Browsing by Author "Miniati, F."

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Evolution of shocks and turbulence in major cluster mergers
(2010-12-14) Paul, Surajit; Iapichino, L.; Miniati, F.; et al.
We performed a set of cosmological simulations of major mergers in galaxy clusters, in order to study the evolution of merger shocks and the subsequent injection of turbulence in the post-shock region and in the intra-cluster medium (ICM). The computations have been performed with the grid-based, adaptive mesh reﬁnement (AMR) hydrodynamical code Enzo, using a reﬁnement criterion especially designed for reﬁning turbulent ﬂows in the vicinity of shocks. When a major merger event occurs, a substantial amount of turbulence energy is injected in the ICM of the newly formed cluster. Our simulations show that the shock launched after a major merger develops an ellipsoidal shape and gets broken by the interaction with the ﬁlamentary cosmic web around the merging cluster. The size of the post-shock region along the direction of shock propagation is of the order of 300 kpc h−¹, and the turbulent velocity dispersion in this region is larger than 100 km s−¹. We performed a scaling analysis of the turbulence energy within our cluster sample. The best ﬁt for the scaling of the turbulence energy with the cluster mass is consistent with M⁵/³, which is also the scaling law for the thermal energy in the self-similar cluster model. This clearly indicates the close relation between virialization and injection of turbulence in the cluster evolution. As for the turbulence in the cluster core, we found that within 2 Gyr after the major merger (the timescale for the shock propagation in the ICM), the ratio of the turbulent to total pressure is larger than 10%, and after about 4 Gyr it is still larger than 5%, a typical value for nearly relaxed clusters. Turbulence at the cluster center is thus sustained for several Gigayears, which is substantially longer than typically assumed in the turbulent re-accelerationmodels, invoked for explaining the statistics of observed radio halos. Striking similarities in the morphology and other physical parameters between our simulations and the ‘symmetrical radio relics’ found at the periphery of the merging cluster Abell 3376 are ﬁnally discussed. In particular, the interaction between the merger shock and the ﬁlaments surrounding the cluster could explain the presence of ‘notch-like’ features at the edges of the double relics.
Evolution of shocks and turbulence in the formation of galaxy clusters embedded in megaparsec-scale filaments
(2011-04-26) Iapichino, L.; Miniati, F.; et al.; Paul, Surajit
Massive structures like cluster of galaxies, embedded in cosmic ﬁlaments, release enormous amount of energy through their interactions. These events are associated with production of Mpc-scale shocks and injection of considerable amount of turbulence, aﬀecting the non-thermal energy budget of the ICM. In order to study this thoroughly, we performed a set of cosmological simulations using the hydrodynamical code Enzo.We studied the formation of clusters undergoing major mergers, the propagation of merger shocks and their interaction with the ﬁlamentary cosmic web. This interaction is shown to pro- duce peripheral structures remarkably similar to giant radio relics observed, for example, in Abell 3376 and Abell 3667. We ﬁnd a relatively long timescale (about 4 Gyr) for turbulence decay in the centre of major merging clusters. This timescale is substantially longer than typically assumed in the turbulent re-acceleration models, invoked for explaining the statistics of observed radio halos.
Radio observations of ZwCl 2341.1+0000: a double radio relic cluster
(2009-10-01) Weeren, R.; Miniati, F.; Bagchi, Joydeep; et al.
infall and mergers of smaller subclusters and galaxy groups. Diﬀuse radio emission, in the form of radio halos and relics, is found in clusters undergoing a merger, indicating that shocks or turbulence associated with the merger are capable of accelerating electrons to highly relativistic energies. Double relics are a rare class of radio sources found in the periphery of clusters, with the two components located symmetrically on the opposite sides of the cluster center. These relics are important probes of the cluster periphery as (i) they provide an estimate of the magnetic ﬁeld strength, and (ii) together with detailed modeling can be used to derive information about the merger geometry, mass, and timescale. Observations of these double relics can thus be used to test the framework of LSS formation. Here we report on radio observations of ZwCl 2341.1+0000, a complex merging structure of galaxies located at z = 0.27, using Giant Metrewave Radio Telescope (GMRT) observations. Aims. The main aim of the observations is to study the nature of the diﬀuse radio emission in the galaxy cluster ZwCl 2341.1+0000. Methods. We carried out GMRT 610, 241, and 157 MHz continuum observations of ZwCl 2341.1+0000. The radio observations are combined with X-ray and optical data of the cluster. Results. The GMRT observations show a double peripheral radio relic in the cluster ZwCl 2341.1+0000. The spectral index is −0.49 ± 0.18 for the northern relic and −0.76 ± 0.17 for the southern relic. We have derived values of 0.48 − 0.93 µGauss for the equipartition magnetic ﬁeld strength. The relics are probably associated with outward traveling merger shock waves.