The VST ATLAS Quasar Survey: Catalogue and Weak Gravitational Lensing Analyses

Abstract

We develop selection criteria for a g<22.5 quasar catalogue based on VST ATLAS+unWISE and aimed at competing with DESI to reach a quasar sky density of 130 deg^−2. Ultimately this catalogue can be used to help select quasar targets for the eROSITA AGN+4MOST Cosmology Redshift Surveys. To guide our selection, we use deep X-ray/optical/NIR/MIR data in the extended William Herschel Deep Field (WHDF) as well as the selections performed on DECaLS and NEOWISE data by DESI Collaboration et al.(2016). Accordingly adjusting our ATLAS+unWISE criteria and applying over the ∼4000 deg^2 ATLAS survey gives us a total number of quasar candidates to g<22.5 of ∼704000 i.e.a sky density of 170 deg^−2 of which ∼85% or 600000 are likely to be 0.7<z<2.3 quasars as confirmed by comparisons with 2QZ, eBOSS and 2QDESp spectroscopic surveys. We find that the QSO sky density at g<22.5 may be as high as 204 deg^−2 with the inclusion of quasars morphologically mis-classified as galaxies that otherwise satisfy the usual ugri and giW1 selections. We assume an efficiency of 60% for these quasar candidates, therefore giving us a final quasar sky density of ∼120 deg^−2. We then cross-correlate a low-contamination subset of this QSO catalogue with g<20.5 galaxy clusters and r<21 galaxies to detect quasar magnification bias caused by weak lensing. From this analysis we confirm previous results of Myers et al. (2003) and Scranton et al. (2005). We also detect lensing of the Cosmic Microwave Background by our quasar sample, finding remarkable agreement between our result and the ones achieved by Geach et al. (2019) and Chehade et al. (2016).