Study of populations of low-mass X-ray binaries in elliptical galaxies

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.