Simulating cosmic rays in clusters of galaxies – II. A unified scheme for radio haloes and relics with predictions of the γ-ray emission

By C. Pfrommer, Torsten A. Ensslin, Volker Springel. 2007

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The thermal plasma of galaxy clusters lost most of its information on how structure formation proceeded as a result of dissipative processes. In contrast , non-equilibrium distributions of cosmic rays (CR) preserve the information about their injection and transport processes and provide thus a unique window of current and past structure formation processes. This information can be unveiled by observations of non-thermal radiative processes, including radio synchrotron, hard X-ray, andγ-ray emission. To explore this, we use high-resolution simulations of a sample of galaxy clusters spanning a mass range of about two orders of magnitudes, and follow self-consistent CR physics on top of the radiative hydrodynamics. We model relativistic electrons that are accelerated at cosmological st ructure formation shocks and those that are produced in hadronic interactions of CRs with ambient gas protons. We find that the CR proton pressure traces the time integrated non-equilibriu m activities of clusters and is modulated by the recent dynamical activities. In contrast, the p ressure of primary shock-accelerated CR electrons resembles current accretion and merging shock waves that break at the shallow cluster potential in the virial regions. The resulting sync hrotron emission is predicted to be polarised and has an inhomogeneous and aspherical spatial distribution which matches the properties of observed radio relics. We propose a unified sch eme for the generation of giant radio halos as well as radio mini-halos that naturally arise s from our simulated synchrotron surface brightness maps and emission profiles. Giant radio h alos are dominated in the centre by secondary synchrotron emission with a transition to the radio synchrotron radiation emitted from primary, shock-accelerated electrons in the cluster periphery. This model is able to explain the regular structure of radio halos by the dominant contribution of hadronically produced electrons. At the same time, it is able to account for th e observed correlation of mergers with radio halos, the larger peripheral variation of the spe ctral index, and the large scatter in the scaling relation between cluster mass and synchrotron emission. Future low-frequency radio telescopes (LOFAR, GMRT, MWA, LWA) are expected to probe the accretion shock regions of clusters and the warm-hot intergalactic medium, depending on the adopted model for the magnetic fields. The hadronic origin of radio halos can be scrutinised by the detection of pion-decay inducedγ-rays following hadronic CR interactions. The high-energyγ-ray emission depends only weakly on whether radiative or non-radiative gas physics is simulated due to the self-regulated nature of the CR cooling processes. Our models predict aγ-ray emission level that should be observable with the GLAST satellite.

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/229023e2872168e7828641a6756c0df8d66263a2

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Citations in the corpus

First results from the IllustrisTNG simulations: radio haloes and magnetic fields (2017, background): “In one scenario, relativistic electrons are produced by hadronic interactions of cosmic ray protons with the ICM (e.g. Blasi & Colafrancesco 1999; Pfrommer et al. 2008; Enßlin et al. 2011).”
SEARCH FOR COSMIC-RAY-INDUCED GAMMA-RAY EMISSION IN GALAXY CLUSTERS (2013, methodology): “The secondary leptons also Compton upscatter with background radiation fields to γ-ray energies, but this emission is always subdominant compared to the γ rays produced by decaying neutral pions (Miniati 2003; Pfrommer et al. 2008; Pinzke & Pfrommer 2010).”
THE FERMI-LAT HIGH-LATITUDE SURVEY: SOURCE COUNT DISTRIBUTIONS AND THE ORIGIN OF THE EXTRAGALACTIC DIFFUSE BACKGROUND (2010, background): “…(e.g. Loeb & Waxman 2000; Miniati 2002; Keshet et al. 2003; Gabici & Blasi 2003), γ -ray emission from galaxy clusters (e.g. Berrington & Dermer 2003; Pfrommer et al. 2008), emission from starburst as well as normal galaxies (e.g. Pavlidou & Fields 2002; Thompson et al. 2007), are among the most…”
Cosmological Shocks in Adaptive Mesh Refinement Simulations and the Acceleration of Cosmic Rays (2008, methodology): “The cosmic ray electron populations are likely sources of radio halos and radio relics in galaxy clusters (Pfrommer et al. 2008; Kim et al. 1989; Giovannini et al. 1993), while the cosmic ray protons may be sources of γ-ray emission through their interactions with gas protons.”
Astrophysical smooth particle hydrodynamics (2009, background): “For the implementation of the effects of cosmic rays into SPH see (Enßlin et al., 2007; Jubelgas et al., 2008; Pfrommer et al., 2008a,b; Sijacki et al., 2008).”