Modelling X-shaped Radio Galaxies: Dynamical and Emission Signatures from the Back-flow model

Abstract

Context. Active galactic nuclei typically show the presence of radio jets ranging from sub-kpc to Mpc scales. Some of these radio galaxies show distortion in their jets, forming tailed or winged sources. X-shaped radio galaxies are a subclass of winged sources formation mechanism of which is still unclear. Aims. The focus of this work is to understand hydro-dynamical back-flows and their role in dynamics and non-thermal emission signatures (in presence of radiative losses and diffusive shock acceleration) during the initial phase of these galaxies. Methods. We have performed relativistic MHD simulations of an under-dense jet travelling in a tri-axial ambient using a hybrid Eulerian-Lagrangian framework to incorporate effects of micro-physical processes. Results. We have demonstrated the dominant role played by pressure gradient in shaping XRGs in thermally dominated cases. We show that the prominence of the formed structure decreases as the jet deviates from the major axis of the ambient. The wing evolution is mainly governed by re-energized particles due to shocks that keep the structure active during the evolution time. The synthetic intensity maps of the radio galaxy show similarities with morphologies that are typically found in observed XRGs. This includes the cases with wider wings than the active lobes. The characteristic emission signatures in terms of its synchrotron spectra and implication of equipartition condition in age estimation are also discussed here. Additionally, we show that discrepancy of age can be attributed to mixing of different aged particle populations. Further, the effect of viewing angle on the difference of spectral index (∆α) of the active lobes and the wings shows a large variation and degenerate behaviour. We have demonstrated the role of diffusive shocks in the obtained variation and have concluded that the ∆α spread is not a dependable characteristic in determining the formation model of XRGs.

A low-frequency sub-arcsecond view of powerful radio galaxies in rich-cluster environments: 3C 34 and 3C 320

“…The simulations of Giri et al (2022) also predict that the turbulent nature of the plasma in wings or diffuse tails will induce in situ re-energisation of particles through local shocks, causing the wing material to survive over longer time scales than what would be expected under a model of only radiative and adiabatic losses. More sensitive observations at high resolution at frequencies greater than 1 GHz that would detect the tails will allow for a highly resolved spectral analysis of the diffuse tails to confirm this scenario, for the case of 3C 34.…”

“…We prefer the explanation given by Capetti et al (2002), who found a striking correlation between the axes of wings and those of host galaxies of radio sources -the wings are preferentially directed along the minor axis of their elliptical host galaxies, and there is a preference towards high projected ellipticity. This naturally allows the backflow to expand laterally due to the steeper pressure gradient in the ICM perpendicular to the jet axis, and has been modelled with numerical simulations (Leahy & Williams 1984;Capetti et al 2002;Giri et al 2022). The HST image in Figure 1 does not conclusively show an elliptical morphology but rather a spheroidal morphology for the host, which McCarthy et al (1995) attribute to a superposition of cluster members in the line of sight.…”

The Dynamics and Energetics of Remnant and Restarting RLAGN

“…X-shaped sources are the opposite of DDRGs, in the sense that the active jets drive at a large angle from the axis of the more diffuse and older lobe plasma. It is not known whether X-shaped and 'winged' sources are truly restarting, as competing models exist for their formation (e.g., large deflection of their back-flow [270][271][272][273]) and as X-shaped sources of the LERG type (which are common in DDRGs) are rare. It should be noted that the formation of X-shaped lobes and restarted activity are not necessarily mutually exclusive, as the source PKS 2014-55 is known to have X-shaped lobes while being a DDRG [274]; thus, it is possible that X-shaped sources are truly a result of deflected backflows rather than restarted jets at a different angle.…”