Astronomy and Astrophysics – Astrophysics
Scientific paper
Jan 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003phdt........11d&link_type=abstract
PhD Thesis, Astrophysics Research Institute, Liverpool John Moores University, UK, 2003.
Astronomy and Astrophysics
Astrophysics
1
Scientific paper
The X-ray emission of four very different galaxy structures have been analyzed: one galaxy group, two clusters and one supercluster. The diffuse emission in the galaxy group HCG 90 is quite faint, and the overall X-ray emission is dominated by the emission from the individual galaxies, with strong interactions between the brightest galaxies in the group. HCG 090 follows the same trend of richer structures both for the σ-T and for the LX-σ relations, supporting the idea that poor and small groups are simply low-mass extensions of clusters. However different trends are observed from the cluster ones, both in the S-T and in the LX-T relations. The analysis of the galaxy cluster RBS 797 revealed two remarkable, almost circular depressions in the X-ray emission, positioned symmetrically with respect to the cluster centre. The bright regions surrounding these cavities have temperatures which are lower than the neighbouring areas. This indicates that the cavities are not shocked but are rising outwards slowly, probably by buoyancy as the jet from the central radio source expands. Radio observations have confirmed the presence of a bright central radio source with extended emission in the directions of the X-ray cavities. An analysis of cluster CL 0939+4713 revealed an elongated shape with two peaks in the X-ray emission. A central hotter region, located between the two peaks, extends in the direction perpendicular to the cluster major axis, therefore marking an area where the gas is compressed and heated, most probably due to the motion of the two sub-clusters towards each other in the course of a merging process. This scenario is supported by other signs of active dynamical activity such as high galaxy velocity dispersion, displacement of the X-ray peaks with respect to the peaks in the galaxy density distribution, a high fraction of post-starburst galaxies, and a peculiar distribution of the early-type galaxies (displaced preferentially along the possible collision axis). The detection of metallicity variations gives hints on the metal enrichment processes. The spatial coincidence of high galaxy density and high metallicity suggests that the enrichment took place quite recently, so that the intra-cluster gas, which moves around with velocities of several hundred km/s, has not yet had enough time to mix. We have then analized the whole region covered by the Shapley supercluster: ROSAT All-Sky Survey is still today a unique tool to analyze structures on such large scales. 11 clusters known in the optical but with no previous X-ray detection were detected and analyzed and 14 new candidate clusters were discovered in the region leading to a mean cluster number density of more than an order of magnitude higher than the mean density of Abell rich clusters. The whole region also shows an extremely high number of interacting and merging structures making it an ideal field to test the general validity of X-ray-optical property relations. As a general tendency, the σ-T, LX-σ and LX-T relations for the Shapley clusters tightly follow the generally observed trends observed, apart from a few poor, low X-ray emitting objects in which strong dynamical activity is highly influencing their whole physical state causing their departure from the general relations. The fact that all the analyzed clusters are located in a very high density region does not therefore lead to a general deviation from the previously observed trends. While, comparing our results with high redhsift samples, we observe no evolution of the cluster gas mass fractions with redshift, a trend of increasing fgas with temperature and X-ray luminosity is indeed observed, pointing to an inconsistency of the ICM properties with self-similarity and providing a possible solution to the discrepancies in the scaling relations between theory and observations.
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