A Study of Quasar Selection in the Supernova Fields of the Dark Energy Survey

Abstract

We present a study of quasar selection using the supernova fields of the Dark Energy Survey (DES). We used a quasar catalog from an overlapping portion of the SDSS Stripe 82 region to quantify the completeness and efficiency of selection methods involving color, probabilistic modeling, variability, and combinations of color/ probabilistic modeling with variability. In all cases, we considered only objects that appear as point sources in the DES images. We examine color selection methods based on the Wide-field Infrared Survey Explorer2 color, a mixture of WISE and DES colors (g − i andiW 1), and a mixture of Vista Hemisphere Survey and DES colors (g − i and i − K ). For probabilistic quasar selection, we used XDQSO, an algorithm that employs an empirical multi-wavelength flux model of quasars to assign quasar probabilities. Our variability selection uses the multi-band χ 2 -probability that sources are constant in the DES Year 1 griz-band light curves. The completeness and efficiency are calculated relative to an underlying sample of point sources that are detected in the required selection bands and pass our data quality and photometric error cuts. We conduct our analyses at two magnitude limits, i<19.8 mag and i<22 mag. For the subset of sources with W1 and W2 detections, the -W W 1 2 color or XDQSOz method combined with variability gives the highest completenesses of >85% for both i-band magnitude limits and efficiencies of >80% to the bright limit and >60% to the faint limit; however, the giW1 and giW1+variability methods give the highest quasar surface densities. The XDQSOz method and combinations of W1W2/giW1/XDQSOz with variability are among the better selection methods when both high completeness and high efficiency are desired. We also present the OzDES Quasar Catalog of 1263 spectroscopically confirmed quasars from three years of OzDES observation in the 30 deg 2 of the DES supernova fields. The catalog includes quasars with redshifts up to z∼4 and brighter than i = 22 mag, although the catalog is not complete up to this magnitude limit.

The QUEST-La Silla AGN Variability Survey: Selection of AGN Candidates through Optical Variability

“…They tested various combinations of color and variability parameters, finding that using a combination of optical colors and variability parameters improves quasar classification efficiency and completeness over the use of colors alone. More recently, Tie et al (2017) used data from the supernova fields of the Dark Energy Survey (DES; Abbott et al 2018) to select quasars by combining color and variability selection methods. All these previous studies have shown the potential of selecting AGN candidates through variability analyses, demonstrating that variability-based techniques can increase considerably the number of AGN candidates in the redshift range where the colors of stars are similar to those of AGN.…”

OzDES multifibre spectroscopy for the Dark Energy Survey: 3-yr results and first data release

“…Briefly, the science objectives of OzDES include obtaining supernova (SN) host-galaxy redshifts for cosmology (e.g., Bazin et al 2011;Campbell et al 2013), spectroscopically classifying active transients, monitoring a sample of active galactic nuclei (AGN) for reverberation mapping (RM -see e.g., Bentz et al 2009;King et al 2015) and potentially for cosmology (Watson et al 2011;King et al 2014), securing redshifts for a wide variety of galaxies to be used for photometric redshift training (e.g., Sánchez et al 2014;Bonnett et al 2016), including a large sample of luminous red galaxies (LRGs; Banerji et al 2015), and using redshifts of selected galaxies to confirm their membership in clusters (e.g., Rozo et al 2016;Rykoff et al 2016). OzDES has already produced several discoveries, including spectroscopy of hundreds of active transients, many new QSOs (Tie et al 2017), and the first FeLoBAL QSO in a post-starburst galaxy (Mudd et al 2016). …”

Quasar Accretion Disk Sizes from Continuum Reverberation Mapping from the Dark Energy Survey

“…This project photometrically and spectroscopically monitors 771 quasars in order to measure the masses of their SMBHs (King et al 2015). These quasars are as faint as 23.6 in g and were selected heterogeneously with a broad range of quasar detection techniques (e.g., Banerji et al 2015;Tie et al 2017). From the combination of these two surveys, we use the first three years of photometry and spectra for the 771 quasars that we continue to monitor (Diehl et al 2016;Childress et al 2017).…”