Computer Science
Scientific paper
Aug 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000phdt........14b&link_type=abstract
Thesis (PhD). UNIVERSITY OF ALABAMA IN HUNTSVILLE, Source DAI-B 61/02, p. 890, Aug 2000, 159 pages.
Computer Science
3
Scientific paper
Extreme ultra-violet and soft X-ray radiation (~60-400 eV) from extragalactic sources suffers from severe Galactic absorption losses, rendering the detection of radiation at these energies a complicated task. Clusters are well known to be X-ray emitters; some EUV and soft X-rays are therefore expected as the low- energy `tail' of this emission. The first detection of excess EUV emission, i.e., above that expected fro the X-ray emitting gas, was achieved only very recently (1996), nearly thirty years after cluster X-rays were detected. Basic dynamic and energetic properties of clusters of galaxies are reviewed in Chapter 2. Clusters analyzed in this Dissertation (A2199, A1795, Coma and Virgo) all lie along directions of low line-of-sight Galactic HI column density (NH <= 1-2 × 1020 cm-2), and diffuse excess EUV emission from them was detected in the 60-200 eV band by the Extreme Ultraviolet Explorer (EUVE). Instrumental issues concerning the Deep Survey detector aboard EUVE are therefore scrutinised in Chapter 3 in order to devise a correct method of data analysis. Relevant issues to the ROSAT Position-sensitive Proportional Counter (PSPC) detector, used for its soft X-ray sensitivity, are also considered. The EUV and soft X-ray excess emission (the cluster soft excess phenomenon, CSE) is found for all four clusters in the sample (Chapter 4). Some of the clusters show a peculiar trend of the CSE emission, resulting in the increase of the relative importance of the CSE emission with radial distance (the soft-excess radial trend, SERT). Two alternative explanations of the CSE emission are explored: non-thermal radiation due to Inverse Compton (IC) scattering of relativistic electrons and radiation from a warm (~106 K) gas. A detailed treatment of IC scattering (off the microwave background field, the MWB) in the cluster environment is described in Chapter 5, where cluster spectra are fitted to IC models. It is initially shown that the soft excess of some clusters (e.g., A2199 and A1795) can be explained in the context of an adiabatic gas and cosmic ray (CR) atmosphere. It is however found that not all clusters allow an interpretation of the CSE emission as non- thermal radiation; in some cases (e.g., Coma, A1795), severe energetic problems, such as equipartition of thermal and non-thermal energy, arise. A more likely interpretation of the CSE is as emission from a second phase of the ICM (warm ~ 106 K gas). The thermal interpretation is possible for all clusters in the sample (Chapter 6) and requires a significant fraction of a cluster's mass to reside in this warm gas. Interesting clues are also gained by the imaging of the excess emission. In particular, analysis of ROSAT PSPC images (Chapter 6) reveals evidence for cold, absorbing gas in clusters of galaxies. It is concluded that a self-consistent interpretation of EUV and soft X-ray emission from clusters is that of an ICM composed of three thermal components: hot (~108 K, the X-ray emitting gas), warm (~10 6 K) and cold (<=105 K) gas.
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