The head-tail radio galaxy and revived fossil plasma in Abell 1775

Abstract

Context. Head-tail radio galaxies are characterized by a head, corresponding to an elliptical galaxy, and two radio jets sweeping back from the head, forming an extended structure behind the host galaxy that is moving through the intracluster medium (ICM). This morphology arises from the interaction between the diffuse radio-emitting plasma and the surrounding environment. Sometimes, in galaxy clusters, revived fossil plasma can be found, and it traces old active galactic nucleus ejecta with a very steep spectrum that has been re-energized through processes in the ICM, unrelated to the progenitor radio galaxy.Aims. We focus on the study of the central region of Abell 1775, a galaxy cluster in an unclear dynamical state at a redshift of z = 0.07203. It hosts two giant radio-loud elliptical galaxies, the head-tail radio galaxy that ‘breaks’ at the position of a cold front detected in the X-rays, filamentary revived fossil plasma, and central diffuse emission. This study aims to investigate and constrain the spectral properties and trends along the head-tail, as well as the revived fossil plasma, to better understand the formation process of the non-thermal phenomena in A1775.Methods. We make use of observations at three frequencies performed with LOFAR at 144 MHz, and new deep uGMRT observations at 400 MHz and at 650 MHz.Results. We observe an overall steepening along the tail of the head-tail radio galaxy. In the radio colour-colour diagram, ageing models reproduce the emission of the head-tail. An unexpected brightness increase at the head of the tail suggests a complex bending of the jets. We derived the equipartition magnetic field and minimum pressure along the tail. We recovered the structure of the revived fossil plasma, which appears as thin filaments with ultra-steep spectra.Conclusions. We show that high-sensitivity, high-resolution observations at low frequencies are essential for detecting the full extent of the tail, enabling a deeper spectral analysis and resolving the structure and spectral properties of revived fossil plasma.