Radio Relics: a joint analysis of surveys and simulations

We present the compilation of a homogeneous sample and analysis of radio relics found in actual and mock sky surveys at 1.4 GHz. We aim at setting constraints on the physics of CR electrons and the magnetic fields in the ICM. As the observational data, we compile an updated catalog of radio relics found in the NRAO VLA Sky Survey. For the synthetic sky survey, we improve the scheme that we developed in (Nuza et al., 2017, MNRAS) by considering the amplification of magnetic fields through shock compression and a more careful consideration of the survey incompleteness towards compact radio relics. We investigate the synchrotron emission model of (Hoeft & Brueggen, 2007, MNRAS) - corresponding to DSA of electrons of ICM electrons from the thermal pool. We are the first to present the results of a likelihood-free parameter inference on ICM properties with approximate Bayesian computation. This approach is based on ranking samples of synthetic sky surveys against the observational data, each trial with an individual set of model parameters. While considerable agreement in detection counts and power-size distribution can be reached, observed relics tend to be farther away and have on average steeper integrated spectral index than their simulated counterparts for all reasonable parameter sets. The estimated fraction of thermalized shock energy put into electrons by the DSA mechanism is log10(xi_e) = 5.0 +- 0.2 at an \ICM magnetic field strength of log10(B_0/μG) = (1.4 +- 0.7) + 0.53^{+0.40}_{-0.49} log10(n_e/10^-4cm-3). The model also suggests that half the relics above a flux density of 3.6 mJy are yet undiscovered. Lastly, we show that an expansion of the modeling including a population of pre-existing CR-electrons in the ICM does not significantly improve the match between model and data. Our work demonstrates how sky surveys and cosmological simulations can be used to infer model parameters; radio relic surveys to study particle acceleration in the ICM.Wir präsentieren eine homogene Zusammenstellung und Analyse von Radiorelikten in durchgeführten und synthetischen Himmelsdurchmusterungen bei 1.4 GHz. Unser Ziel ist es, die Physik der kosmischen Strahlung und die magnetischen Eigenschaften des Haufengases (engl. intracluster medium, ICM) zu bestimmen. Als Datenbasis stellen wir einen aktualisierten Katalog von Radiorelikten innerhalb der NRAO VLA Sky Survey (Condon et al., 1998) zusammen. Für die synthetische Himmelsdurchmusterung verbessern wir den von uns in Nuza et al. (2017) entwickelten Ansatz, indem wir die Verstärkung von Magnetfeldern durch Stoßkompression und die Unvollständigkeit der Durchmusterung gegenüber kompakten Radiorelikten sorgfältiger betrachten. Wir untersuchen das Synchrotronemissionsmodell von Hoeft and Brüggen (2007), welches diffuse Stoßbeschleunigung (engl. diffusive shock acceleration, DSA) von thermischen Elektronen des Haufengases beschreibt. Wir führen erstmals eine likelihood-freie Parameterabschätzung der Eigenschaften des Haufengases mittels einer Approximativen Bayesianischen Berechnung durch. Diese Abschätzungsmethode basiert auf dem Auswählen von synthetischen Himmelsdurchmusterungen nach größtmöglicher Ähnlichkeit zu den Beobachtungsdaten. Während eine deutliche Übereinstimmung in Detektionshäufigkeit und Leuchtkraft- Größenverteilung von Relikten erzielt werden kann, tendieren beobachtete Relikte weiter zur Peripherie der Galaxienhaufen und weisen ein im Mittel steileres Spektrum auf. Der geschätzte Anteil der thermalisierten Stoßwellenenergie, welcher durch DSA in die Elektronen fließt, ist log10(xi_e) = 5.0 +- 0.2 bei Magnetfeldstärken des Haufengases von log10(B_0/μG) = (1.4 +- 0.7) + 0.53^{+0.40}_{-0.49} log10(n_e/10^-4cm-3). Das Modell lässt darauf schließen, dass die Hälfte der Relikte mit spektralen Flussdichten über 3.6 mJy noch unentdeckt ist. Zuletzt zeigen wir, dass eine Erweiterung des Modells mit bereits vorhandenen hoch-relativistischen Elektronen im Haufengas die Übereinstimmung mit der Datenbasis nicht signifikant verbessert. Unsere Arbeit zeigt, wie die Kombination von Himmelsdurchmusterungen und kosmologischen Simulationen zur Modellbildung genutzt werden kann, Radiorelikte im Speziellen für Modelle der Teilchenbeschleunigung und Magnetfelder im Haufengas

