The Dragonfly Galaxy. III. Jet Brightening of a High-redshift Radio Source Caught in a Violent Merger of Disk Galaxies

Abstract

The Dragonfly galaxy (MRC 0152-209), the most infrared-luminous radio galaxy at redshift z ∼ 2, is a merger system containing a powerful radio source and large displacements of gas. We present kiloparsec-resolution data from the Atacama Large Millimeter/submillimeter Array and the Very Large Array of carbon monoxide (6−5), dust, and synchrotron continuum, combined with Keck integral field spectroscopy. We find that the Dragonfly consists of two galaxies with rotating disks that are in the early phase of merging. The radio jet originates from the northern galaxy and brightens when it hits the disk of the southern galaxy. The Dragonfly galaxy therefore likely appears as a powerful radio galaxy because its flux is boosted into the regime of high-z radio galaxies by the jet–disk interaction. We also find a molecular outflow of (1100 ± 550) M ⊙ yr−1 associated with the radio host galaxy, but not with the radio hot spot or southern galaxy, which is the galaxy that hosts the bulk of the star formation. Gravitational effects of the merger drive a slower and longer-lived mass displacement at a rate of (170 ± 40) M ⊙ yr−1, but this tidal debris contains at least as much molecular gas mass as the much faster outflow, namely M H2 = (3 ± 1) × 109 (α CO/0.8) M ⊙. This suggests that both the active-galactic-nucleus-driven outflow and mass transfer due to tidal effects are important in the evolution of the Dragonfly system. The Keck data show Lyα emission spread across 100 kpc, and C iv and He ii emission across 35 kpc, confirming the presence of a metal-rich and extended circumgalactic medium previously detected in CO(1–0).

CO Survey of High-z Radio Galaxies, Revisited with the Atacama Large Millimeter/submillimeter Array: Jet–Cloud Alignments and Synchrotron Brightening by Molecular Gas in the Circumgalactic Environment

“…Moreover, the 4.7 and 8.2 GHz measurements of the "lobe" predominantly reflect the hot-spot regions (Section 3.2), which are the working surface where the lobe interacts with the CGM at subrelativistic speeds. Although some Doppler boosting likely still occurs for the main hot-spot region, this typically results in a brightening of the flux density by a factor of at most a few (Komissarov & Falle 1996), i.e., less than what we observe for MRC 0114-211, MRC 0156-252, MRC 2048-272, and also MRC 0152-209 (Dragonfly Galaxy; Lebowitz et al 2023).…”

“…Under this scenario that jet-cloud interactions brighten the radio synchrotron emission, our observed alignments between radio sources and CO-emitting gas reservoirs could in part be due to intrinsically fainter radio sources being pushed into the flux-selected samples of high-z radio galaxies (see also Eales 1992). This scenario was also suggested in the recent study of the Dragonfly Galaxy (MRC 0152-209) by Lebowitz et al (2023). The scenario where jet-cloud interactions convert jet power into radio luminosity also agrees with studies of mostly ionized and neutral gas around low-z radio sources (e.g., van Breugel et al 1985;Fosbury et al 1998;Villar-Martín et al 1999Tadhunter et al 2000;Morganti et al 2002Morganti et al , 2011Murthy et al 2020) and has been proposed to explain the relatively large fraction of young and compact radio sources seen among starbursting radio galaxies at low-z (Tadhunter et al 2011).…”

“…1. MRC 0152-209 (Dragonfly Galaxy): For the Dragonfly Galaxy, Lebowitz et al (2023) concluded in a recent paper that the radio source brightens as it interacts with the molecular gas in the disk of a merging companion galaxy (but see also Zhong et al 2023). This means that the Dragonfly Galaxy follows the trend seen in Figure 4, with S main /S counter ∼13 at 4.7 GHz.…”

“…Based on a K-S test, we show that, at a statistical significance of 99.5%, there is a difference in this ratio of the lobe brightness between high-z radio galaxies with CO detected in the environment and the larger population of high-z radio galaxies. This suggests that the cold gas affects the properties of the radio source, and that the alignment effect may in part be caused by intrinsically fainter radio sources, the brightness of which is enhanced enough to enter the flux-selected samples of high-z radio galaxies (see also Lebowitz et al 2023). The radio source may also affect the molecular gas reservoir though enrichment and cooling, but this is more speculative from our data.…”

“…This ATCA survey revealed that five out of 13 high-z radio galaxies in the sample (38%) contain detectable amounts of CO(1-0) in the halo environment, preferentially aligned along the radio axis. One of these systems, MRC 0152-209 (also known as the "Dragonfly Galaxy"), was observed with the Atacama Large Millimeter/ submillimeter Array (ALMA) at high resolution in CO(6-5), which revealed that the radio jet aligns and interacts with molecular gas in the disk of a merging companion galaxy (Lebowitz et al 2023). For three other systems observed as part of the ATCA survey, namely MRC 0114-211, MRC 0156-252, and MRC 2048-272, all of the CO(1-0) emission was detected in a single molecular gas reservoir, located beyond the brightest edge of the radio source.…”