The Outer Disks of S0 and Spiral Galaxies

The dominant visible component of S0 and spiral galaxies is the disk: a flattened collection of stars (and often gas and dust) in approximately circular orbits about the galaxy center. How disks form and how they grow over cosmic time are active areas of research. I am particularly interested in the overall radial structure of stellar disks, and in what happens to them at the edges of galaxies.

Michael Pohlen, John Beckman, and I discovered that galaxy disks have three main radial structures outside the bulge/bar region (Erwin et al. 2005, 2008, building on the two-element classification scheme of Freeman 1970):

1. Type I: Exponential decline with a single scale length out to the limits of observation.
2. Type II ("truncations"): Broken exponentials with a "down-bending" break, so that the outer part of the disk fades away much more quickly than the inner part does. These are responsible for the so-called "truncations" seen in edge-on galaxies since the late 1970s.
3. Type III ("antitruncations"): Also broken exponentials, but with an "up-bending" break, so that the outer disk has a shallower profile and fades away more slowly. As we argued in Erwin et al. (2005), some of these are due to luminous stellar halos dominating the light at large distances — but some are clearly due to an extended disk.



Current work in this area includes comparison of stellar radial profiles with radial profiles of star formation in the same galaxies, as part of the HAGGIS project, in order to test models for how the different disk profile types form and evolve.