludes quasar host galaxies in addition to bona fide stars. While this excluded population is comparatively rare (∼3-4 per square degrees), it may hold interesting clues regarding galaxy and quasar physics. Therefore, we present a target selection strategy that was implemented to recover these missing active galactic nuclei (AGN) from the BGS sample. The design of the selection criteria was both motivated and confirmed using spectroscopy. The resulting BGS-AGN sample is uniformly distributed over the entire DESI footprint. According to DESI survey validation data, the sample comprises 93% quasi-stellar objects (QSOs), 3% narrow-line AGN or blazars with a galaxy contamination rate of 2%, and a stellar contamination rate of 2%. Peaking around redshift z = 0.5, the BGS-AGN sample is intermediary between quasars from the rest of the BGS and those from the DESI QSO sample in terms of redshifts and AGN luminosities. The stacked spectrum is nearly identical to that of the DESI QSO targets, confirming that the sample is dominated by quasars. We highlight interesting small populations reaching z > 2, which are either faint quasars with nearby projected companions or very bright quasars with strong absorption features including the Lyα forest, metal absorbers, and/or broad absorption lines.

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Author(s): Juneau S, Canning R, Alexander DM, Pucha R, Fawcett VA, Myers AD, Moustakas J, Ruiz-Macias O, Cole S, Pan Z, Aguilar J, Ahlen S, Alam S, Bailey S, Brooks D, Chaussidon E, Circosta C, Claybaugh T, Davis TM, Dawson K, de la Macorra A, Dey A, Doel P, Fanning K, Forero-Romero JE, Gaztanaga E, Gontcho A Gontcho S, Gutierrez G, Hahn C, Honscheid K, Kehoe R, Kisner T, Kremin A, Lambert A, Landriau M, Le Guillou L, Manera M, Martini P, Meisner A, Miquel R, Munoz-Gutierrez A, Nie J, Palanque-Delabrouille N, Percival WJ, Poppett C, Prada F, Ravoux C, Rezaie M, Rossi G, Sanchez E, Schlafly EF, Schlegel D, Schubnell M, Seo H, Silber J, Siudek M, Sprayberry D, Tan T, Tarle G, Yeche C, Zhou Z, Zou H

Publication type: Article

Publication status: Published

Journal: Astronomical Journal

Year: 2025

Volume: 169

Issue: 3

Print publication date: 03/03/2025

Online publication date: 19/02/2025

Acceptance date: 23/12/2024

Date deposited: 10/03/2025

ISSN (print): 0037-8720

Publisher: American Astronomical Society

URL: https://doi.org/10.3847/1538-3881/adabc9

DOI: 10.3847/1538-3881/adabc9

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