High redshift blazars

TL;DR

High redshift blazars, characterized by relativistic jets pointing towards us, are key to understanding the early universe's massive black holes and their evolution, especially through hard X-ray observations.

Contribution

This paper highlights the importance of hard X-ray surveys in detecting powerful high-z blazars and their role in studying early black hole growth.

Findings

High-z blazars peak in the MeV band, making hard X-ray observations crucial.

Powerful high-z blazars likely host supermassive black holes exceeding a billion solar masses.

Searches for high-z blazars complement radio-quiet quasar studies in understanding black hole evolution.

Abstract

Blazars are sources whose jet is pointing to us. Since their jets are relativistic, the flux is greatly amplified in the direction of motion, making blazars the most powerful persistent objects in the Universe. This is true at all frequencies, but especially where their spectrum peaks. Although the spectrum of moderate powerful sources peaks in the ~GeV range, extremely powerful sources at high redshifts peak in the ~MeV band. This implies that the hard X-ray band is the optimal one to find powerful blazars beyond a redshift of ~4. First indications strongly suggest that powerful high-z blazars harbor the most massive and active early black holes, exceeding a billion solar masses. Since for each detected blazars there must exist hundreds of similar, but misaligned, sources, the search for high-z blazars is becoming competitive with the search of early massive black holes using radio-quiet quasars. Finding how the two populations of black holes (one in jetted sources, the other in radio-quiet objects) evolve in redshift will shed light on the growth of the most massive black holes and possibly on the feedback between the central engine and the rest of the host galaxy.