Variability selected high-redshift quasars on SDSS Stripe 82

TL;DR

This paper presents a variability-based method for selecting high-redshift quasars in SDSS Stripe 82, significantly increasing the density of identified quasars and improving future selection strategies using sparser data.

Contribution

The authors developed and validated a variability-based quasar selection method that outperforms color-based strategies, enhancing high-redshift quasar detection in SDSS Stripe 82.

Findings

Achieved a quasar density of 24.0 deg^{-2} in Stripe 82.

Recovered 90% of known high-redshift quasars with only 8% false positives.

Including variability improves selection efficiency, especially for Broad Absorption Line quasars.

Abstract

The SDSS-III BOSS Quasar survey will attempt to observe z>2.15 quasars at a density of at least 15 per square degree to yield the first measurement of the Baryon Acoustic Oscillations in the Ly-alpha forest. To help reaching this goal, we have developed a method to identify quasars based on their variability in the u g r i z optical bands. The method has been applied to the selection of quasar targets in the SDSS region known as Stripe 82 (the Southern equatorial stripe), where numerous photometric observations are available over a 10-year baseline. This area was observed by BOSS during September and October 2010. Only 8% of the objects selected via variability are not quasars, while 90% of the previously identified high-redshift quasar population is recovered. The method allows for a significant increase in the z>2.15 quasar density over previous strategies based on optical (ugriz) colors, achieving a density of 24.0 deg^{-2} on average down to g~22 over the 220 deg^2 area of Stripe 82. We applied this method to simulated data from the Palomar Transient Factory and from Pan-STARRS, and showed that even with data that have sparser time sampling than what is available in Stripe 82, including variability in future quasar selection strategies would lead to increased target selection efficiency in the z>2.15 redshift range. We also found that Broad Absorption Line quasars are preferentially present in a variability than in a color selection.