Khagol Bulletin # 136 (Apr 2025) - ENG

The evolution of a jet through the host's ISM follows a set of evolutionary sequences (as shown in Figure 1). First is the confined phase, where the jet remains trapped within the ISM as it interacts vigorously with the dense gas clouds. The jet-plasma is diverted to low density channels through the clouds, percolating into the ISM, resulting in the formation of the so-called “Flood-Channel” evolution. This is the most important phase in terms 2005, Sutherland and Bicknell et al. 2007). These were later improved upon by more detailed 3D simulations including relativistic solvers and a wide variety of physics modules (Wagner et al. 2011, 2012, Mukherjee et al. 2016, 2017, 2018b). Although specific individual results often vary between different simulations, due to different choices of parameters, all studies show a set of general outcomes. | 07 | KHAG L | No. 136 - APRIL 2025 Feedback from supermassive blackholes (SMBH) in large early type galaxies has been strongly established as a major influencer of galaxy evolution. From a historical perspective, Bremsstrahlung driven cooling of gas in galactic (or cluster) environments has been posited to form cooling flows, which has been observed as early as late 1970s (Fabian et al. 1977). However, the fate of the gas cooled below X-ray emitting temperatures ( ≤ 1-2 keV) was left uncertain due to lack of distinct observational signatures (Peterson et al. 2001). This prompted considerations of re-heating of the gas by some mechanism, with feedback from the an Active Galactic Nuclei (AGN) being a viable source (Croton et al. 2006). The classical notion of AGN feedback considers a dual mode effect over the lifetime of a massive galaxy: a) Quasar/Establishment mode: AGN driven outflows regulate gas content and in turn SMBH and galaxy mass. The connection between feeding and feedback from the SMBH is predicted to set up the observed blackhole and bulge mass correlations. b) Radio/Maintenance mode: Large scale r ad i o j e t s hea t up t he amb i en t environment, preventing cooling flows and limiting gas supply to the central regions, thus regulating mass build up. In this dual mode scenario, the role of relativistic jets have been largely confined to their impact on extra-galactic gas. However, in the recent decades, a large body of studies have demonstrated both from theory and observations that jets can have a significant impact on the ISM of the host galaxy. This makes the earlier dual mode distinction ambiguous in some cases, requiring re-thinking of the traditional definitions (see detailed reviews by Harrison and Ramos Almeida 2024 and Mukherjee 2025 for more technical comments). In the mid and late 1970s, there were several seminal developments of theoretical models to explain the dynamics and emission from extra-galactic relativistic jets such as the `twin-exhaust' and beam models, the B-Z jet-launch mechanism, diffusive shock acceleration models, which defined the studies of jets and non-thermal emission in future (Blandford and Rees 1974, Scheur 1974, Blandford and Ostriker 1978). The first detailed 2D simulations of propagation of hypersonic jet beams and their structures were presented in 1982 (Norman et al. 1982). Since then simulations to study dynamics of relativistic jets have increased in sophistication. While the early simulations primarily focused on the magnetohydrodynamics (MHD) of these jets, later more holistic models explored realistic simulation setups that explored the impact of such jets drilling through the ambient interstellar medium (ISM) of their host. The earliest suggestions of jets interacting with intervening gas clouds was proposed to explain the observed knots in the jet of Cen A, as early as 1979 (Blandford and Konigl 1979). Direct observational evidence of jets impacting ambient gas were found from joint observations of radio jets and ionised gas outflows in the early 80s (e.g. Butcher et al. 1980, Heckman et al. 1982, Heckman et al. 1984 and others). Further investigations of such notions were also prompted by discoveries of a class of radio sources characterised by a turnover in their radio spectrum, the so called GPS (Gigahertz-Peaked Spectrum) and CSS (Compact Steep Spectrum) radio galaxies. Such galaxies were often found to be gas rich with high rotation measures and low polarisation, all indicating jet-gas interaction as the potential cause of jet compactness. The early simulations of jets interacting with an inhomogeneous ISM were performed in the 1990s (De Young et al. 1990), who performed 2D simulations of jets moving through a random distribution of spherical clouds. Later, more realistic fractal descriptions of clouds were modelled in such simulations (Saxton et al. Research Highlights Relativistic jet feedback on host galaxies Figure 1. Evolution of a jet through a dense kiloparsec scale gas disk (fromSimulation B of Mukherjee et al. 2018b), depicting the three phases of evolution. The 3D visualisation shows the temperature of the gas and the jet tracer in blue. The red-coloured contours trace the cocoon of hot gas expanding into the ISM. Post break-out, the hot pressurized cocoon spreads over the ISMand engulfs it fromthe upper and lower regions.