Speaker
Description
From extrapolations of contemporaneous X-ray active galactic nuclei (AGN) luminosity functions, the eROSITA all-Sky Survey (eRASS) is expected to contain ~100 X-ray ultra-luminous quasars, that emitted their light when the universe was less than a billion years old, at z>5.6. In the luminosity regime probed by eROSITA at these early times, the powerful X-ray output of these quasars is driven by rapid accretion onto black holes, often boosted by non-thermal jet contributions. eRASS reionization-era quasars are thus powerful probes of the evolution of black hole growth and active galactic nuclei (AGN) jet demographics. Theses objects should further shed light on the later stages of reionization and are prime subjects for feedback studies in young host galaxies, before building up a complex merger history and before the peak of star-formation. However, this tip-of-the-iceberg population is hidden away in a haystack of cool Galactic stellar objects, which share their red colours in the deepest optical/IR imaging surveys, but have a sky density that is many sorders of magnitude higher.
I will present the sample of all luminous quasars at z>5.5 with spectroscopic redshifts in the eRASS hemisphere. It was assembled by cross-matching all single-epoch and the stacked eRASS eRASS:4-5 to the latest compilation of ~400 quasars known at these cosmic time.
In addition, I have designed a quasar selection pipeline combining eRASS X-ray data with optical/IR imaging data from the Dark Energy Survey DR2, the Vista Hemisphere Survey DR5 and CatWISE2020. The core selection method relies on optical/IR SED template fitting and X-ray aperture photometry. The method led to discovery of two of the most X-ray luminous quasars at late cosmic dawn in eRASS: a blazar at z=5.6 and an X-ray variable broad-absorption line quasar at z=5.7.
High-redshift eRASS quasars appear in general X-ray over-luminous with respect to their disk UV emission. These findings hint at the existence of an aox-LUV outlier population that could not be sampled in the smaller footprints of previous X-ray surveys. I will discuss implications for the AGN population as a whole and the dominant emission mechanisms in these sources.
