Globular Clusters (GCs)
Globular clusters are groups of roughly spherical, densely packed stars. They are thought to have formed at the same time as most galaxies and the stars which make them up are some of the oldest known–thus GCs are an excellent probe of galaxy formation and evolution. They have a high central stellar density, meaning collisions between stars are actually possible in their cores (in most environments such collisions are so rare as to be deemed impossible). There is no evidence of presence of a black hole in their core and their mass to light ratio is consistent with no or little dark matter content.
There are around 200 known GCs in the Milky Way, whereas Virgo A (M87) has over 12,000. The Andromeda Galaxy (M31) is thought to contain 500-1,000s and this past week I read a paper focusing on the GC system of M31 out to a much-higher-than-previously-studied radius of 120 kpc1. For a sense of scale see the image of the Milky Way to the right, which is about 30 kpc wide, 4 kpc tall (excluding the more extended Galactic halo), and we are 8 kpc from the center. Andromeda is thought to be 20-31 kpc in size. That is to say, 120 kpc is a pretty great distance, especially when we consider that the M31 GCs studied prior to this paper were primarily within 55 kpc.
The updated sample of M31 GCs used by the authors are consistent with the scenario of many GCs being associated with tidal streams in the outer regions of M31 as the authors find no radial color or metallicity gradient at large radii. Such gradients would be expected from accretion.
In addition the authors find a flattening in the number density radial profile occurring at 30 kpc. This is interesting as Abadi, Navarro and Stenmetz2 show that this flattening occurs in galaxies that grow through combination of star-formation and accretion and the point of transition (in this case 30 kpc) is the radius after which most of the matter was accreted. This was the first time that this has been seen in M31 and is a result of the extended sample to radii more than three times larger than earlier profiles.
1. A. P. Huxor, A. M. N. Ferguson, N. R. Tanvir, M. J. Irwin, A. D. Mackey, R. A. Ibata, T. Bridges, S. C. Chapman, & G. F. Lewis (2011). Exploring the Properties of the M31 Halo Globular Cluster System MNRAS arXiv: 1102.0403v1
2. Abadi, M., Navarro, J., & Steinmetz, M. (2006). Stars beyond galaxies: the origin of extended luminous haloes around galaxies Monthly Notices of the Royal Astronomical Society, 365 (3), 747-758 DOI: 10.1111/j.1365-2966.2005.09789.x