Dynamics in M31's halo
Mark Fardal, University of Massachusetts
The Andromeda Galaxy (M31) is the closest large galaxy to our Milky Way. It's similar enough to the Milky Way that people often refer to the two galaxies as twins. The stars in M31 are much farther away than those in the Milky Way (about 2 million light years), but it has the great advantage that you can survey the whole galaxy in a fair, even fashion. Compared to more distant galaxies, M31 is special because you can see what's going on it in ways that are just impossible further out.

Like other galaxies (we think), M31 was built up in a messy, chaotic way, and the outskirts of the galaxy show the traces of this. The image to the left, from the PAndAS survey, shows what you get if you count individual stars far out around M31. (The colors show stars of different elemental composition, which helps us trace different structures.) Stars from individual galaxies have been torn off by M31's gravity, leaving debris streams and clouds far beyond M31's visible extent. The orbits of the debris streams can help us figure out the amount and distribution of the mass in M31 that creates its gravity. And the different structures give us a sample of the many galaxies that helped to build up M31's visible components. Unfortunately, many of these structures are superposed on top of each other, making it tough to figure out what's going on.

To help solve this, I have been using computer simulations of M31's debris streams. I collaborate with Martin Weinberg (UMass), Raja Guhathakurta (UC Santa Cruz), Arif Babul (U. Victoria), and other people in the PAndAS and SPLASH collaborations. We use detailed observations of the stars, which tell us precisely how fast the stars are moving and their elemental, and in some cases also their distance which gives us a three-dimensional picture of the M31 halo. We run simulations of satellite disruption in the gravity field of M31, and compare these in detail to the observations.

The Giant Southern Stream

The most prominent feature in the M31 halo is the Giant Southern Stream, which is the red long feature extending to the bottom left in the images above. This stream was discovered in 2001 by Rodrigo Ibata and collaborators. Although clearly fairly large, at first it was unclear how many stars belonged to this object and whether it connected to other features In 2007 I published a model that explained this stream as just the trailing part of a large debris stream that wrapped several times around M31, producing the features seen in red in the PAndas image above.

A recent version of this general model is shown at left (or better see this animation). We have now studied the motion of stars in various parts of these red features, and the general agreement with the model looks very good. This model was recently used by van der Marel and collaborators, who were studying the sideways motion of stars in M31, and thus needed to correct for the motion of stars in the giant stream as well as the disk and spheroid of M31 itself. This allowed them to forecast the future collision of M31 with the Milky Way. Don't worry, you have a few billion years to prepare...


Last modified: Thu May 31 14:55:07 EDT 2012