Master thesis defense by Søren Hegaard Pallesen

Constraining the Dark Matter Distribution in Dwarf Galaxies Using Extragalactic Stellar Streams

Abstract

The current best cosmological model, the $\Lambda$ Cold Dark Matter model, successfully explains the large scale structures of the Universe. However, this model fails to explain some small scale observations. One key example of this discrepancy between theory and observations is the core-cusp problem. Theoretically, we expect steep inner density profiles for dark matter halos, while observations often find flat inner density profiles.

In this project, we use extragalactic stellar streams and other tidal features to constrain the inner and outer slope of the dark matter halo radial density profile as well as other dark matter halo properties. Stellar streams are narrow, elongated stellar systems orbiting a host galaxy. They form when the stars in a smaller satellite galaxy are tidally stripped by the stronger gravitational pull from the more massive host galaxy. The new extragalactic stream fitting code, \texttt{X-Stream}, enables the exploration of a huge parameter space regarding stream morphology and dark matter properties using only the 2D spatial information of extragalactic streams. We focus our studies on fitting a model to the tidal features around the dwarf galaxy NGC 300. Dwarf galaxies contain a large fraction of dark matter compared to more massive galaxies, making them excellent systems to investigate the dark matter density profile. We find a model of NGC 300's tidal features treating them as one combined structure originating from the same satellite. The model constrains, in particular, the progenitor and halo mass to $\log_{10}(M_\textrm{prog}/M_\odot) = 8.3^{+0.8}_{-0.7}$ and $\log_{10}(M_\textrm{halo}/M_\odot) = 11.2\pm 0.8$. Furthermore, we constrain the radial density profile for NGC 300 with the stream model.

Additionally, we add the capability to evaluate multiple radial velocity observation to \texttt{X-stream}. We test the method with simulated streams and find better constraints using only one or two radial velocity observation. The stream around NGC 4449 has multiple radial velocity observations, which we use to create an optimal stream model.

Supervisors

Sarah Pearson, Steen H Hansen 

Censor

Hans Kjeldsen