Special Issue "Exploring the Outskirts of Galaxy Clusters"

A special issue of Galaxies (ISSN 2075-4434).

Deadline for manuscript submissions: closed (15 November 2016).

Special Issue Editors

Guest Editor
Prof. Dr. Stefano Ettori Website E-Mail
INAF - Osservatorio Astronomico di Bologna, via P. Gobetti 93/3, 40129 Bologna, Italy
Interests: clusters of galaxies; intracluster medium; dark matter; cosmology
Guest Editor
Dr. Dominique Eckert Website E-Mail
Astronomy Department, University of Geneva 16, ch. d’Ecogia, CH-1290 Versoix, Switzerland

Special Issue Information

Dear Colleagues,

One of the major scientific goals for Athena, the concept for the next-generation European X-ray telescope, recently selected by the European Space Agency(ESA) as a second Large Mission, is to determine how baryons assemble and dynamically evolve into galaxy clusters. The outskirts of galaxy clusters are where the connection between the highest peaks in the comic matter density and the large-scale structure is established. Until recently, more than about 70% of the cluster’s volume, where most of the mass accretion processes onto the main halo is occurring, has remained essentially unexplored. In these regions, the distribution of hot gas (traced with both X-ray and SZ signal) is expected to be clumpy and asymmetric, with non-negligible effects from non-thermal processes (e.g., turbulence, bulk motions, magnetic fields, particle acceleration) that can be also investigated through radio observations. The overall dark matter distribution has started to be mapped through the weak lensing signal and the galaxy distribution, by which the evolution of the properties of the accreting galaxies as the environment changes (from low to high density regions) can also be traced.

This Special Issue of Galaxies is mainly based on the contributions presented during the Symposium 6 at the European Week of Astronomy and Space Science (4-5 July, 2016, Athens, Greece) that has been organized with the aim to discuss the physics and the problems concerning the peripheries of the galaxy clusters, bringing together a number of observers and theorists working on such issues to present up-to-date results from optical, X-ray, SZ data, and numerical simulations, to propose future directions of investigations and to speculate on the prospects of next-generation instruments.

Dr. Dominique Eckert
Dr. Stefano Ettori
Guest Editors

Manuscript Submission Information

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Published Papers (7 papers)

