Browsing by Author "Paul, Surajit"

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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.
Giant Ringlike Radio Structures Around Galaxy Cluster Abell 3376
(2007-11-09) Bagchi, Joydeep; Durret, Florence; Paul, Surajit; et al.
In the current paradigm of cold dark matter cosmology, large-scale structures are assembling through hierarchical clustering of matter. In this process, an important role is played by megaparsec (Mpc)-scale cosmic shock waves, arising in gravity-driven supersonic ﬂows of intergalactic matter onto dark matter-dominated collapsing structures such as pancakes, ﬁlaments and clusters of galaxies. Here we report Very Large Array telescope observations of giant (∼ 2 Mpc ×1.6 Mpc), ring-shaped non-thermal radio emitting structures, found at the outskirts of the rich cluster of galaxies Abell 3376. These structures may trace the elusive shock waves of cosmological large scale matter ﬂows, which are energetic enough to power them. These radio sources may also be the acceleration sites where magnetic shocks are possibly boosting 1cosmic-ray particles with energies of up to 1018 to 1019 electron volts.
High & ultra-high energy cosmic ray acceleration in structure-formation shocks in Abell 3376 galaxy cluster
(2005-09-01) Bagchi, Joydeep; Paul, Surajit; Durret, Florence; et al.
We describemultiwavelength optical, X-ray and radio evidences of energeticmergers in southern galaxy cluster Abell 3376 (A3376, redshift z=0.046, X-ray luminosity "#$&'($' ( ). Here we report the rare discovery of gigantic, Mpc-scale non-thermal radio structures (‘radio-arcs’), shaped like a pair of bow-shock fronts on the merger axis. We argue for the shock acceleration origin of the radio emission and discuss the possibilty of acceleration of very high energy cosmic ray particles in this and similar clusters
Turbulence modeling and the physics of the Intra-cluster medium
(2010-07-11) Niemeyer, Jens C.; Paul, Surajit; et al.; Iapichino, L.
The effective modeling of the stirring and development of turbulent ﬂows in grid-based hydrodynamical simulations is computationally challenging. Here we present two possible ways to tackle the problem: ﬁrst, we consider the use of the adaptive mesh reﬁnement (AMR), applying novel reﬁnement criteria which are optimized to follow the evolution of a turbulent ﬂow. In a second step, the AMR is combined with a subgrid scale (SGS) model for the unresolved turbulence, thus resulting in a new numerical technique called FEARLESS (Fluid mEchanics with Adaptively Reﬁned Large Eddy SimulationS). FEARLESS performs both the adaptive reﬁnement of the regions where turbulent ﬂows develop and a consistent coupling of the SGS turbulence with the resolved scales, and is argued to be a suitable tool in simulations of turbulent clumped ﬂows. The results of galaxy cluster simulations, performed with the new tool, give rise to several interesting implications with regard to the physics of these objects, and to the numerical methods employed for their exploration in computational cosmology.