Identifying Quasars from the DESI Bright Galaxy Survey

TL;DR

This paper develops a new selection strategy to recover quasars and active galactic nuclei from the DESI Bright Galaxy Survey, enhancing the sample with spectroscopically confirmed AGN, including high-redshift quasars, for cosmological and astrophysical research.

Contribution

The paper introduces a novel target selection method to identify quasars within the DESI BGS, improving the completeness of AGN samples beyond existing star/galaxy separation criteria.

Findings

The BGS-AGN sample is 93% quasars, with 3% narrow-line AGN or blazars.

The sample is uniformly distributed across the DESI footprint.

The stacked spectrum confirms the sample is dominated by quasars.

Abstract

The Dark Energy Spectroscopic Instrument (DESI) cosmology survey includes a Bright Galaxy Survey (BGS) which will yield spectra for over ten 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 Lyman-apha forest, metal absorbers and/or broad absorption lines.