Simulating cosmic rays in clusters of galaxies - III. Non-thermal scaling relations and comparison to observations

Complementary views of galaxy clusters in the radio synchrotron, hard X-ray inverse Compton, and high-energy gamma-ray regimes are critical in calibrating them as high-precision cosmological probes. We present predictions for scaling relations between cluster mass and these non-thermal observables. To this end, we use high-resolution simulations of a sample of galaxy clusters spanning a mass range of almost two orders of magnitudes, and follow self-consistent cosmic ray physics on top of the radiative hydrodynamics. Calibrating the magnetic fields of our model with Faraday rotation measurements (RM), the synchrotron emission of our relativistic electron populations matches the radio synchrotron luminosities and morphologies of observed giant radio halos and mini-halos surprisingly well. Using the complete sample of the brightest X-ray clusters observed by ROSAT in combination with our gamma-ray scaling relation, we predict GLAST will detect about ten clusters allowing for Eddington bias due to the scatter in the scaling relation. The brightest gamma-ray clusters are Ophiuchus, Fornax, Coma, A3627, Perseus, and Centaurus. We provide an absolute lower flux limit for the gamma-ray emission of Coma in the hadronic model which can be made tighter for magnetic field values derived from RM values to match the GLAST sensitivity, providing thus a unique test for the possible hadronic origin of radio halos. Our predicted hard X-ray emission, due to inverse Compton emission of shock accelerated and hadronically produced relativistic electrons, falls short of the detections in Coma and Perseus by a factor of 50. This casts doubts on the inverse Compton interpretation and reinforces the known discrepancy of magnetic field estimates from Faraday RM values and those obtained by combining synchrotron and inverse Compton emission. [abridged]Comment: 15 pages, 4 figures, to be published by MNRAS, added concept of minimum gamma-ray emission from clusters in the hadronic model and improved discussion on hard X-ray emission, full resolution version available at http://www.cita.utoronto.ca/~pfrommer/Publications/CRs_scaling.pd

Introduction to extragalactic sources of very high-energy photons

The launch of the Fermi gamma-ray space telescope and the imaging air Cerenkov telescopes H.E.S.S., MAGIC, and VERITAS have substantially transformed our knowledge of gamma-ray sources in the last decade. The extragalactic gamma-ray sky is teeming with blazars, which are active galactic nuclei whose jet is directed at us. Additionally, there are radio galaxies, starburst and spiral galaxies, and gamma-ray bursts, albeit with smaller numbers. Galaxy clusters have not yet been observed in gamma rays. Here, I will introduce the different gamma-ray emission processes and review what they may tell us about these objects and the underlying acceleration mechanisms. Beyond the study of these fascinating objects, TeV gamma rays from blazars probe the integrated star formation history of the universe. Studies of TeV blazar spectra may provide us with insights into intergalactic magnetic fields or alternatively, may lead us to infer the existence of a novel mechanism that heats the intergalactic medium at late times (for redshifts z<3) and impacts the Lyman-alpha forest and late-time structure formation. The TeV gamma-ray emission may also allow us to probe fundamental physics such as the structure of space time.Comment: 10 pages, 5 multi-panel figures. Introduction also aimed at non-experts. Proceedings of the conference "Rencontres de Moriond 2013: Very High Energy Phenomena in the Universe" (La Thuile, March 9th - 16th, 2013

Cosmic-ray hydrodynamics: Alfv\'en-wave regulated transport of cosmic rays

Star formation in galaxies appears to be self-regulated by energetic feedback processes. Among the most promising agents of feedback are cosmic rays (CRs), the relativistic ion population of interstellar and intergalactic plasmas. In these environments, energetic CRs are virtually collisionless and interact via collective phenomena mediated by kinetic-scale plasma waves and large-scale magnetic fields. The enormous separation of kinetic and global astrophysical scales requires a hydrodynamic description. Here, we develop a new macroscopic theory for CR transport in the self-confinement picture, which includes CR diffusion and streaming. The interaction between CRs and electromagnetic fields of Alfv\'enic turbulence provides the main source of CR scattering, and causes CRs to stream along the magnetic field with the Alfv\'en velocity if resonant waves are sufficiently energetic. However, numerical simulations struggle to capture this effect with current transport formalisms and adopt regularization schemes to ensure numerical stability. We extent the theory by deriving an equation for the CR momentum density along the mean magnetic field and include a transport equation for the Alfv\'en-wave energy. We account for energy exchange of CRs and Alfv\'en waves via the gyroresonant instability and include other wave damping mechanisms. Using numerical simulations we demonstrate that our new theory enables stable, self-regulated CR transport. The theory is coupled to magneto-hydrodynamics, conserves the total energy and momentum, and correctly recovers previous macroscopic CR transport formalisms in the steady-state flux limit. Because it is free of tunable parameters, it holds the promise to provide predictable simulations of CR feedback in galaxy formation.Comment: 34 pages, 6 figures, minor revision to match the accepted version to be published in MNRA

