PhD ceremony Mr. M.A. Breddels: Orbit-based dynamical models of local group dwarf spheroidal galaxies

PhD ceremony: Mr. M.A. Breddels, 11.00 uur, Academiegebouw, Broerstraat 5, Groningen

Dissertation: Orbit-based dynamical models of local group dwarf spheroidal galaxies

Promotor(s): prof. A. Helmi

Faculty: Mathematics and Natural Sciences

In this thesis we have developed new models to explain the motions of stars in nearby dwarf spheroidal galaxies. With these models we can test the distribution of dark matter in these systems and establish the match to the observations.

The inner parts of the dark matter distribution inside a galaxy are of special interest since the cosmological model predicts that the density should strongly increase in the inner regions, and this sometimes seems to be in conflict with the observations of other (larger) galaxies. The question is now, do these dwarf spheroidal galaxies follow the cosmological predictions?

Earlier work claimed that the cosmological model failed on these systems. However, from our models we learned that it is not possible to determine very accurately the distribution of dark matter in the inner regions of these galaxies. Nonetheless, we showed that we can very accurately determine the distribution of dark matter at a particular radius. Thanks to these results, we have now found a more robust way (and free of biases which affected earlier work) to compare the cosmological predictions to the observations.

Even small galaxies like those studied in the thesis are complex systems. In particular Sculptor has been proposed to host two dynamical components using the chemical information from its stars. Thanks to our models, we have been able to demonstrate that these two populations indeed exist, and are truly dynamically different without the need for the chemical information.