Discovery of two M 32 twins in Abell 1689
Infante, L.; Blakeslee, J. P.; Hilker, M.; Benítez, N.; Ford, H.; Hertling, G.; Mieske, S.; Zekser, K.
Germany, Chile, United States, Spain
Abstract
The M 31 satellite galaxy M 32 has long been considered an object of unique properties, being the most extreme example of the very rare compact elliptical galaxy class. Here we present the spectroscopic discovery of two M 32 twins in the massive galaxy cluster Abell 1689. As these objects are so rare, this is an important step towards a better understanding of the nature of compact galaxies. The two M 32 twins had first been detected within our photometric search for ultra compact dwarf galaxy (UCDs) candidates in A1689 (Mieske et al. \cite{Mieske04b}, AJ, 128, 1529) with the Advanced Camera for Surveys (ACS). Their luminosities (MV≃ -17 mag) are very similar to M 32 and their surface brightness profiles are consistent with that of M 32 projected to A1689's distance. From our ACS imaging we detect several fainter compact galaxy candidates with luminosities intermediate between M 32 and the Fornax UCDs. If spectroscopically confirmed as cluster members, this would almost close the gap in the mag-μ plane between the region of UCDs and the compact ellipticals, implying a sequence of faint compact galaxies well separated from that of dwarf ellipticals.
Based on observations obtained at the European Southern Observatory, Chile (Observing Programme 273.B-5008).