Detection of the cosmological evolution of the Doppler factor for blazars jets

TL;DR

This study reveals a positive cosmic evolution of the Doppler factor in blazar jets, indicating that relativistic jets are more prevalent at high redshifts, which impacts our understanding of supermassive black hole growth.

Contribution

It provides the first measurement of the redshift dependence of the Doppler factor in blazars using gamma-ray spectra and optical light curves, demonstrating an evolving jet velocity over cosmic time.

Findings

Doppler factor scales as (1+z)^{0.8} from gamma-ray spectra

Doppler factor scales as (1+z)^{1.1} from optical light curves

Higher evolution index observed in low-luminosity sources

Abstract

The Doppler factor ($\delta$) is a fundamental quality for the relativistic jets from active galactic nuclei (AGNs). It is also fundamental to assessing the number of the entire population of the jetted AGN at high redshift ($z$), and therefore is important for tracing the growth of supermassive black holes (SMBHs) across cosmic time. Here we present the identification of the positive cosmic evolution of the Doppler factor in {\it Fermi}-detected bright $\gamma-$ray blazars. The redshift dependence of the Doppler factor, $\delta\propto(1+z)^{0.8}$, is measured from the observed characteristic energies in the gamma-ray spectra of 141 blazars. Moreover, the analysis of the characteristic timescales derived from modeling the long-term optical light curves of 89 blazars with Gaussian process regression gives $\delta\propto(1+z)^{1.1}$, but with a larger scatter. \textbf{Note that each index is derived from the entire sample, representing an average evolution. Interestingly, the index itself also appears to evolve, with low-luminosity sources showing stronger evolution, as indicated by a larger index.} This detection straightly suggests that relativistic jets from AGNs are much more common at high redshifts than what is previously estimated.