2021ApJ...909..198H

We report on the discovery of a mysterious ultra-steep-spectrum (USS) synchrotron source in the galaxy cluster A2877. We have observed the source with the Murchison Widefield Array at five frequencies across 72-231 MHz and found the source to exhibit strong spectral curvature over this range, as well as the steepest known spectra of a synchrotron cluster source, with a spectral index across the central three frequency bands of α=-5.97_–0.48^+0.40. Higher-frequency radio observations, including a deep observation with the Australia Telescope Compact Array, fail to detect any of the extended diffuse emission. The source is approximately 370 kpc wide and bears an uncanny resemblance to a jellyfish with two peaks of emission and long tentacles descending south toward the cluster center. While the "USS Jellyfish" defies easy classification, we here propose that the phenomenon is caused by the reacceleration and compression of multiple aged electron populations from historic active galactic nucleus (AGN) activity, a so-called "radio phoenix," by an as yet undetected weak cluster-scale mechanism. The USS Jellyfish adds to a growing number of radio phoenix in cool-core clusters with unknown reacceleration mechanisms; as the first example of a polyphoenix, however, this implies that the mechanism is on the scale of the cluster itself. Indeed, we show that in simulations, emission akin to the USS Jellyfish can be produced as a short-lived transient phase in the evolution of multiple interacting AGN remnants when subject to weak external shocks.