Central dynamics of globular clusters
Abstract
This dissertation explores some aspects of the central dynamics of globular
clusters. Surface brightness profiles are measured with a newly developed
technique that yields accurate, high resolution density profiles. The technique
uses integrated light measured with a robust statistical estimator, and it is
applied to images obtained by the Hubble Space Telescope. Surface brightness
profiles are presented for 39 globular clusters belonging to the Milky Way,
21 to the LMC, 5 to the SMC and 4 to the Fornax dwarf galaxy. Results
show that the central structure of some globular clusters departs from the
predictions made by classic dynamical models. When the distribution of central
logarithmic slopes is analyzed, instead of finding a bimodal distribution
between flat cores and steep cusps (as expected for post core-collapse clusters),
a continuous distribution of central slopes is observed. A new sub-class
of objects is found that have intermediate slopes between flat cores and the
expected post-core collapse central slope. In total, 45% of the sample is not
consistent with having King-type profiles in the center. Omega Centauri, the
largest Galactic globular cluster, is one of the objects that deviates from a flat
core, having a central logarithmic slope clearly different than zero. In order to
further explore the dynamical state of this cluster, central kinematic measurements
are obtained. Spectroscopic measurements come from the GMOS-IFU
on the Gemini-south telescope. Line-of-sight velocity dispersions from integrated
spectra are measured in an area of 5×5 arcseconds around the center
and also 14 arcseconds away. A clear rise in dispersion from 18.5 to 23 km s−1
is observed between the outer and the central fields. The observed velocity
dispersion profile is compared with dynamical models containing central black
holes of various masses. Observations are best explained by the presence of
an intermediate-mass black hole of 4 × 104M⊙ at the center. It is crucial to
investigate the central regions of globular clusters in great detail in order to
find the causes for the observed photometric and kinematic peculiarities.
Department
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