Discovery of two M 32 twins in Abell 1689

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. 2004b, AJ, 128, 1529) with the Advanced Camera for Surveys (ACS). Their luminosities (M V ≃ -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.