The impact of the CMB on the evolution of high-z blazars

Abstract

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 ≲ z ≲ 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 extended ROentgen Survey with an Imaging Telescope Array, could help to further constrain the observed evolution at even higher redshifts (up to z ∼ 6–7).