The Impact of the CMB on the Evolution of high-z Blazars

TL;DR

This paper investigates whether the inverse Compton scattering of CMB photons by relativistic electrons in blazar jets explains the observed increase in X-ray-to-radio luminosity ratios with redshift and the differing space density evolutions in X-ray and radio-selected blazars.

Contribution

It demonstrates that the IC/CMB model can account for the redshift evolution of luminosity ratios and the different cosmological evolutions in blazar samples, supported by analysis of complete samples across a wide redshift range.

Findings

IC/CMB explains the luminosity ratio trend with redshift.

The model accounts for different space density evolutions.

Predictions align with observations up to z~6-7.

Abstract

Different works have recently found an increase of the average X-ray-to-radio luminosity ratio with redshift in the blazar population. We evaluate here whether the inverse Compton interaction between the relativistic electrons within the jet and the photons of the Cosmic Microwave Background (IC/CMB) can explain this trend. Moreover, we test whether the IC/CMB model can also be at the origin of the different space density evolutions found in X-ray and radio selected blazar samples. By considering the best statistically complete samples of blazars selected in the radio or in the X-ray band and covering a large range of redshift (0.5$\lesssim$ z$\lesssim$ 5.5), we evaluate the expected impact of the CMB on the observed X-ray emission on each sample and then we compare these predictions with the observations. We find that this model can satisfactorily explain both the observed trend of the X-ray-to-radio luminosity ratios with redshift and the different cosmological evolutions derived from the radio and X-ray band. Finally, we discuss how currently on-going X-ray missions, like eROSITA, could help to further constrain the observed evolution at even higher redshifts (up to z$\sim$6-7).