The great Kite in the sky: A LOFAR observation of the radio source in Abell 2626

Abstract

Context. The radio source at the center of the galaxy cluster Abell 2626, also known as the Kite, stands out for its unique morphology composed of four symmetric arcs. Previous studies have probed the properties of this source at different frequencies and its interplay with the surrounding thermal plasma, but the puzzle of its origin is still unsolved. Aims. We use a new LOw-Frequency ARray (LOFAR) observation from the LOFAR Two-meter Sky Survey at 144 MHz to investigate the origin of the Kite. Methods. We present a detailed analysis of the new radio data, which we combined with archival radio and X-ray observations. We produced a new, resolved spectral index map of the source with a resolution of 7″ and we studied the spatial correlation of radio and X-ray emission to investigate the interplay between thermal and nonthermal plasma. Results. The new LOFAR data changed our view of the Kite because we discovered two steep-spectrum (α < −1.5) plumes of emission connected to the arcs. The spectral analysis shows, for the first time, a spatial trend of the spectrum along the arcs with evidence of curved synchrotron spectra and a spatial correlation with the X-ray surface brightness. On the basis of our results, we propose that the Kite was originally an X-shaped radio galaxy whose fossil radio plasma, after the end of the activity of the central active galactic nucleus, has been compressed as a consequence of motions of the thermal plasma encompassing the galaxy. The interplay between the compression and advection of the fossil plasma, with the restarting of the nuclear activity of the central galaxy, could have enhanced the radio emission of the fossil plasma producing the arcs of the Kite. We also present the first, low-frequency observation of a jellyfish galaxy in the same field, in which we detect extended, low-frequency emission without a counterpart at higher frequencies.

LoTSS jellyfish galaxies

“…For star-forming galaxies experiencing RPS, tails of synchrotron emission may be observed as these cosmic rays are stripped from the galaxy disk. In fact LOFAR has already detected extended emission at 144 MHz around a known jellyfish galaxy in Abell 2626 (Poggianti et al 2019b;Ignesti et al 2020). Additionally, previous works have observed RPS features in radio continuum within nearby clusters (primarily around 1.4 GHz, Gavazzi & Jaffe 1987;Miller et al 2009;Murphy et al 2009;Vollmer et al 2009;Chen et al 2020;Müller et al 2021).…”

The galaxy group NGC 507: newly detected AGN remnant plasma transported by sloshing

“…Finally, in some cases such as the galaxy group Nest200047 (Brienza et al 2021) and Abell 2626 (Ignesti et al 2020b), filaments of radio emission likely represent regions where old AGN remnant plasma gets accumulated under the influence of buoyancy combined with cluster/group weather. Recent simulations by Vazza et al (2021) show indeed how the magnetised remnant plasma from AGN can be shredded into a multitude of filamentary structures and dispersed over large spatial scales under the influence of the cluster dynamics.…”

Role of Magnetic Fields in Ram Pressure Stripped Galaxies

“…JW100 also shows an extended, non-thermal radio emission. Although a detailed polarimetric analysis has not been carried out yet, its emission has been detected in a wider range of frequencies than JO206, extending from 144 MHz to 5.5 GHz [44][45][46]. A combined analysis has recently been investigated within the GASP collaboration involving, for the first time, LOFAR, MeerKAT and VLA observations [24].…”

“…Finally, the advent of deep radio surveys made possible by the new-generation facilities such as LOFAR and SKA in the future provide us the opportunity to investigate the properties of jellyfish magnetic fields on larger samples of galaxies and to evaluate the diversity of the cluster potential onto the connection of the magnetic field configuration and strength to the star-forming regions. Moreover, as outlined in [27,28,46], low-frequency LOFAR observations provide us an unique insight into the low-energy electrons travelling along the stripped tails and the physical phenomena that drive their evolution. Future synergies between LOFAR, MeerkAT, VLA, and SKA will allow extending this kind of analysis to larger samples of galaxies, thus deepening our knowledge of these fascinating systems.…”