Phd Award 2012 of the AG
The 2012 Phd Award of the Astronomische Gesellschaft goes to
Julius Donnert
for his work on
Giant Radio Halos
Radio Halos are Mpc sized diffuse objects observed exclusively in
merging clusters of galaxies. This non-thermal emission is due to
relativistic electrons interacting with the intra-cluster magnetic
field. The injection of these particles in shocks and galactic outflows
is believed to be localised. At the same time the life-time of
synchrotron bright electrons in the ICM is short. Therefore the sheer
size of radio halos poses the puzzle to how nature maintains a
cluster-wide CR electron population.
Hadronic models solve this problem with in-situ injection of CR
electrons through hadronic interactions of long lived CR protons with
the ICM. However, over the past years it has become clear that these
models are disfavoured by observations. A more complete approach,
reacceleration models, are predicted to elegantly solve many problems
encountered with other models, at the cost of increased complexity.
We present a study based on constrained cosmological MHD simulations and
an analytic description for steady-state CR protons and electrons in the
ICM. We find that pure hadronic models are not able to explain the full
complexity of observed radio halo characteristics. Specifically the
transient nature of the emission (on-off) and the curved non-thermal
spectra found in the COMA cluster. We predict upper limits in the
gamma-ray regime for the most extreme of these models. Today these
models are ruled out by recent non-detections of the COMA cluster by the
FERMI satellite.
In a second step we use a novel implementation of a Fokker-Planck solver
to follow the dynamics of CR electrons in postprocessing to a direct
simulation of a cluster merger. As an explorative approach we use
hadronic injection of CR electrons to seed the population. We find that
this mechanism does not only produce transient emission, but also gives
the complex non-thermal radio spectra observed in some clusters and CR
proton densities compatible with recent gamma-ray observations.
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