Reconciling radio relic observations and simulations: The NVSS sample

The diffusive shock acceleration scenario is usually invoked to explain radio relics, although the detailed driving mechanism is still a matter of debate. Our aim is to constrain models for the origin of radio relics by comparing observed relic samples with simulated ones. Here we present a framework to homogeneously extract the whole sample of known radio relics from NVSS so that it can be used for comparison with cosmological simulations. In this way, we can better handle intrinsic biases in the analysis of the radio relic population. In addition, we show some properties of the resulting NVSS sample relics such as the correlation between relic shape and orientation with respect to the cluster. Also, we briefly discuss the typical relic surface brightness and its relation to projected cluster distance and relic angular sizes.Comment: 4 pages, 2 figures. Proceedings of "The many facets of extragalactic radio surveys: towards new scientific challenges" (EXTRA-RADSUR2015). 20-23 October 2015. Bologna, Ital

Can cluster merger shocks reproduce the luminosity and shape distribution of radio relics?

Radio relics in galaxy clusters are believed to trace merger shock fronts. If cosmological structure formation determines the luminosity, size and shape distributions of radio relics, then merger shocks need to be lighted up in a homogeneous way. We investigate if a mock relic sample, obtained from zoomed galaxy cluster simulations, is able to match the properties of relics measured in the NRAO VLA Sky Survey (NVSS). We compile a list of all radio relics known to date and homogeneously measure their parameters in all NVSS images and apply the same procedure to relics in our simulations. Number counts in the mock relic sample increase more steeply towards lower relic flux densities, suggesting an incompleteness of NVSS in this regime. Overall, we find that NVSS and mock samples show similar properties. However, large simulated relics tend to be somewhat smaller and closer to the cluster centre than observed ones. Besides this, the mock sample reproduces very well-known correlations for radio relics, in particular those relating the radio luminosity with the largest linear size and the X-ray luminosity. We show that these correlations are largely governed by the sensitivity of the NVSS observations. Mock relics show a similar orientation with respect to the direction to the cluster centre as the NVSS sample. Moreover, we find that their maximum radio luminosity roughly correlates with cluster mass, although displaying a large scatter. The overall good agreement between NVSS and the mock sample suggests that properties of radio relics are indeed governed by merger shock fronts, emitting in a homogeneous fashion. Our study demonstrates that the combination of mock observations and data from upcoming radio surveys will allow us to shed light on both the origin of radio relics and the nature of the intracluster medium.Fil: Nuza, Sebastian Ernesto. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; Argentina. Universidad de Buenos Aires; ArgentinaFil: Gelszinnis, Jakob. Thüringer Landessternwarte; AlemaniaFil: Hoeft, Matthias. Thüringer Landessternwarte; AlemaniaFil: Yepes, Gustavo. Universidad Autónoma de Madrid; Españ

Reconciling radio relic observations and simulations: The NVSS sample

The diffusive shock acceleration scenario is usually invoked to explain radio relics, although the detailed driving mechanism is still a matter of debate. Our aim is to constrain models for the origin of radio relics by comparing observed relic samples with simulated ones. Here we present a framework to homogeneously extract the whole sample of known radio relics from NVSS so that it can be used for comparison with cosmological simulations. In this way, we can better handle intrinsic biases in the analysis of the radio relic population. In addition, we show some properties of the resulting NVSS sample relics such as the correlation between relic shape and orientation with respect to the cluster. Also, we briefly discuss the typical relic surface brightness and its relation to projected cluster distance and relic angular sizes