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Zhang, Zhongli (2012): Study of populations of low-mass X-ray binaries in elliptical galaxies. Dissertation, LMU München: Faculty of Physics
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Abstract

The aim of this thesis is to study the environmental dependence of populations of low-mass X-ray binaries (LMXBs) in early-type galaxies. The dissertation is mainly based on archival data of the Chandra observatory, complemented by the infrared data of the Spitzer observatory as well as by the results of the Hubble Space Telescope and ground based optical observations. The dissertation investigates the dependence of LMXB populations on the stellar density, velocity dispersion and the age of the stellar population. To investigate the LMXB dependence on the stellar density and velocity dispersion, we studied different sub populations of low-mass X-ray binaries -- dynamically formed systems in globular clusters and in the nucleus of M31, and presumably primordial X-ray binaries in the fields of galaxies. We found that the luminosity distributions of globular cluster and field LMXBs differ throughout the entire luminosity range. The fraction of faint (Lx < E37 erg/s) sources in globular clusters is ~ 4 times smaller than in the field population. This may present a challenge for the models suggesting that the entire LMXB population was formed dynamically in globular clusters and then expelled to the field due to dynamical interactions or as globular clusters dissolve. The luminosity function of dynamically formed sources in the nucleus of M31 is similar to that of globular cluster sources at the faint end but differs at the bright end, that the M31 nucleus hosting significantly fewer bright sources than globular clusters (and field population). The difference between their luminosity distributions is likely caused by the factor of ~ 10-20 difference in stellar velocities in globular clusters and galactic nuclei, which leads to different dynamical formation channels. In order to investigate the LMXB dependence on the stellar age we collected a sample of 20 nearby massive E/S0 galaxies covering a broad range of stellar ages from ~ 1 to > 10 Gyrs. We found statistically significant correlation of the specific frequency of LMXBs (number per unit stellar mass) with the age of the host galaxy and its globular cluster content. Overall, older galaxies tend to have more X-ray binaries per unit stellar mass than the younger ones. This can be explained as a combined effect of two factors. On one hand, there appears to be an intrinsic correlation of the specific number of low-mass X-ray binaries in the field with the age of the stellar population. On the other hand, the older galaxies tend to have richer globular cluster population and therefore a larger number of dynamically formed X-ray sources associated with them. We also found a significant difference in the luminosity functions of low-mass X-ray binaries in younger and older galaxies, with the former having a flatter luminosity distribution and hosting more bright sources. This behavior is unrelated to the difference between luminosity distributions of the globular cluster and field sources and reflects the intrinsic age dependence of the LMXB populations.