On the complexity and the information content of cosmic structures

The emergence of cosmic structure is commonly considered one of the most complex phenomena in Nature. However, this complexity has never been defined nor measured in a quantitative and objective way. In this work we propose a method to measure the information content of cosmic structure and to quantify the complexity that emerges from it, based on Information Theory. The emergence of complex evolutionary patterns is studied with a statistical symbolic analysis of the datastream produced by state-of-the-art cosmological simulations of forming galaxy clusters. This powerful approach allows us to measure how many bits of information are necessary to predict the evolution of energy fields in a statistical way, and it offers a simple way to quantify when, where and how the cosmic gas behaves in complex ways. The most complex behaviors are found in the peripheral regions of galaxy clusters, where supersonic flows drive shocks and large energy fluctuations over a few tens of million years. Describing the evolution of magnetic energy requires at least a twice as large amount of bits than for the other energy fields. When radiative cooling and feedback from galaxy formation are considered, the cosmic gas is overall found to double its degree of complexity. In the future, Cosmic Information Theory can significantly increase our understanding of the emergence of cosmic structure as it represents an innovative framework to design and analyze complex simulations of the Universe in a simple, yet powerful way.Comment: 15 pages, 14 figures. MNRAS accepted, in pres

Do radio relics challenge diffusive shock acceleration?

Radio relics in galaxy clusters are thought to be associated with powerful shock waves that accelerate particles via diffusive shock acceleration (DSA). Among the particles accelerated by DSA, relativistic protons should outnumber electrons by a large factor. While the relativistic electrons emit synchrotron emission detectable in the radio band, the protons interact with the thermal gas to produce gamma-rays in hadronic interactions. Using simple models for the propagation of shock waves through clusters, the distribution of thermal gas and the efficiency of DSA, we find that the resulting hadronic $\gamma$-ray emission lies very close or above the upper limits from the FERMI data on nearby clusters. This suggests that the relative acceleration efficiency of electrons and protons is at odds with predictions from DSA. The inclusion of re-accelerated "fossil" particles does not seem to solve the problem. Our study highlights a possible tension of the commonly assumed scenario for the formation of radio relics and we discuss possible solutions to the problem.Comment: 7 pages, 3 figures. Updated version to match with the published version in MNRA

The Complexity and Information Content of Simulated Universes

The emergence of a complex, large-scale organisation of cosmic matter into the Cosmic Web is a beautiful exemplification of how complexity can be produced by simple initial conditions and simple physical laws. In the epoch of Big Data in astrophysics, connecting the stunning variety of multi-messenger observations to the complex interplay of fundamental physical processes is an open challenge. In this contribution, I discuss a few relevant applications of Information Theory to the task of objectively measuring the complexity of modern numerical simulations of the Universe. When applied to cosmological simulations, complexity analysis makes it possible to measure the total information necessary to model the cosmic web. It also allow us to monitor which physical processes are mostly responsible for the emergence of complex dynamical behaviour across cosmic epochs and environments, and possibly to improve mesh refinement strategies in the future.Comment: 26 pages, 10 figures. Preprint version of chapter accepted for "Intelligent Astrophysics", published in the book series https://www.springer.com/series/10624 "Emergence, Complexity and Computation (Editors I. Zelinka, M. Brescia & D. Baron

Testing cosmic-ray acceleration with radio relics: a high-resolution study using MHD and tracers

Weak shocks in the intracluster medium may accelerate cosmic-ray protons and cosmic-ray electrons differently depending on the angle between the upstream magnetic field and the shock normal. In this work, we investigate how shock obliquity affects the production of cosmic rays in high-resolution simulations of galaxy clusters. For this purpose, we performed a magneto-hydrodynamical simulation of a galaxy cluster using the mesh refinement code \enzo. We use Lagrangian tracers to follow the properties of the thermal gas, the cosmic rays and the magnetic fields over time. We tested a number of different acceleration scenarios by varying the obliquity-dependent acceleration efficiencies of protons and electrons, and by examining the resulting hadronic $\gamma$-ray and radio emission. We find that the radio emission does not change significantly if only quasi-perpendicular shocks are able to accelerate cosmic-ray electrons. Our analysis suggests that radio emitting electrons found in relics have been typically shocked many times before $z=0$. On the other hand, the hadronic $\gamma$-ray emission from clusters is found to decrease significantly if only quasi-parallel shocks are allowed to accelerate cosmic-ray protons. This might reduce the tension with the low upper limits on $\gamma$-ray emission from clusters set by the \textit{Fermi}-satellite.Comment: 16 pages, 17 Figures, accepted for publication by MNRA

