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.