1	Dark Matter Mike Brotherton Professor of Astronomy, University of Wyoming Author of Star Dragon and Spider Star
2	The Father of Dark Matter In 1933, Fritz Zwicky checked out the Coma Cluster. The galaxies were flying around too fast (as measured by the Doppler effect) for their visible mass to keep them together, so he proposed dark matter was present.
3	The Mother of Dark Matter A few decades later, Vera Rubin started to notice FLAT rotation curves in spiral galaxies.
4	Flat Rotation Curves – so what? Following Rieke, images from Bennett and Pryke
5	The Nature of Dark Matter
6	WIMPs??? “WIMP” = Weakly interactive massive particle Neutrinos? Seem to have mass, but too small. Axions? From Wikipedia, “The axion is a hypothetical elementary particle postulated by Peccei-Quinn theory in 1977 to resolve the strong-CP problem in quantum chromodynamics (QCD).” As yet, not detected (axions are predicted to change to and from photons in the presence of strong magnetic fields, and this property is used for creating experiments to detect axions)
7	MaCHOs “MaCHO” = Massive Compact Halo Object Just trying to explain flat rotation curves with things like black holes, brown dwarfs, etc. These are “Baryonic,” made from conventional stuff on the periodic table (like people, planets, etc.) Can be probed via gravitational microlensing
8	Baryonic Dark Matter
9	Are we sure Dark Matter is real? Astronomers have argued that dark matter can explain the strange motions of galaxies in clusters and stars in galaxies, but that it can’t be normal stuff Could we just have gotten gravity wrong on large scales? “MoND” = Modified Newtonian Dynamics Viable alternative, until 2006… Need to look more into the phenomenon of gravitational lensing on larger scales
10	Probing Dark Matter with Distant Quasars: Gravitational Lensing
11	Gravitational Lensing of Quasars
12	Gravitational Lensing
13	Hot Gas in Clusters of Galaxies
14	The Bullet Cluster Given what we know about gravitational lensing (tracing the total mass in blue), hot X-ray gas in (the dominant baryonic mass, red), we can show that dark matter exists in at least one system:
15	The Bullet Cluster Lensing of background galaxies seen in the optical images lets the mass distribution be mapped. The X-rays trace the hot gas, the dominant source of baryons in this cluster merger. They don’t line up! Why? Dark Matter seems to not interact with itself the way diffuse gas does during a cluster collision.
16	The Bullet Cluster
17	Summary That Zwicky Bastard was right! Dark Matter does indeed seem to be real, thank you Bullet Cluster. The majority of matter, dark or otherwise, is “non-baryonic” exotic stuff, and we don’t know for sure what it is. It’s likely flying through this room right this instant in huge amounts. WIMPs, not MaCHOs.
18	Voyage to an alien space station in a planet made of dark matter…