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Abstract Feedback from active galactic nuclei is a key process in the evolution of massive halos in the Universe. New observational information on feedback is crucial for improving the implementation of the physics in numerical models. In this work, we apply a novel image-manipulation technique, termed “X-arithmetic,” to a sample of 15 galaxy clusters and groups deeply observed with Chandra. This technique decomposes perturbations in feedback-dominated regions into images excluding either (1) weak shocks and sound waves, (2) bubbles inflated by jets, or (3) cooling and slow gas motions (isobaric perturbations), enabling efficient spatial identification of these features without involving spectroscopic analysis. We confirm the nature of previously (spectroscopically) identified features and newly establish the origin of other structures. We find that feedback produces multiple shocks in groups and massive galaxies, but only one to two shocks in clusters. Prominent isobaric structures are abundant around inner cavities in clusters, compared to almost no such structures in groups. These differences suggest that feedback effects are stronger in smaller-mass systems, possibly due to the shallower gravitational potential of groups or more violent feedback. Follow-up spectroscopy, guided by the X-arithmetic results, suggests that earlier-identified “isothermal shocks” could be a mix of isobaric and adiabatic structures. We applied X-arithmetic to galaxy cluster simulations, demonstrating its straightforward application and future potential for testing the feedback physics details in simulations. Our feasibility study shows that imaging data from future X-ray observatories like AXIS will be ideal for expanding X-arithmetic application to a larger sample of objects. 

ABSTRACT A self-similar spherical collapse model predicts a dark matter (DM) splashback and accretion shock in the outskirts of galaxy clusters while missing a key ingredient of structure formation – processes associated with mergers. To fill this gap, we perform simulations of merging self-similar clusters and investigate their DM and gas evolution in an idealized cosmological context. Our simulations show that the cluster rapidly contracts during the major merger and the splashback radius rsp decreases, approaching the virial radius rvir. While in the self-similar model rsp depends on a smooth mass accretion rate parameter Γs, our simulations show that in the presence of mergers, rsp responds to the changes in the total mass accretion rate Γvir, which accounts for both mergers and smooth accretion. The scatter of the Γvir − rsp/rvir relation indicates a generally low Γs ∼ 1 in clusters in cosmological simulations. In contrast to the DM, the hot gaseous atmospheres significantly expand by the merger-accelerated (MA-) shocks formed when the runaway merger shocks overtake the outer accretion shock. After a major merger, the MA-shock radius is larger than rsp by a factor of up to ∼1.7 for Γs ≲ 1 and is ∼rsp for Γs ≳ 3. This implies that (1) mergers could easily generate the MA-shock-splashback offset measured in cosmological simulations, and (2) the smooth mass accretion rate is small in regions away from filaments where MA-shocks reside. We further discuss the shapes of the DM haloes, various shocks, and contact discontinuities formed at different epochs of the merger, and the ram-pressure stripping in cluster outskirts. 

ABSTRACT Galaxy clusters grow primarily through the continuous accretion of group-scale haloes. Group galaxies experience preprocessing during their journey into clusters. A star-bursting compact group, the Blue Infalling Group (BIG), is plunging into the nearby cluster A1367. Previous optical observations reveal rich tidal features in the BIG members, and a long H α trail behind. Here, we report the discovery of a projected ∼250 kpc X-ray tail behind the BIG using Chandra and XMM–Newton observations. The total hot gas mass in the tail is ∼7 × 1010 M⊙ with an X-ray bolometric luminosity of ∼3.8 × 1041 erg s−1. The temperature along the tail is ∼1 keV, but the apparent metallicity is very low, an indication of the multi-T nature of the gas. The X-ray and H α surface brightnesses in the front part of the BIG tail follow the tight correlation established from a sample of stripped tails in nearby clusters, which suggests the multiphase gas originates from the mixing of the stripped interstellar medium (ISM) with the hot intracluster medium (ICM). Because thermal conduction and hydrodynamic instabilities are significantly suppressed, the stripped ISM can be long lived and produce ICM clumps. The BIG provides us a rare laboratory to study galaxy transformation and preprocessing.