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Simulating the  gamma-ray emission from galaxy clusters: a universal cosmic ray spectrum and spatial distribution
Stockholm University, Faculty of Science, Department of Physics. (CoPS)
CITA, Toronto.
(English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966Article in journal (Refereed) Accepted
Abstract [en]

Entering a new era of high-energy gamma-ray experiments, there is an exciting quest for the first detection of gamma-ray emission from clusters of galaxies. To complement these observational efforts, we use high-resolution simulations of a broad sample of galaxy clusters, and follow self-consistent cosmic ray (CR) physics using an improved spectral description. We study CR proton spectra as well as the different contributions of the pion decay and inverse Compton emission to the total flux and present spectral index maps. We find a universal spectrum of the CR component in clusters with surprisingly little scatter across our cluster sample. The spatial CR distribution also shows approximate universality; it depends however on the cluster mass. This enables us to derive a semi-analytic model for both, the distribution of CRs as well as the pion-decay gamma-ray emission that results from hadronic CR interactions with ambient gas protons. In addition, we provide an analytic framework for the inverse Compton emission that is produced by shock-accelerated CR electrons and valid in the full gamma-ray energy range. Combining the complete sample of the brightest X-ray clusters observed by ROSAT with our gamma-ray scaling relations, we identify the brightest clusters for the gamma-ray space telescope Fermi and current imaging air Cherenkov telescopes (MAGIC, HESS, VERITAS). We reproduce the result in Pfrommer (2008), but provide somewhat more conservative predictions for the fluxes in the energy regimes of Fermi and imaging air Cherenkov telescopes when accounting for the bias of `artificial galaxies' in cosmological simulations. We find that it will be challenging to detect cluster gamma-ray emission with Fermi after the second year but this mission has the potential of constraining interesting values of the shock acceleration efficiency after several years of surveying.

National Category
Astronomy, Astrophysics and Cosmology
Research subject
Theoretical Astrophysics
URN: urn:nbn:se:su:diva-42452OAI: diva2:346429
Available from: 2010-09-01 Created: 2010-08-31 Last updated: 2010-09-03Bibliographically approved
In thesis
1. Gamma-Ray Emission from Galaxy Clusters: DARK MATTER AND COSMIC-RAYS
Open this publication in new window or tab >>Gamma-Ray Emission from Galaxy Clusters: DARK MATTER AND COSMIC-RAYS
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The quest for the first detection of a galaxy cluster in the high energy gamma-ray regime is ongoing, and even though clusters are observed in several other wave-bands, there is still no firm detection in gamma-rays.

To complement the observational efforts we estimate the gamma-ray contributions from both annihilating dark matter and cosmic-ray (CR) proton as well as CR electron induced emission.

Using high-resolution simulations of galaxy clusters, we find a universal concave shaped CR proton spectrum independent of the simulated galaxy cluster. Specifically, the gamma-ray spectra from decaying neutral pions, which are produced by CR protons, dominate the cluster emission. Furthermore, based on our derived flux and luminosity functions, we identify the galaxy clusters with the brightest galaxy clusters in gamma-rays. While this emission is challenging to detect using the Fermi satellite, major observations with Cherenkov telescopes in the near future may put important constraints on the CR physics in clusters.

To extend these predictions, we use a dark matter model that fits the recent electron and positron data from Fermi, PAMELA, and H.E.S.S. with remarkable precision, and make predictions about the expected gamma-ray flux from nearby clusters. In order to remain consistent with the EGRET upper limit on the gamma-ray emission from Virgo, we constrain the minimum mass of substructures for cold dark matter halos. In addition, we find comparable levels of gamma-ray emission from CR interactions and dark matter annihilations without Sommerfeld enhancement.

Place, publisher, year, edition, pages
Stockholm: Department of Physics, Stockholm University, 2010. 117 p.
Galaxy clusters, gamma-rays, cosmic-rays, dark matter
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Theoretical Physics
urn:nbn:se:su:diva-42453 (URN)978-91-7447-141-0 (ISBN)
Public defence
2010-10-01, FB53, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 13:00 (English)
At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Accepted.Available from: 2010-09-09 Created: 2010-08-31 Last updated: 2010-09-03Bibliographically approved

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