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Issue A&A
Volume 407, Number 3, September I 2003
Page(s) L73 - L77
Section Letters
DOI http://dx.doi.org/10.1051/0004-6361:20031088



A&A 407, L73-L77 (2003)
DOI: 10.1051/0004-6361:20031088

Letter

Probing the cosmic ray population of the giant elliptical galaxy M 87 with observed TeV $\gamma$-rays

C. Pfrommer and T. A. Enßlin

Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str.1, Postfach 1317, 85741 Garching, Germany
(Received 27 May 2003 / Accepted 15 July 2003 )

Abstract
We examine the cosmic ray proton (CRp) population within the giant elliptical galaxy M 87 using the TeV  $\gamma$-ray detection of the HEGRA collaboration. In our scenario, the $\gamma$-rays are produced by decaying pions which result from hadronic CRp interactions with thermal gas of the interstellar medium of M 87. By comparing the $\gamma$-ray emission to upper limits from EGRET, we constrain the spectral index of the CRp population to $\alpha_\mathrm{GeV}^\mathrm{TeV}<2.275$ within our scenario. Both the expected radial $\gamma$-ray profile and the required amount of CRp support this hadronic scenario. The accompanying radio mini-halo of hadronically originating cosmic ray electrons is outshone by the synchrotron emission of the relativistic jet of M 87 by one order of magnitude. According to our predictions, the future GLAST mission should allow us to test this hadronic scenario.


Key words: $\gamma$-rays: theory -- cosmic rays -- galaxies: individual: M 87 -- galaxies: cooling flows -- intergalactic medium -- radiation mechanisms: non-thermal

Offprint request: C. Pfrommer, pfrommer@mpa-garching.mpg.de

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