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Research

Open AccessArticle
Extended Radio Emission in the Perhipheral Regions of the Shapley Concentration Core
Galaxies 2017, 5(1), 16; https://doi.org/10.3390/galaxies5010016 - 14 Feb 2017
Cited by 1
Abstract
The Shapley Concentration (SC) is a galaxy supercluster (few tens of degrees) in the Local Universe (<z>∼0.048) which is currently undergoing cluster mergers and group accretion. It is a diversified environment, with cluster complexes in advanced evolutionary stage, groups of clusters in the [...] Read more.
The Shapley Concentration (SC) is a galaxy supercluster (few tens of degrees) in the Local Universe (<z>∼0.048) which is currently undergoing cluster mergers and group accretion. It is a diversified environment, with cluster complexes in advanced evolutionary stage, groups of clusters in the very early stages of merger, fairly massive clusters with ongoing accretion activity, and smaller groups located in filaments. These features make the SC an ideal place to observe the signatures of the formation of large-scale structures in the Universe. As a matter of fact, the SC has been observed over a broad range of frequencies with the most important observatories, allowing for a unique multiband study. In this paper, we will present new results from an ongoing study of the Shapley Concentration Core, which is being carried out with the Giant Metrewave Radio Telescope (GMRT). Our work confirms the role played by radio observations in disentangling the details of the accretion and merging processes, and delivers a wealth of information in regions well outside the cluster cores. In particular, we will report on the discovery of a relic in the region between the two clusters A 3558 and A 3562, and of the radio properties of the brightest galaxy in the peripheral cluster A 3556. Full article
(This article belongs to the Special Issue Exploring the Outskirts of Galaxy Clusters)
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Open AccessFeature PaperArticle
Using the Outskirts of Galaxy Clusters to Determine Their Mass Accretion Rate
Galaxies 2016, 4(4), 79; https://doi.org/10.3390/galaxies4040079 - 20 Dec 2016
Abstract
We explore the possibility of using the external regions of galaxy clusters to measure their mass accretion rate (MAR). The main goal is to provide a method to observationally investigate the growth of structures on the nonlinear scales of galaxy clusters. We derive [...] Read more.
We explore the possibility of using the external regions of galaxy clusters to measure their mass accretion rate (MAR). The main goal is to provide a method to observationally investigate the growth of structures on the nonlinear scales of galaxy clusters. We derive the MAR by using the mass profile beyond the splashback radius, evaluating the mass of a spherical shell and the time it takes to fall in. The infall velocity of the shell is extracted from N-body simulations. The average MAR returned by our prescription in the redshift range z = [ 0 , 2 ] is within 20%–40% of the average MAR derived from the merger trees of dark matter haloes in the reference N-body simulations. Our result suggests that the external regions of galaxy clusters can be used to measure the mean MAR of a sample of clusters. Full article
(This article belongs to the Special Issue Exploring the Outskirts of Galaxy Clusters)
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Open AccessArticle
Asymmetric Star Formation Efficiency Due to Ram Pressure Stripping
Galaxies 2016, 4(4), 77; https://doi.org/10.3390/galaxies4040077 - 13 Dec 2016
Cited by 2
Abstract
Previous works have shown that a dense cluster environment affects satellite galaxy properties and accelerates or truncates their evolutionary processes. In this work, we use the EAGLE simulation to study this effect, dissecting the galaxies in two halves: the one that is falling [...] Read more.
Previous works have shown that a dense cluster environment affects satellite galaxy properties and accelerates or truncates their evolutionary processes. In this work, we use the EAGLE simulation to study this effect, dissecting the galaxies in two halves: the one that is falling directly to the cluster (leading half) and the one behind (trailing half). Considering all galaxies within the virial radius of the most massive groups and clusters of the simulation ( M h a l o > 10 13 . 8 [ M ] ), we find that on average the leading half presents an enhancement of the star formation rate with respect to the trailing half. We conclude that galaxies falling into the intra-cluster medium experience a boost in star-formation in their leading half due to ram pressure. Sparse observations of jellyfish galaxies have revealed visually the enhancement of the star formation in the leading half. In order to confirm this effect statistically using observations, different cases must be investigated using the simulation as a test dataset. Full article
(This article belongs to the Special Issue Exploring the Outskirts of Galaxy Clusters)
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Open AccessArticle
Galaxy Cluster Outskirts from the Thermal SZ and Non-Thermal Synchrotron Link
Galaxies 2016, 4(4), 73; https://doi.org/10.3390/galaxies4040073 - 29 Nov 2016
Cited by 1
Abstract
Galaxy cluster merger shocks are the main agent for the thermalization of the intracluster medium and the energization of cosmic ray particles in it. Shock propagation changes the state of the tenuous intracluster plasma, and the corresponding signal variations are measurable with the [...] Read more.