The impact of magnetic fields on cold streams feeding galaxies

High redshift, massive halos are observed to have sustained, high star formation rates, which require that the amount of cold gas in the halo is continuously replenished. The cooling time scale for the hot virialized halo gas is too long to provide the source of cold gas. Supersonic, cold streams have been invoked as a mechanism for feeding massive halos at high redshift and deliver the cold gas required for continued star formation at the rates observed. This mechanism for replenishing the cold gas reservoir is motivated by some cosmological simulations. However, the cold streams are likely to be subject to the supersonic version of the Kelvin-Helmholtz instability (KHI), which eventually leads to stream disruption. Cosmological simulations have yet to obtain the spatial resolution required for understanding the detailed stability properties of cold streams. In this paper, we consider instead an idealized model of magnetized cold streams that we spatially resolve. Using linear theory we show how magnetic fields with dynamically important field strengths do not inhibit the KHI but rather enhance its growth rate. We perform nonlinear simulations of magnetized stream disruption and find that magnetic fields can nevertheless increase stream survival times by suppressing the mixing rate of cold gas with the circumgalactic medium. We find that magnetic fields can allow streams to survive $\sim 2-8$ times longer and, consequently, that streams $\sim 2-8$ times thinner can reach the central galaxy if the magnetic field strength is $\sim 0.3-0.8 \mu$G.Comment: 19 pages, 15 figures, accepted for publication in MNRAS, animations of Figures 4 and 5 at http://tberlok.dk/movies/coldstream.htm

A Phenomenological Model for the Intracluster Medium that matches X-ray and Sunyaev-Zel'dovich observations

Cosmological hydrodynamical simulations of galaxy clusters are still challenged to produce a model for the intracluster medium that matches all aspects of current X-ray and Sunyaev-Zel'dovich observations. To facilitate such comparisons with future simulations and to enable realistic cluster population studies for modeling e.g., non-thermal emission processes, we construct a phenomenological model for the intracluster medium that is based on a representative sample of observed X-ray clusters. We create a mock galaxy cluster catalog based on the large collisionless N-body simulation MultiDark, by assigning our gas density model to each dark matter cluster halo. Our clusters are classified as cool-core and non cool-core according to a dynamical disturbance parameter. We demonstrate that our gas model matches the various observed Sunyaev-Zel'dovich and X-ray scaling relations as well as the X-ray luminosity function, thus enabling to build a reliable mock catalog for present surveys and forecasts for future experiments. In a companion paper, we apply our catalogs to calculate non-thermal radio and gamma-ray emission of galaxy clusters. We make our cosmologically complete multi-frequency mock catalogs for the (non-)thermal cluster emission at different redshifts publicly and freely available online through the MultiDark database (www.multidark.org).Comment: Accepted for publication in MNRAS. 8 pages, 4 figures. This article draws heavily from arXiv:1207.6410. Updated to match the published version. Latest version updated to match the Erratum published after the correction of the mock catalog

On the Physics of Radio Halos in Galaxy Clusters: Scaling Relations and Luminosity Functions

The underlying physics of giant and mini radio halos in galaxy clusters is still an open question. We find that mini halos (such as in Perseus and Ophiuchus) can be explained by radio-emitting electrons that are generated in hadronic cosmic ray (CR) interactions with protons of the intracluster medium. By contrast, the hadronic model either fails to explain the extended emission of giant radio halos (as in Coma at low frequencies) or would require a flat CR profile, which can be realized through outward streaming and diffusion of CRs (in Coma and A2163 at 1.4 GHz). We suggest that a second, leptonic component could be responsible for the missing flux in the outer parts of giant halos within a new hybrid scenario and we describe its possible observational consequences. To study the hadronic emission component of the radio halo population statistically, we use a cosmological mock galaxy cluster catalog built from the MultiDark simulation. Because of the properties of CR streaming and the different scalings of the X-ray luminosity (L_X) and the Sunyaev-Zel'dovich flux (Y) with gas density, our model can simultaneously reproduce the observed bimodality of radio-loud and radio-quiet clusters at the same L_X as well as the unimodal distribution of radio-halo luminosity versus Y; thereby suggesting a physical solution to this apparent contradiction. We predict radio halo emission down to the mass scale of galaxy groups, which highlights the unique prospects for low-frequency radio surveys (such as the LOFAR Tier 1 survey) to increase the number of detected radio halos by at least an order of magnitude.Comment: Accepted for publication in MNRAS. 22 pages, 9 figures. This article supersedes arXiv:1207.6410. Updated to match the published version. Latest version updated to match the Erratum published after the correction of the mock catalog

The Cosmological Impact of Luminous TeV Blazars II: Rewriting the Thermal History of the Intergalactic Medium

