4MOST ChANGES: Catalog of high-redshift quasar candidates (4.5 < $z$ < 7) selected with SED fitting

Abstract

The identification of high-redshift quasars ($z > 4.5$) is critical for studying the early Universe, supermassive black hole growth, and cosmic reionization. Most known high-redshift quasars are located in the northern hemisphere, leaving the southern sky largely unexplored. As part of the 4-meter Multi-Object Spectroscopic Telescope (4MOST) and Chilean AGN/Galaxy Extragalactic Survey (ChANGES) S1604 survey, we aim to create a large catalog of high-redshift quasar candidates in the southern hemisphere using multiwavelength photometry and Spectral Energy Distribution (SED) fitting, with the goal of spectroscopic follow-up with 4MOST. We construct a multi-band photometric catalog by combining optical data from DELVE DR2 and DECaLS DR10, near-infrared data from VHS DR5 with an additional field of VIKING, mid-infrared data from AllWISE, and optical astrometry from Gaia DR3. After applying morphological and color-based cuts, we perform SED fitting using quasar and brown dwarf templates. Statistical outputs, including $χ^2$, BIC, and F-test are used to rank and select candidates. Our final catalog contains 6104 high-redshift quasar candidates within $4.5 < z < 7$, with detections in 7 or more photometric bands satisfying our SED-based selection criteria (BIC > 0 and F-test > 10). Spectroscopic validation using NTT/EFOSC2 and Palomar/NGPS confirmed 3 high-redshift quasars at $z > 5$ out of 6 observed candidates.