| Summary: | 博士 === 國立中央大學 === 天文研究所 === 92 === We investigate theoretical models for the radio halo and hard X-ray (HXR)
excess in the Coma galaxy cluster. Time-independent and time-dependent
re-acceleration models for relativistic electrons have been carried out to
study the formation of the radio halo and HXR excess. In these models, the
relativistic electrons are injected by merger shocks and re-accelerated by
ensuing violent turbulence. The effects of different Mach numbers of the merger
shocks on the radio and HXR excess emission are also investigated. We adopt 6
uG as the central magnetic field and reproduce the observed radio spectra
via the synchrotron emission. We also obtain a central "plateau" in the radio
spectral-index distribution, which have been observed in radio emission
distribution. Our models can also produce the observed HXR excess emission via
the inverse Compton scattering of the cosmic microwave background photons. We
find that only the merger shocks with Mach numbers around 1.6--2 can produce
results in agreement with both the radio and HXR emission in the Coma cluster.
We also investigate the evolution and number distribution of radio halos in
galaxy clusters. Without re-acceleration or regeneration, the relativistic
electrons responsible for the diffuse radio emission will lose their energy via
inverse-Compton and synchrotron losses in a rather short time, and radio halos
will have lifetimes ~ 0.1 Gyr. Radio halos could last for ~ Gyr if a
significant level of re-acceleration is involved. The lifetimes of radio halos
would be comparable with the cosmological time if the radio-emitting electrons
are mainly the secondary electrons generated by pion decay following
proton-proton collisions between cosmic-ray protons and the thermal
intra-cluster medium within the galaxy clusters. Adopting both observational
and theoretical constraints for the formation of radio halos, we calculate the
formation rates and the comoving number density of radio halos in the
hierarchical clustering scheme. Comparing with observations, we find that the
lifetimes of radio halos are ~ Gyr. Our results indicate that a
significant level of re-acceleration is necessary for the observed radio halos
and the secondary electrons may not be a dominant origin for radio halos.
We have proposed a method to calculate the real effect of non-isothermality on
the Sunyaev-Zel'dovich effect (SZE). The non-isothermal effect is
conventionally approximated by an emission-weighted temperature with the
isothermal assumption or only considered the influence of the non-isothermal
Compton y-parameter. We have compared the calculated SZE with those estimated
by these two approximative methods for non-isothermal clusters with and without
cooling flows. Two temperature profiles, the hybrid model and polytropic model,
are considered for the clusters without cooling flows. For investigating the
effect of cooling flows on the SZE, the A1835 cluster is taken for example.
Temperature profiles in galaxy clusters strongly affect the SZE and
consequently the estimated values of the Hubble constant. Different profiles
result in very different error ranges for estimating the Hubble constant,
~ -3%--+10% for the hybrid model and ~ -10%--+40% for the
polytropic model. Specially, the effect of cooling flows on determining the
value of the Hubble constant is dramatic, ~ +45% for A1835, when the
isothermal emission-weighted temperature is adopted.
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