# CMB-induced radio quenching of high-redshift jetted AGNs with highly magnetic hotspots

## TL;DR

This study investigates how the Cosmic Microwave Background (CMB) causes high-redshift jetted AGNs to appear radio-quenched by analyzing X-ray and radio data, revealing CMB quenching effects in specific radio galaxies.

## Contribution

It provides observational evidence for CMB-induced radio quenching in high-redshift AGNs through detailed spectral energy distribution modeling and analysis of X-ray and radio data.

## Key findings

- CMB quenching effectively suppresses extended radio emission in high-z AGNs.
- Hotspot magnetic energy density exceeds CMB energy density, indicating hotspots are the source of synchrotron emission.
- Inverse Compton scattering of CMB photons dominates the X-ray emission from lobes.

## Abstract

In an effort to understand the cause of the apparent depletion in the number density of radio-loud AGNs at $z>3$, this work investigates the viability of the so-called Cosmic Microwave Background (CMB) quenching mechanism of intrinsically jetted, high-z AGNs, whereby Inverse Compton scattering of CMB photons off electrons within the extended lobes results in a substantial dimming of the lobe synchrotron emission at GHz frequencies, while simultaneously boosting their diffuse X-ray signal. We focus on five $z>3.5$ radio galaxies that have sufficiently deep Chandra exposure (> 50 ks) to warrant a meaningful investigation of any extended X-ray emission. For those objects with evidence for statistically significant extended X-ray lobes (4C 41.17 and 4C 03.24), we combine the Chandra measurements with literature data at lower frequencies to assemble the systems' Spectral Energy Distributions (SEDs), and utilize state-of-the-art SED modelling (Ghisellini et al. 2015) -- including emission from the disk, torus, jet, hotspots, and lobes -- to infer their physical parameters. For both radio galaxies, the magnetic energy density in the hotspots is found to exceed the energy density in CMB photons, wheres the opposite is true for the lobes. This implies that any extended synchrotron emission likely originates from the hotspots themselves, rather than the lobes. Conversely, Inverse Compton scattering of CMB photons dominates the extended X-ray emission from the lobes, which are effectively "radio-quenched". As a result, CMB quenching is effective in these systems in spite of the fact that the observed X-ray to radio luminosity ratio does not bear the signature $(1+z)^4$ dependence of the CMB energy density.