A JVLA, GMRT, and XMM study of Abell 795: Large-scale sloshing and a candidate radio phoenix

¹ Dipartimento di Fisica e Astronomia, Università di Bologna, Via Gobetti 93/2, 40129 Bologna, Italy
² Istituto Nazionale di Astrofisica – Istituto di Radioastronomia (IRA), Via Gobetti 101, 40129 Bologna, Italy
³ Istituto Nazionale di Astrofisica – Istituto di Astrofisica Spaziale e Fisica cosmica (IASF), Via A. Corti 12, I-20133 Milano, Italy
⁴ Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM de Paris-Saclay, 91191 Gif-sur-Yvette, France
⁵ University of California Observatories/Lick Observatory, Department of Astronomy and Astrophysics, Santa Cruz, CA 95064, USA
⁶ INAF-Osservatorio di Astrofisica e Fisica dello Spazio, Via Gobetti 101, I-40129 Bologna, Italy

^⋆ Corresponding author; nicolo.rotella@studio.unibo.it

Received: 21 January 2025
Accepted: 25 March 2025

Abstract

Aims. The galaxy cluster Abell 795 (z = 0.1374) is known from previous works for the presence of extended (≈200 kpc) radio emission with a steep spectral index of an unclear origin surrounding the brightest cluster galaxy (BCG), and for the sloshing signatures visible in Chandra observations of its cool core. Our purpose is to investigate the nature of the extended radio emission and its possible link with the dynamical state of the intracluster medium (ICM) on large scales (≈1 Mpc).

Methods. We used new JVLA 1.5 GHz, as well as archival GMRT 325 MHz and XMM-Newton X-ray observations to study the cluster with a thermal and nonthermal approach.

Results. From the XMM surface brightness analysis, we detected an azimuthally asymmetric excess reaching around 650 kpc from the center of Abell 795. The excess appears to follow the sloshing spiral previously detected, but with the existing XMM data it is not possible to confirm its classification as a large-radius cold front in Abell 795. Furthermore, the X-ray data allowed us to detect the hot gas from a previously unknown galaxy group at a projected distance of ≈7.4′ (1 Mpc) northwest of Abell 795. Its surface brightness radial profile is well-fitted with a β model of slope β = 0.52 ± 0.17, and the spectral analysis reveals a thermal plasma of temperature kT = 1.08 ± 0.08 keV and metallicity Z = 0.13 ± 0.06 Z_⊙. We discuss the possibility that this galaxy group is the perturber that initiated sloshing in Abell 795, and we show that the velocity distribution of member galaxies supports the dynamically unrelaxed nature of Abell 795. The analysis of JVLA 1.5 GHz and GMRT 325 MHz images confirms the presence of extended radio emission with the largest linear size ≈200 kpc, preferentially extended toward southwest and terminating in a sub-component (“SW blob”). We measured the spectral indices between 325 MHz and 1.5 GHz, finding α_Ext = −2.24 ± 0.13 for the diffuse extended emission, and α_SWb = −2.10 ± 0.13 for the SW blob. These ultra-steep spectral index values, coupled with the complex morphology and cospatiality with the radio-loud active galactic nuclei (AGN) present in the BCG, suggest that this extended emission could be classified as a radio phoenix, possibly arising from adiabatic compression of an ancient AGN radio lobe due to the presence of sloshing motions.