On the quenching of LRD X-ray emission by both Compton-thick gas and high accretion rates

TL;DR

This study models how dense, Compton-thick gas cocoons and high accretion rates suppress X-ray emission in high-redshift supermassive black hole candidates, explaining their X-ray faintness.

Contribution

It provides the first quantitative predictions of X-ray attenuation in LRDs based on optical and near-infrared spectral constraints, highlighting the roles of gas column density and metallicity.

Findings

X-ray faintness requires Compton-thick gas with N_H~10^25 cm^-2 and moderate metallicity.

High intrinsic X-ray emission would make LRDs detectable, implying they are not chemically pristine.

Low metallicity objects could be detected even with low intrinsic X-ray luminosity.

Abstract

Little Red Dots (LRDs), candidate high-redshift supermassive black holes accreting in dense gas, remain undetected in X-rays. In previous work, we provided the first quantitative models that reproduce the optical and near-infrared spectra of LRDs with the Sirocco radiative transfer code, thereby constraining the properties of the surrounding gas. Here, we use those constraints to predict the X-ray attenuation produced by dense gas cocoons, and explore its dependence on Balmer-break strength, metallicity, intrinsic X-ray SED, and observed bandpass as a function of redshift. We find that the X-ray constraints are very tight, requiring both extinction by a Compton-thick gas column $N_{\rm H}\sim10^{25}\,{\rm cm}^{-2}$ with moderate metallicity ($0.05$-$0.1\,Z_\odot$) and intrinsically weak X-ray emission (bolometric to X-ray luminosity ratio, $k_{\rm bol,X}\gtrsim 30$) as observed in high accretion rate, narrow-line AGN, to make LRDs sufficiently faint to evade detection. Intrinsically bright X-ray emitters as seen in typical broad-line AGN would be detected even behind the typical modest metallicity, Compton-thick gas columns inferred from the optical spectra. Very low metallicity objects could be detected in X-rays even with low intrinsic X-ray luminosities, suggesting that LRDs are not (currently) chemically pristine.