The challenge of detecting intracluster filaments with Faraday Rotation

The detection of filaments in the cosmic web will be crucial to distinguish between the possible magnetogenesis scenarios and future large polarization surveys will be able to shed light on their magnetization level. In this work, we use numerical simulations of galaxy clusters to investigate their possible detection. We compute the Faraday Rotation signal in intracluster filaments and compare it to its surrounding environment. We find that the expected big improvement in sensitivity with the SKA-MID will in principle allow the detection of a large fraction of filaments surrounding galaxy clusters. However, the contamination of the intrinsic Faraday Rotation of background polarized sources will represent a big limiter to the number of objects that can be significantly detected. We discuss possible strategies to minimize this effect and increase the chances of detection of the cosmic web with the large statistics expected from future surveys.Comment: 16 pages, accepted to Galaxie

Properties of Cosmological Filaments extracted from Eulerian Simulations

Using a new parallel algorithm implemented within the VisIt framework, we analysed large cosmological grid simulations to study the properties of baryons in filaments. The procedure allows us to build large catalogues with up to $\sim 3 \cdot 10^4$ filaments per simulated volume and to investigate the properties of cosmic filaments for very large volumes at high resolution (up to $300^3 ~\rm Mpc^3$ simulated with $2048^3$ cells). We determined scaling relations for the mass, volume, length and temperature of filaments and compared them to those of galaxy clusters. The longest filaments have a total length of about $200 ~\rm Mpc$ with a mass of several $10^{15} M_{\odot}$. We also investigated the effects of different gas physics. Radiative cooling significantly modifies the thermal properties of the warm-hot-intergalactic medium of filaments, mainly by lowering their mean temperature via line cooling. On the other hand, powerful feedback from active galactic nuclei in surrounding halos can heat up the gas in filaments. The impact of shock-accelerated cosmic rays from diffusive shock acceleration on filaments is small and the ratio of between cosmic ray and gas pressure within filaments is of the order of $\sim 10-20$ percent.Comment: 27 pages, 24 figures, accepted for publication in Monthly Notices of the Royal Astronomical Society Main Journa

Evolution of vorticity and enstrophy in the intracluster medium

Turbulence generated by large-scale motions during structure formation affects the evolution of the thermal and non-thermal components of the intracluster medium. As enstrophy is a measure of the magnitude of vorticity, we study the generation and evolution of turbulence by analysing the Lagrangian history of enstrophy. For this purpose we combine cosmological simulations carried out with the ENZO-code with our Lagrangian post-processing tool CRaTer. This way we are able to quantify the individual source terms of enstrophy in the course of the accretion of groups onto galaxy clusters. Here we focus on the redshift range from $z=1$ to $z=0$. Finally, we measure the rate of dissipation of turbulence and estimate the resulting amplification of intracluster magnetic fields. We find that compressive and baroclinic motions are the main sources of enstrophy, while stretching motions and dissipation affect most of the ensuing enstrophy evolution. The rate of turbulent dissipation is able to sustain the amplification of intracluster magnetic fields to observed levels.Comment: 14 pages, 17 Figures, accepted for publication in MNRA

The impact of the SZ effect on cm-wavelength (1-30 GHz) observation of galaxy cluster radio relics

(Abridged) Radio relics in galaxy clusters are believed to be associated with powerful shock fronts that originate during cluster mergers, and are a testbed for the acceleration of relativistic particles in the intracluster medium. Recently, radio relic observations have pushed into the cm-wavelength domain (1-30 GHz) where a break from the standard synchrotron power-law spectrum has been found, most noticeably in the famous 'Sausage' relic. In this paper, we point to an important effect that has been ignored or considered insignificant while interpreting these new high-frequency radio data, namely the contamination due to the Sunyaev-Zel'dovich (SZ) effect that changes the observed synchrotron flux. Even though the radio relics reside in the cluster outskirts, the shock-driven pressure boost increases the SZ signal locally by roughly an order of magnitude. The resulting flux contamination for some well-known relics are non-negligible already at 10 GHz, and at 30 GHz the observed synchrotron fluxes can be diminished by a factor of several from their true values. Interferometric observations are not immune to this contamination, since the change in the SZ signal occurs roughly at the same length scale as the synchrotron emission, although there the flux loss is less severe than single-dish observations. We present a simple analytical approximation for the synchrotron-to-SZ flux ratio, based on a theoretical radio relic model that connects the non-thermal emission to the thermal gas properties, and show that by measuring this ratio one can potentially estimate the relic magnetic fields or the particle acceleration efficiency.Comment: Updated to the accepted version. Includes major text modifications and a correction to the numerical coefficient in Eq. 15. Results and conclusions are unchange