From 100 MHz to 10 GHz: Unveiling the spectral evolution of the X-shaped radio galaxy in Abell 3670

TL;DR

This study investigates the spectral evolution of the X-shaped radio galaxy MRC 2011-298 in Abell 3670 using multi-frequency radio observations, revealing jet reorientation, age differences in lobes and wings, and possible black hole coalescence effects.

Contribution

First detailed spectral analysis of MRC 2011-298 across multiple frequencies, confirming jet reorientation and jet precession, and linking observations to formation models involving black hole mergers.

Findings

Spectral steepening from lobes to wings

Maximum radiative age ~80 Myr, wings older by >30 Myr

Jet reorientation by ~90 degrees and precession with ~10 Myr period

Abstract

X-shaped radio galaxies (XRGs) are characterised by two pairs of misaligned lobes, namely the active lobes hosting radio jets and the wings. None of the formation mechanisms proposed so far can exhaustively reproduce the diverse features observed among XRGs. The emerging evidence is the existence of sub-populations of XRGs forming via different processes. The brightest cluster galaxy in Abell 3670 (A3670) is a dumbbell system hosting the XRG MRC 2011-298. The morphological and spectral properties of this interesting XRG have been first characterised through Karl G. Jansky Very Large Array (JVLA) data at 1-10 GHz. In the present work, we followed-up MRC 2011-298 with the upgraded Giant Metrewave Radio Telescope (uGMRT) at 120-800 MHz to further constrain its properties and origin. We carried out a detailed spectral analysis sampling different spatial scales. Integrated radio spectra, spectral index maps, radio colour-colour diagrams, and radiative age maps of both the active lobes and prominent wings have been employed to test the origin of the source. We confirm a progressive spectral steepening from the lobes to the wings. The maximum radiative age of the source is $\sim 80$ Myr, with the wings being older than the lobes by $\gtrsim 30$ Myr in their outermost regions. The observed properties are in line with an abrupt reorientation of the jets by $\sim 90$ deg from the direction of the wings to their present position. This formation mechanism is further supported by the comparison with numerical simulations in the literature, which additionally highlight the role of hydrodynamic processes in the evolution of large wings such as those of MRC 2011-298. Potentially, the coalescence of supermassive black holes could have triggered the spin-flip of the jets. Moreover, we show that the S-shape of the radio jets is likely driven by precession with a period $P \sim 10$ Myr.