The Universe is opaque to extragalactic very high-energy gamma rays (VHEGRs, E>100 GeV) because they annihilate and pair produce on the extragalactic background light. The resulting ultra-relativistic pairs are assumed to lose energy through inverse Compton scattering of CMB photons. In Broderick et al. (2011, Paper I of this three paper series), we argued that instead powerful plasma instabilities in the ultra-relativistic pair beam dissipate the kinetic energy of the TeV-generated pairs locally, heating the intergalactic medium (IGM). Here, we explore the effect of this heating upon the thermal history of the IGM. We collate the observed extragalactic VHEGR sources to determine a local VHEGR heating rate and correct for the pointed nature of VHEGR observations using Fermi observations of high and intermediate peaked BL Lacs. Because the local extragalactic VHEGR flux is dominated by TeV blazars, we tie the TeV blazar luminosity density to the quasar luminosity density, and produce a VHEGR heating rate as a function of redshift. This heating is relatively homogeneous for z<~4 with increasing spatial variation at higher redshift (order unity at z~6). This new heating process dominates photoheating at low redshift and the inclusion of TeV blazar heating qualitatively and quantitatively changes the structure and history of the IGM. TeV blazars produce a uniform volumetric heating rate that is sufficient to increase the temperature of the mean density IGM by nearly an order of magnitude, and at low densities by substantially more, naturally producing an inverted equation of state inferred by observations of the Ly-alpha forest, a feature that is difficult to reconcile with standard reionization models. Finally, we close with a discussion on the possibility of detecting this hot low-density IGM, but find that such measurements are currently not feasible. (abridged)Comment: 23 pages, 14 figures, update that matches the final version in the Ap

Turbulence and Particle Acceleration in Giant Radio Halos: the Origin of Seed Electrons

About 1/3 of X-ray-luminous clusters show smooth, unpolarized radio emission on ~Mpc scales, known as giant radio halos. One promising model for radio halos is Fermi-II acceleration of seed relativistic electrons by turbulence of the intracluster medium (ICM); Coulomb losses prohibit acceleration from the thermal pool. However, the origin of seed electrons has never been fully explored. Here, we integrate the Fokker-Planck equation of the cosmic ray (CR) electron and proton distributions in a cosmological simulations of cluster formation. For standard assumptions, structure formation shocks lead to a seed electron population which produces too centrally concentrated radio emission. Instead, we present three realistic scenarios that each can reproduce the spatially flat radio emission observed in the Coma cluster: (1) the ratio of injected turbulent energy density to thermal energy density increase significantly with radius, as seen in cosmological simulations. This generates a flat radio profile even if the seed population of CRs is steep with radius. (2) Self-confinement of energetic CR protons can be inefficient, and CR protons may stream at the Alfven speed to the cluster outskirts when the ICM is relatively quiescent. A spatially flat CR proton distribution develops and produces the required population of secondary seed electrons. (3) The CR proton to electron acceleration efficiency K_ep ~ 0.1 is assumed to be larger than in our Galaxy (K_ep ~ 0.01), due to the magnetic geometry at the shock. The resulting primary electron population dominates. Due to their weaker density dependence compared to secondary electrons, these primaries can also reproduce radio observations. These competing non-trivial solutions provide incisive probes of non thermal processes in the high-beta ICM.Comment: 6 pages, 2 figures. Submitte

Giant radio relics in galaxy clusters: reacceleration of fossil relativistic electrons?

Many bright radio relics in the outskirts of galaxy clusters have low inferred Mach numbers, defying expectations from shock acceleration theory and heliospheric observations that the injection efficiency of relativistic particles plummets at low Mach numbers. With a suite of cosmological simulations, we follow the diffusive shock acceleration as well as radiative and Coulomb cooling of cosmic ray electrons during the assembly of a cluster. We find a substantial population of fossil electrons. When reaccelerated at a shock (through diffusive shock acceleration), they are competitive with direct injection at strong shocks and overwhelmingly dominate by many orders of magnitude at weak shocks, Mach < 3, which are the vast majority at the cluster periphery. Their relative importance depends on cooling physics and is robust to the shock acceleration model used. While the abundance of fossils can vary by a factor of ~10, the typical reaccelerated fossil population has radio brightness in excellent agreement with observations. Fossil electrons with 1 < gamma < 100 (10 < gamma < 10^4) provide the main seeds for reacceleration at strong (weak) shocks; we show that these are well-resolved by our simulation. We construct a simple self-similar analytic model which assumes steady recent injection and cooling. It agrees well with our simulations, allowing rapid estimates and physical insight into the shape of the distribution function. We predict that LOFAR should find many more bright steep-spectrum radio relics, which are inconsistent with direct injection. A failure to take fossil cosmic ray electrons into account will lead to erroneous conclusions about the nature of particle acceleration at weak shocks; they arise from well-understood physical processes and cannot be ignored.Comment: Published in MNRAS, 25 pages, 20 figures. Cluster region of interest changed to (0.8-1.0) R_200, new figure added, figures and references updated. Conclusions unchange

Simultaneous Localization and Layout Model Selection in Manhattan Worlds

In this paper, we will demonstrate how Manhattan structure can be exploited to transform the Simultaneous Localization and Mapping (SLAM) problem, which is typically solved by a nonlinear optimization over feature positions, into a model selection problem solved by a convex optimization over higher order layout structures, namely walls, floors, and ceilings. Furthermore, we show how our novel formulation leads to an optimization procedure that automatically performs data association and loop closure and which ultimately produces the simplest model of the environment that is consistent with the available measurements. We verify our method on real world data sets collected with various sensing modalities