Astronomy and Astrophysics – Astronomy
Scientific paper
Jul 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001phdt........70r&link_type=abstract
Ph.D. Thesis, Ludwig-Maximilians-Universität München, (2001)
Astronomy and Astrophysics
Astronomy
14
Galaxy Clusters, Cosmology, X-Ray Astronomy
Scientific paper
An X-ray selected and X-ray flux-limited sample comprising the 63 X-ray brightest galaxy clusters in the sky (excluding the galactic band, called HIFLUGCS) has been constructed based on the ROSAT All-Sky Survey. The flux limit has been set at 2x10^-11 erg/s/cm^2 in the energy band 0.1-2.4 keV. It has been shown that a high completeness is indicated by several tests. Due to the high flux limit this sample can be used for a variety of applications requiring a statistical cluster sample without any corrections to the effective survey volume.
Mainly high quality pointed observations have been used to determine fluxes and physical cluster parameters. It has been shown that a tight correlation exists between the X-ray luminosity and the gravitational mass using HIFLUGCS and an extended sample of 106 galaxy clusters. The relation and its scatter have been quantified using different fitting methods. A comparison to theoretical and numerical predictions shows an overall agreement. This relation may be directly applied in large X-ray cluster surveys or dark matter simulations for conversions between X-ray luminosity and gravitating mass.
Data from the performance verification phase of the recently launched X-ray satellite observatory XMM-Newton on the galaxy cluster Abell 1835 has been analyzed, in order to test the assumption of isothermality of the cluster gas in the outer parts applied throughout the work. It has been found that the measured outer temperature profile is consistent with being isothermal. In the inner regions a clear drop of the temperature by a factor of two has been found.
Physical properties of the cluster sample have been studied by analyzing relations between different cluster parameters. The overall properties are well understood but in detail deviations from simple expectations have been found. It has been found that the gas mass fraction (fgas) does not vary as a function of intracluster gas temperature. For galaxy groups (kTx < 2 keV), however, a steep drop of fgas has been observed. No clear trend of a variation of the shape of the surface brightness profile, i.e. beta, has been observed as a function of temperature. The Lx-Tx relation has been found to be steeper than expected from simple self similar models, as has been found by previous authors. But no clear deviations from a power law shape down to kTx = 0.7 keV have been found. The Mt-Tx relation found here is steeper than expected from self similar models and its normalization is lower compared to hydrodynamic simulations, in agreement with previous findings. Suggested scenarios to account for these deviations, including heating and cooling processes, and observational difficulties have been described. It appears that a blend of different effects, possibly including a variation of mean formation redshift with system mass, is needed to account for the observations presented here.
Using HIFLUGCS the gravitational mass function has been determined for the mass interval 3.5x10^13 < M200 < 5.2x10^15 h50^-1 Msun. Comparison with Press-Schechter mass functions has yielded tight constraints on the mean matter density in the universe and the amplitude of density fluctuations. The large covered mass range has allowed to put constraints on the parameters individually. Specifically it has been found that OmegaM = 0.12^{+0.06}_{-0.04} and sigma8 = 0.96^{+0.15}_{-0.12} (90% c.l. statistical uncertainty). This result is consistent with two more estimates of OmegaM obtained in this work using different methods. The mean intracluster gas fraction of the 106 clusters in the extended sample combined with predictions from the theory of nucleosynthesis indicates OmegaM < 0.34. The cluster mass to light ratio multiplied by the mean luminosity density implies OmegaM 0.15. Various tests for systematic uncertainties have been performed, including comparison of the Press-Schechter mass function with the most recent results from large N-body simulations, yielding deviations smaller than the statistical uncertainties. For comparison the best fit OmegaM values for fixed sigma8 values have been determined yielding the relation sigma8 = 0.43OmegaM^-0.38.
The mass function has been integrated to obtain the fraction of the total gravitating mass in the universe contained in galaxy clusters. Normalized to the critical density it has been found that Omega_Cluster = 0.012^{+0.003}_{-0.004} for cluster masses larger than 6.4^{+0.7}_{-0.6}x10^13 h50^-1 Msun. With the value for OmegaM determined here this implies that about 90% of the mass in the universe resides outside virialized cluster regions. Similarly it has been found that the fraction of the total gravitating mass which is contained in the intracluster gas, Omega_b,Cluster = 0.0015^{+0.0002}_{-0.0001} h50^-1.5 for gas masses larger than 6.9^{+1.4}_{-1.5}x10^12 h50^{-5/2}Msun, is very small.
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