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Glowing in the Cold - New Theory for Mysterious Shining in Clusters of Galaxies

Scientists at the Max-Planck-Institute for Astrophysics have proposed a new explanation for the radio emission from cold cores within the centers of galaxy clusters. High-energy reactions of protons travelling approximately with the speed of light (cosmic ray protons) which interact with the dense gas in the cluster centers are proposed to be responsible for the emission. These reactions produce electrons that shine in the radio band. Simultaneously produced gamma ray emission should be detectable with future telescopes and thus serves as a test for the proposed scenario.

Perseus galaxy cluster

Figure 1: Cold core of the Perseus galaxy cluster. The blow-up (right) shows the compressed gas. It is visible through its X-ray emission. (Left exposure: ROSAT X-ray satellite, right exposure: Chandra X-ray satellite; image courtesy of NASA / IoA / A. Fabian et al.).

Perseus radio mini-halo

Figure 2: An extended radio source (a so-called radio mini-halo) in the Perseus galaxy cluster which has a diameter of about 500,000 light years. This image shows the color-coded radio emission at 1.4 GHz and was taken by the American VLA telescope; image courtesy of Pedlar et al. (1990).

highenergetic particle reaction

Figure 3: This diagram shows the high-energy particle reaction in the proposed scenario. A proton traveling with approximately the speed of light (cosmic ray proton, CRp) collides with a gas proton of the galaxy cluster and produces an electrically charged pion (π+). This pion decays successively into a muon (μ), into neutrinos (ν), and finally into an electron or positron (e), dependent on the charge of the pion. This electron (or positron) moves on a spiral orbit around the magnetic field lines in the cluster of galaxies and generates radio emission which is visible on Earth with radio telescopes like the VLA. A neutral pion (π0) has the same probability as a charged one of being generated. The neutral pion decays into two high-energy photons. This gamma-ray emission can be detected with Cerenkov telescopes and should be detectable in future satellite missions like GLAST. Images shown courtesy of GLAST / Spectrum Astro and NRAO / AUI / NSF.

radio brightness profile

Figure 4: The radio brightness of the Perseus galaxy cluster as a function of distance from the centre of the cluster (logarithmic representation). The comparison of the observed radio data (blue crosses, Pedlar et al. (1990)) and the new theoretical model shows a perfect consistency.

Clusters of galaxies are the largest gravitationally bound objects in the Universe. They consist of hundreds of galaxies and hot hydrogen gas which emits X-ray radiation. They are created through violent mergers of groups and smaller clusters of galaxies. During the phase of merging enormous amounts of energy is released, which lead to a multitude of extended radio phenomena, e.g. the formation of radio halos and radio relics (see Radio Ghosts in Galaxy Cluster Collisions). In the following hundreds of millions of years the galaxy clusters settle down again and the gas in the cluster centers cools, collapses, and becomes compressed. This forms so-called cold cores like that in the Perseus galaxy cluster (cf. Fig. 1). One whould expect that in the settled clusters which have not received a new input of energy in the last hundreds of million years no such extended radio emission to be observable. However, radio telescopes detect diffuse emission in such objects like the Perseus galaxy cluster (cf. Fig. 2). Where does the required energy for the emission come from?

In the scenario proposed by Christoph Pfrommer and Torsten En▀lin cosmic ray protons serve as an energy storage medium. These particles are able to store a part of the energy released by group-cluster and cluster-cluster collisions in the relatively thin gas inside clusters. This energy can be stored for billions and billions of years and will not be emitted until the fast cosmic ray protons collide with gas particles. The denser the gas within the centers of clusters of galaxies the more frequent these particles collide. During such collisions the fast protons create mostly pions. Many of the pions decay into electrons and positrons which gyrate around magnetic field lines of the cluster of galaxies. Because of the rapid circular motion the electrons radiate their energy which can in turn be detected by radio telescopes after a long cosmic journey (cf. Fig. 3).

Calculations show that comparatively few cosmic ray protons are necessesary to reproduce the observed radio glowing. Moreover, there exists an astonishingly good consistency between the predicted radio brightness distribution using this model and the measured radio data (cf. Fig. 4).

The described particle interactions should also release high-energetic gamma ray emission which can be detected on Earth using Cerenkov telescopes. The recent gamma ray observation of the Virgo galaxy cluster using such a telescope (HEGRA) could be a first indication of the correctness of this scenario. Conclusive information about this model is being expected with the launch of the future satellite mission GLAST (start date 2006).

Christoph Pfrommer & Torsten Enßlin

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