Description
High-redshift (z > 5) quasars provide a unique window into the early growth of supermassive black holes, their coevolution with host galaxies, and the evolving ionisation state of the early Universe. However, traditional high-z quasar selection methods, such as colour-based selection, suffer from high false-positive rates and require resource-intensive spectroscopic follow-up. Efficient selection techniques are needed to effectively leverage the anticipated data volume from deep, wide-field surveys LSST and Euclid. We explore optical and infrared spectral energy distribution (SED) fitting as a method for both selecting and characterising high-z quasars, with a focus on obtaining accurate photometric redshifts. Our approach involves fitting candidate objects with SED models for quasars as well as foreground contaminants, such as low-redshift galaxies and cool dwarf stars. This allows us to remove contaminants and characterise key quasar properties, including luminosity, reddening, and host galaxy contributions. We present results from tests on a spectroscopically confirmed sample of quasars from the Million Quasars catalogue, as well as new quasar candidates identified using this method in multiwavelength data. Specifically, we combine eROSITA X-ray sources with DECam optical, VISTA near-infrared, and WISE mid-infrared broadband photometry. Additionally, we introduce an improved model for intergalactic absorption from neutral hydrogen, which reduces bias in photometric redshift estimates at z > 4. This methodology shows strong potential for identifying high-z quasars and reliably measuring their properties, offering an efficient approach for studying quasars in the early universe.
