Testing X-Ray Scaling Relations with a Sample of Chandra Galaxy Groups and Detailed Analysis of A2244 with Chandra and Suzaku

Helen J. Eckmiller

Abstract

 

Groups and clusters of galaxies are valuable tools for cosmology, especially with respect to their mass and other global properties. Individual clusters are also useful to study the complex gravitational and non-gravitational physics involved in their formation. The first part of this work deals with a sample of galaxy groups and its use for cosmology, while in the second part a single cluster is analyzed to gain information about its individual properties and formation history.

The first part is motivated as follows. Well-determined scaling relations between X-ray observables and cluster mass are essential for using large cluster samples to constrain fundamental cosmological parameters. Scaling relations between cluster masses and observables, such as the luminosity-temperature, mass-temperature, luminosity-mass relations, have been investigated extensively, however the question of whether these relations hold true also for poor clusters and groups remains unsettled. Some evidence supports a "break" at the low end of the group/cluster mass range, possibly caused by the stronger influence of non-gravitational physics on low-mass systems. The main goal of this part of the thesis is to test local scaling relations for the low-mass range in order to check whether or not there is a systematic difference between clusters and groups, and to thereby extend this method of reliable and convenient cluster mass determination for future large samples down to the group regime.

A statistically complete sample of 112 X-ray galaxy groups was compiled, 26 of which have usable Chandra data. Temperature, metallicity, and surface brightness profiles were created for these 26 groups, and used to determine the main physical quantities and scaling relations. The group properties were then compared to those of the HIFLUGCS clusters, as well as several other group and cluster samples. Radial profiles for the individual objects and scaling relations of the whole sample are presented (Lx-T, M-T, Lx-M, Mg-M, M-Yx, Lx-Yx, fg-T). Temperature and metallicity profiles behave universally, except for the core regions. The Lx-T, M-T, Lx-M, Mg-M, M-Yx, and Lx-Yx relations of the group sample are generally in good agreement with the clusters. The Lx-T relation steepens for T< 3 keV which could point to a larger impact of heating mechanisms on cooler systems. There is a significant drop in the gas mass fraction below 1 keV, as well as a correlation with radius, which indicates the ICM is less dominant in groups compared to clusters and the galaxies have a stronger influence on the global properties of the system. In all relations the intrinsic scatter for groups is larger than for clusters, which appears not to be correlated with merger activity but could be due to scatter caused by baryonic physics in the group cores. The importance of selection effects is also demonstrated. In summary, there is some evidence for a similarity break between groups and clusters. However this does not have a strong effect on the scaling relations.

In the second part of this thesis, the detailed analysis of A2244 is presented, which is a HIFLUGCS cluster about 6 keV in temperature. Data from both the Chandra and Suzaku telescopes was used. Because of its low and stable background, Suzaku is an excellent observatory to study the cluster gas out to the virial radius, for instance to look for possible deviations from hydrostatic equilibrium. At the same time, Suzaku can be used to investigate the distribution of heavy elements, which holds the key to the chemical enrichment history of the system.

Detailed radial temperature and metallicity profiles were created, as well as profiles in the north, south, east, and west directions. The abundance profiles of several groups of heavy elements (Fe/Ni, O/Ne, Mg, and Si/S/Ar/Ca) were also investigated. The Chandra data was used to determine the surface brightness profile, as well as the total mass and gas mass. There is a gentle drop in temperature and a peak in metallicity in the inner regions. There is another nearby cluster to the west of A2244 that needs to be considered in the radial temperature and metallicity profiles, but does not show any direct signs of interacting with A2244.

The temperature in the northern and eastern directions is ~2 keV higher than in the southern and western directions, which may be due to material falling into the cluster along a filament. The metallicity profiles are in agreement in all directions.

The Fe/Ni abundance follows the general metallicity gradient, dropping off to ~0.2 in the outskirts. Mg and Si/S/Ar/Ca fall off more steeply with radius. Both the Mg/Fe and Si/Fe ratios, which are usually expected to be flat, decrease with radius. This would indicate that either the emission lines are too faint to be detected in the outer regions of the cluster, or the ICM enrichment mainly took place in the cluster center and the metals were continually mixed into the cluster volume. The O abundance appears to strongly increase with radius, which is probably a spurious effect. There is also a possible excess in type II metals in the 2'-4' region where both O and Mg are enhanced.

The temperature and metallicity profiles of Chandra and Suzaku agree well in the cluster center as well as the outer parts of the cluster, however there is a significant offset in the 4'-8' region. This is not explained by calibration, nor by PSF smearing, nor by different background estimations, so it is most likely due to the different areas observed by the two telescopes, since Chandra does not have complete radial coverage.

The Chandra mass measurement may be underestimated due to incomplete coverage of the cluster outskirts. The mass result is lower even than other X-ray determinations, and as a consequence the gas mass fraction is strongly overestimated in the outskirts. The independent caustic method that takes into account the kinematics of the cluster galaxies gives a total mass that is consistent with previous publications, but may be slightly overestimated due to substructure. Re-observing this cluster in X-rays to get more complete coverage of the outskirts and additional independent mass measurements (e. g. gravitational lensing, SZ) would be useful to resolve this issue.

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© Universitäts- und Landesbibliothek Bonn | Published: 22.08.2012