BLAZ4R and the eROSITA view of z>4 blazars

Abstract

We present BLAZ4R, the first living catalog of confirmed $z>4$ blazars, with a focus on the contribution of eROSITA, on board of the Spectrum Roentgen Gamma (SRG) spacecraft. Blazars at $z>4$ are rare but powerful probes of how active supermassive black holes evolve in connection to relativistic jets, in the first 2 billion years of cosmic history. At these redshifts, X-ray observations are essential for constraining blazars jet power and orientation, enabling effective trace of their parent population. The all-sky surveys provided by eROSITA ensure X-ray detection for BLAZ4R sources and, in some cases, allow spectral and temporal studies of their jetted emission. BLAZ4R includes 54 confirmed blazars, characterized through their X-ray properties, radio spectra and morphology, and multiwavelength profiles. We confirm that jetted sources are significantly more numerous relative to non-jetted counterparts at high-$z$, and that blazars (and therefore the overall jetted population) do not exhibit significantly different features compared to the total active galactic nuclei population in the early Universe. Fast accretion processes that involve relativistic jets are clearly required to justify the existence of fully formed jetted AGN at $z>4$. However, the diverse multiwavelength properties characterizing BLAZ4R do not yet allow us to identify the specific signatures of these processes. We will continue updating BLAZ4R to search for such signatures and ultimately understand the early formation of jetted AGN.