A tale of giants stealing from dwarfs

Cold Dark Matter simulations predict 10-100x more dwarf satellite galaxies than are observed. Some of these 'missing satellites' may have been accreted, along with their globular clusters (GCs), by giant galaxies (Cote et al. 1998). But examples of dwarfs in the early stages of disruption have remained elusive. However images of the Tadpole galaxy by the ACS on HST contained a background spiral with a candidate 'disrupting dwarf' (ie two tails were seen corning from the dwarf). A 1hr Keck spectrum of the spiral (V = 18) and dwarf (V = 23) confirmed the physical association and gave a redshift of 0.15. We fit Sersic profiles to the surface brightness and found the dwarf has properties similar to a dE galaxy with blue central colors (Forbes et al. 2003). Comparison with simulations suggests the original dwarf had a disk, ie a dIn galaxy (Mayer et al. 2001). As it orbits, tails of stars are pulled off by tidal forces and the galaxy is accreted due to dynamical friction. As the starburst fades and it continues to lose stars it may resemble a dSph galaxy. The disrupted stars eventually add a metal-poor population to the giant galaxy halo. We would also expect the dwarf to contain some 25 GCs but these would have V > 29 and hence too faint to be seen in the ACS image. This observation illustrates the transformation of a dIn to dE and possibly dSph via tidal stripping, and may help explain the dwarf galaxy morphology-density relation. Dwarfs can be transformed from one type into another if they get too close to a giant.