Speaker
Payel Das
Description
Massive elliptical galaxies, like all assemblies of stars in our Universe, reside in extended haloes consisting of dark matter. They are believed to have been formed by multiple mergers between spirals and smaller elliptical galaxies.
We would like to find their mass distribution to probe their dark matter content and to obtain a picture of the orbits of the stars. Our focus is the outer halo, where the dark matter is thought to dominate and the orbits of the stars have longer dynamical timescales therefore better preserving past events in the formation of the galaxy.
Usually the orbits of the stars are found from models that simultaneously calculate the gravitational potential and the 3-D positions and velocities of the stars, by fitting to data that only tells us about the projected 2-D positions and velocities of the stars. Therefore if we could obtain the mass distribution independently, we would both obtain an insight into the dark matter content of these structures and more stringent constraints on the motion of the stars.
A method that has been used to obtain mass distributions of massive ellipti¬cal galaxies is the modelling of the large hot gas haloes trapped in their potential. The gas emits thermal bremsstrahlung radiation that can be detected in the X¬ray part of the spectrum. If the gas is relatively undisturbed we can assume that the radiative pressure of the gas is balanced by the gravitational force and therefore determine the mass profile.
We have developed a new non-parametric method to obtain the range of mass distributions consistent with the X-ray observations for six massive, elliptical galaxies. We have found evidence for massive dark matter haloes that contribute 70-80% to the total mass at distances of 50 kpc from the centre of the galaxies. We are now creating models of the galaxy using the derived mass profiles to probe the motions of the stars deep into the halo.