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

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).

Accepted for publication in ApJ (19 pages, 6 figures)

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