Nonthermal phenomena in the center of Abell 1775

An 800 kpc head-tail, revived fossil plasma and slingshot radio halo^⋆

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300, RA, Leiden, The Netherlands
e-mail: botteon@strw.leidenuniv.nl
² Naval Research Laboratory, 4555 Overlook Avenue SW, Code 7213, Washington, DC 20375, USA
³ INAF – IASF Milano, Via A. Corti 12, 20133 Milano, Italy
⁴ INAF – IRA, Via P. Gobetti 101, 40129 Bologna, Italy
⁵ ASTRON, the Netherlands Institute for Radio Astronomy, Postbus 2, 7990, AA Dwingeloo, The Netherlands
⁶ SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584, CA Utrecht, The Netherlands
⁷ Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
⁸ Thüringer Landessternwarte, Sternwarte 5, 07778 Tautenburg, Germany
⁹ GEPI, Observatoire de Paris, CNRS, Université Paris Diderot, 5 Place Jules Janssen, 92190 Meudon, France
¹⁰ Centre for Radio Astronomy Techniques and Technologies, Department of Physics and Electronics, Rhodes University, Grahamstown 6140, South Africa

Received: 7 December 2020
Accepted: 2 March 2021

Abstract

Context. Thermal gas in the center of galaxy clusters can show substantial motions that generate surface-brightness and temperature discontinuities known as cold fronts. The motions may be triggered by minor or off-axis mergers that preserve the cool core of the system. The dynamics of the thermal gas can also generate radio emission from the intra-cluster medium (ICM) and impact the evolution of clusters’ radio sources.

Aims. We aim to study the central region of Abell 1775, a system in an ambiguous dynamical state at z = 0.072 which is known to host an extended head-tail radio galaxy, with the goal of investigating the connection between thermal and nonthermal components in its center.

Methods. We made use of a deep (100 ks) Chandra observation accompanied by LOFAR 144 MHz, GMRT 235 MHz and 610 MHz, and VLA 1.4 GHz radio data.

Results. We find a spiral-like pattern in the X-ray surface brightness that is mirrored in the temperature and pseudo-entropy maps. Additionally, we characterize an arc-shaped cold front in the ICM. We interpret these features in the context of a slingshot gas tail scenario. The structure of the head-tail radio galaxy “breaks” at the position of the cold front, showing an extension that is detected only at low frequencies, likely due to its steep and curved spectrum. We speculate that particle reacceleration is occurring in the outer region of this tail, which in total covers a projected size of ∼800 kpc. We also report the discovery of revived fossil plasma with ultra-steep spectrum radio emission in the cluster core together with a central diffuse radio source that is bounded by the arc-shaped cold front.

Conclusions. The results reported in this work demonstrate the interplay between thermal and nonthermal components in the cluster center and the presence of ongoing particle reacceleration in the ICM on different scales.