A galaxy cluster in the innermost Zone of Avoidance, close to the radio phoenix VLSS J2217.5+5943
Authors: Kollatschny W. , Meusinger H. , Hoeft M. , Hill G. J. , Ochmann M. W. , Zeimann G. , Froebrich D. , Bhagat S. 
Published: June 2021

Context.Galaxy clusters grow via mergers with other clusters and groups. Extended regions of diffuse radio emission with a steep radio spectral index are thought to be indicators of such merger events. Extended radio sources with a significantly curved spectrum and a complex morphology have been found in several galaxy clusters. It has been proposed that these so-called radio phoenices are witnesses of cluster mergers and of the presence of active galactic nuclei prior to the merger. Shock fronts or turbulence induced by the mergers are believed to reenergize plasma emitted in the past active phase of a galaxy.

Aims.The steep spectrum radio source VLSS J2217.5+5943 shows a complex filamentary morphology and a curved spectrum. Therefore, the source has previously been classified as a radio phoenix. However, no galaxy cluster associated with this radio source had confidently been detected until now because the source is located in the direction of the innermost zone of the Galactic plane atb = +2.4°, the innermost Zone of Avoidance (ZoA). The main aim of this work is to identify galaxies that are part of a cluster at the location of VLSS J2217.5+5943, determine their redshifts, and analyze their connection with the radio source. The confirmation of a cluster would corroborate the classification of the radio source as a radio phoenix and demonstrate that extended, diffuse radio sources are useful indicators of the presence of a galaxy cluster, in particular in the innermost ZoA.

Methods.We analyzed archival observations in the near infrared and mid infrared (Spitzer) to select the galaxies in the immediate neighborhood of the radio source. A sample of 23 galaxies were selected as candidate cluster members. Furthermore, we carried out deep integral field spectroscopy covering 6450 to 10 500 Å with the red unit of theHobby-EberlyTelescope second generation low resolution spectrograph (LRS2-R). We also reanalyzed archival GMRT observations at 325 and 610 MHz.

Results.We selected 23 galaxies within a radius of 2.5 arcmin, centered on RA = 22h17m​.5, Dec = +59° 43′ (J2000). Spectra were obtained for three of the brightest galaxies. For two galaxies we derived redshifts ofz = 0.165 andz = 0.161, based on NaD absorption and TiO band heads. Their spectra correspond to E-type galaxies. Both galaxies are spatially associated with VLSS J2217.5+5943. The spectrum of the third galaxy, which is slightly farther from the radio source, indicates a LINER spectral type atz = 0.042. It is apparently a foreground galaxy with respect to the cluster we identified.

Conclusions.VLSS J2217.5+5943 is associated with a massive galaxy cluster at redshiftz = 0.163 ± .003, supporting its classification as a radio phoenix. The intrinsic properties of the radio source, computed for the cluster redshift, are in good agreement with those of other known radio phoenices. The identification of the galaxy cluster demonstrates that far-red spectroscopy with LRS2-R succeeds in determining the redshift of galaxies in the innermost ZoA. Moreover, it confirms that radio sources can be useful indicators of the presence of galaxy clusters in the ZoA.