Blazar nature of high-z radio-loud quasars

TL;DR

This study uses X-ray observations to classify high-redshift radio-loud quasars as blazars, revealing that massive jetted active galactic nuclei are likely dominant at z≥4, impacting our understanding of early supermassive black hole evolution.

Contribution

It provides the first systematic X-ray based classification of high-z radio-loud quasars, highlighting the prevalence of jetted AGN at early cosmic times.

Findings

Five sources confirmed as blazars based on X-ray data

Jets are common in high-z quasars, influencing black hole growth models

Radio-loudness may not reliably indicate high-energy jet activity at high redshift

Abstract

We report on the Swift/XRT observation and classification of eleven blazar candidates at $z>4$. These sources were selected as part of a sample of extremely radio-loud quasars, in order to focus on quasars with jets oriented roughly close to our line-of-sight. Deriving their viewing angles and their jets bulk Lorentz factors was crucial for a strict blazar classification, and it was possible only thanks to X-ray observations. Out of eleven sources, five show strong and hard X-ray fluxes, that allow their blazar classification, two are uncertain, three host relativistic jets that we observe just outside their beaming cone (i.e. are not strictly blazars), while one went undetected by Swift/XRT. Following this approach, we were able to trace the $>10^9M_\odot$ active supermassive black hole population hosted in jetted active galactic nuclei. At $z\geq4$ the massive jetted sources are likely predominant in the overall quasar population: this calls for a deep review of our understanding of the first supermassive black holes formation and evolution. Jets are indeed key actors in fast accretion, and must be searched for across the whole high redshift quasar population. A note of caution must be added: radio-loudness and in general radio features at high redshifts seem not to perfectly reflect high-energy properties. A strong effect due to interaction with CMB radiation is surely in place, that quenches the radio emission with respect to the X-rays, but also more frequent occasions for the jet to be bent seem to play a relevant role in this matter. Classifications and population studies thus must be carefully performed, in order not to be confused by these inconsistencies.