CO survey of high-z radio galaxies, revisited with ALMA: Jet-cloud Alignments and Synchrotron Brightening by Molecular Gas in the Circumgalactic Environment

TL;DR

This study uses ALMA observations to explore the alignment of radio jets with molecular gas in high-redshift galaxies, revealing that molecular gas can enhance radio brightness and is often found along jet axes beyond the main lobes.

Contribution

It provides new observational evidence linking molecular gas distributions with radio jet properties in distant galaxies, highlighting the impact of the circumgalactic medium on radio source brightness.

Findings

Molecular gas aligns with radio jets beyond the main lobes.

Radio brightness increases when jets encounter cold molecular gas.

Significant molecular gas exists in the circumgalactic environment, often with low excitation.

Abstract

Powerful radio sources associated with super-massive black holes are among the most luminous objects in the Universe, and are frequently recognized both as cosmological probes and active constituents in the evolution of galaxies. We present alignments between radio jets and cold molecular gas in the environment of distant radio galaxies, and show that the brightness of the radio synchrotron source can be enhanced by its interplay with the molecular gas. Our work is based on CO J>1 observations with the Atacama Large Millimeter/submillimeter Array (ALMA) of three radio galaxies with redshifts in the range 1.4 < z < 2.1, namely MRC 0114-211 (z = 1.41), MRC 0156-252 (z = 2.02), and MRC 2048-272 (z = 2.05). These ALMA observations support previous work that found molecular gas out to 50 kpc in the circumgalactic environment, based on a CO(1-0) survey performed with the Australia Telescope Compact Array (ATCA). The CO emission is found along the radio axes but beyond the main radio lobes. When compared to a large sample of high-z radio galaxies from the literature, we find that the presence of this cold molecular medium correlates with an increased flux-density ratio of the main vs. counter lobe. This suggest that the radio lobe brightens when encountering cold molecular gas in the environment. While part of the molecular gas is likely related to the interstellar medium (ISM) from either the host or a companion galaxy, a significant fraction of the molecular gas in these systems shows very low excitation, with r$_{2-1/1-0}$ and r$_{3-2/1-0}$ values $\lesssim$0.2. This could be part of the circumgalactic medium (CGM).