The Science and the Survey
While we don’t know what dark matter is, it is relatively simple to understand how it interacts, since only gravity is important. But we can’t see dark matter, we can only see normal matter (“baryons”). And normal matter is a lot more complicated - heating, cooling, star formation all affect baryons. So now the question becomes “How does the normal matter arrange itself within the dark matter ‘framework’?”
These images and the movie are from the work of J. Bullock and K. Johnston. Click on their names to visit their websites.
Here is where surveys like PAndAS come in. We’ve already seen that lots of dark matter haloes merge together. Some of these will contain stars and gas, and be observed as “dwarf galaxies”. As they merge together and get ripped apart, the stars will also get pulled out, into large “stellar streams”. These streams can survive for a long time, and act as “fossilised” remnants of the formation of the Galaxy. By observing these streams (and other stellar substructures), counting how many there are, quantifying their properties and measuring how they are moving, we can reconstruct the formation of the Galaxy. By comparing what we see to what various models predict, we can understand where our models are going wrong and improve our understanding of galaxy evolution.
The top three panels show three different realistations of galaxies like the Milky Way and Andromeda. All of the features you see are vast stellar structures (dwarf galaxies, streams etc). Do real galaxies look like this? PAndAS is examining Andromeda to find out if they do. Whatever the answer, PAndAS will help improve these models and identify where they are going wrong.
Galaxy Formation Models II - LIGHTSIDE