Galaxy cluster merger shocks are the main agent for the thermalization of the intracluster medium and the energization of cosmic ray particles in it. Shock propagation changes the state of the tenuous intracluster plasma, and the corresponding signal variations are measurable with the current generation of X-ray and Sunyaev–Zel’dovich (SZ) effect instruments. Additionally, non-thermal electrons (re-)energized by the shocks sometimes give rise to extended and luminous synchrotron sources known as radio relics, which are prominent indicators of shocks propagating roughly in the plane of the sky. In this short review, we discuss how the joint modeling of the non-thermal and thermal signal variations across radio relic shock fronts is helping to advance our knowledge of the gas thermodynamical properties and magnetic field strengths in the cluster outskirts. We describe the first use of the SZ effect to measure the Mach numbers of relic shocks, for both the nearest (Coma) and the farthest (El Gordo) clusters with known radio relics. Full article
(This article belongs to the Special Issue Exploring the Outskirts of Galaxy Clusters)
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Open AccessArticle
Studying the Effect of Shock Obliquity on the γ-ray and Diffuse Radio Emission in Galaxy Clusters
Galaxies 2016, 4(4), 71; https://doi.org/10.3390/galaxies4040071 - 25 Nov 2016
Cited by 6
Abstract
Observations of diffuse radio emission in galaxy clusters indicate that cosmic-ray electrons are accelerated on Mpc scales. However, protons appear to be accelerated less efficiently since their associated hadronic γ-ray emission has not yet been detected. Inspired by recent particle-in-cell simulations, [...] Read more.
Observations of diffuse radio emission in galaxy clusters indicate that cosmic-ray electrons are accelerated on Mpc scales. However, protons appear to be accelerated less efficiently since their associated hadronic γ-ray emission has not yet been detected. Inspired by recent particle-in-cell simulations, we study the cosmic-ray production and its signatures under the hypothesis that the efficiency of shock acceleration depends on the Mach number and on the shock obliquity. For this purpose, we combine ENZO cosmological magneto-hydrodynamical simulations with a Lagrangian tracer code to follow the properties of the cosmic rays. Our simulations suggest that the distribution of obliquities in galaxy clusters is random to first order. Quasi-perpendicular shocks are able to accelerate cosmic-ray electrons to the energies needed to produce observable radio emission. However, the γ-ray emission is lowered by a factor of a few, ∼3 , if cosmic-ray protons are only accelerated by quasi-parallel shocks, reducing (yet not entirely solving) the tension with the non-detection of hadronic γ-ray emission by the Fermi-satellite. Full article
(This article belongs to the Special Issue Exploring the Outskirts of Galaxy Clusters)
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Open AccessArticle
Relic—Shock Connection in Abell 115
Galaxies 2016, 4(4), 68; https://doi.org/10.3390/galaxies4040068 - 23 Nov 2016
Abstract
Giant radio relics are arc-shaped diffuse sources with Mpc-scale found in the peripheries of some dynamically disturbed galaxy clusters. According to the leading scenario of relic formation, shock waves occurring in merger events amplify the local magnetic field and (re)accelerate particles. However, Mach [...] Read more.
Giant radio relics are arc-shaped diffuse sources with Mpc-scale found in the peripheries of some dynamically disturbed galaxy clusters. According to the leading scenario of relic formation, shock waves occurring in merger events amplify the local magnetic field and (re)accelerate particles. However, Mach numbers associated with merger shocks are typically low, and hence inefficient at accelerating particles from the thermal pool. We analyzed a deep Chandra observation (334 ks) to study the relic region in the cluster Abell 115. Temperature and surface brightness profiles taken across the relic both show a clear discontinuity, which is consistent with a shock. This result supports the relic–shock connection and represents a test case to study the origin of radio relics. In this particular case, we suggest that a re-acceleration scenario is more suitable. The relic morphology and position are consistent with a shock produced in an off-axis merger between clusters with different masses. Full article
(This article belongs to the Special Issue Exploring the Outskirts of Galaxy Clusters)
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Open AccessArticle
On the Non-Thermal Energy Content of Cosmic Structures
Galaxies 2016, 4(4), 60; https://doi.org/10.3390/galaxies4040060 - 02 Nov 2016
Cited by 2
Abstract
(1) Background: the budget of non-thermal energy in galaxy clusters is not well constrained, owing to the observational and theoretical difficulties in studying these diluted plasmas on large scales; (2) Method: we use recent cosmological simulations with complex physics in order to connect [...] Read more.
(1) Background: the budget of non-thermal energy in galaxy clusters is not well constrained, owing to the observational and theoretical difficulties in studying these diluted plasmas on large scales; (2) Method: we use recent cosmological simulations with complex physics in order to connect the emergence of non-thermal energy to the underlying evolution of gas and dark matter; (3) Results: the impact of non-thermal energy (e.g., cosmic rays, magnetic fields and turbulent motions) is found to increase in the outer region of galaxy clusters. Within numerical and theoretical uncertainties, turbulent motions dominate the budget of non-thermal energy in most of the cosmic volume; (4) Conclusion: assessing the distribution non-thermal energy in galaxy clusters is crucial to perform high-precision cosmology in the future. Constraining the level of non-thermal energy in cluster outskirts will improve our understanding of the acceleration of relativistic particles and of the origin of extragalactic magnetic fields. Full article
(This article belongs to the Special Issue Exploring the Outskirts of Galaxy Clusters)
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