X-Ray Nuclei in Radio Galaxies: Exploring the Roles of Hot and Cold Gas Accretion

TL;DR

This study uses X-ray observations to differentiate the accretion modes of radio galaxy nuclei, revealing that low-excitation sources are jet-dominated with inefficient accretion, while high-excitation sources involve radiatively efficient accretion disks.

Contribution

It introduces a model linking hot and cold gas accretion to the excitation states of radio galaxies, clarifying their X-ray and optical properties and their relation to jet-environment interactions.

Findings

Low-excitation sources lack accretion-related X-ray emission.

High-excitation sources show heavily absorbed, radiatively efficient accretion disks.

Accretion of hot IGM gas powers low-excitation sources, cold gas powers high-excitation sources.

Abstract

We present results from Chandra and XMM-Newton spectroscopic observations of the nuclei of z<0.5 radio galaxies and quasars from the 3CRR catalog, and examine in detail the dichotomy in the properties of low- and high-excitation radio galaxies. The X-ray spectra of low-excitation sources (those with weak or absent optical emission lines) are dominated by unabsorbed emission from a parsec-scale jet, with no contribution from accretion-related emission. These sources show no evidence for an obscuring torus, and are likely to accrete in a radiatively inefficient manner. High-excitation sources (those with prominent optical emission lines), on the other hand, show a significant contribution from a radiatively efficient accretion disk, which is heavily absorbed in the X-ray when they are oriented close to edge-on with respect to the observer. However, the low-excitation/high-excitation division does not correspond to the FRI/FRII division: thus the Fanaroff-Riley dichotomy remains a consequence of the interaction between the jet and the hot-gas environment through which it propagates. Finally, we suggest that accretion of the hot phase of the IGM is sufficient to power all low-excitation radio sources, while high-excitation sources require an additional contribution from cold gas that in turn forms the cold disk and torus. This model explains a number of properties of the radio-loud active galaxy population, and has important implications for AGN feedback mechanisms.