Examining the Self Interaction of Dark Matter through Brightest Cluster Galaxy Offsets
Physical and Biological Sciences
PHYS 182
Matter in our universe is divided into two categories: light and dark. All of the matter that we can see, from the molecules in your hand to the farthest stars and galaxies, is made of light matter. Physicists have spent centuries understanding how light matter interacts with itself. For example, we know that charged particles interact through the electromagnetic force, and we know that massive particles interact through gravity. Dark matter is not as well defined. Since it's discovery almost 50 years ago, most physicists can only agree on three facts about dark matter: (1) it does not interact with light (hence "dark" matter), (2) it has mass (and thus interacts with other massive particles through gravity), and (3) dark matter makes up about 5/6 of the matter in our universe. This thesis focuses on adding one more fact to that list: that dark matter interacts with itself beyond gravity.
Gravity is a singularly attractive force, unlike, say, electromagnetism, where if you put two positive charges next to each other they will repel. These inwards and outwards forces are constantly at odds with each other, and sometimes gravity wins. When this happens, a black hole is formed; all of the mass is as concentrated to one point, giving it a density profile which peaks strongly at the center of the mass distribution. This is in contrast to objects like our sun, which have mass and volume because there are forces other than gravity which push against complete gravitational collapse. Similarly, if dark matter interacts with itself, it will be much less dense at its center, whereas if it does not interact with itself, it will have a much sharper, black-hole-like density profile. These two models (known as Self-Interacting Dark Matter (SIDM) and Cold Dark Matter (CDM) ) are tested in this thesis.
We do this by looking at large concentrations of dark matter in the cosmos, which are called dark matter halos. Specifically, we focus on the halos that surround clusters of galaxies. Galaxy clusters are useful because we know how to find the center of the dark matter distributions, and near those centers we also know that there is a large, bright galaxy called the Brightest Cluster Galaxy (BCG). If the BCG and dark matter center of mass are in the same place, it is more likely that the dark matter distribution has a concentrated center which has caught the BCG and is dragging it around as it moves. If the BCG and dark matter center are offset, however, this is an indication that dark matter has a less sharp density profile, which allows the BCG to shift around inside the SIDM potential. Using data from the Dark Energy Survey and the Chandra X-Ray Telescope, we find that BCGs are significantly offset from their dark matter centers, giving strong observational evidence that dark matter interacts with itself.
