Ultracompact dwarf galaxies in the Fornax Cluster

By utilizing the large multiplexing advantage of the Two-degree Field spect rograph on the Anglo-Australian Telescope, we have been able to obtain a co mplete spectroscopic sample of all objects in a predefined magnitude range, 16.5 < b(j) < 19.7 regardless of morphology, in an area toward the center of the Fornax Cluster of galaxies. Among the unresolved or marginally resol ved targets, we have found five objects that are actually at the redshift o f the Fornax Cluster; i.e., they are extremely compact dwarf galaxies or ex tremely large star clusters. All five have absorption-line spectra. With in trinsic sizes of less than 1."1 HWHM (corresponding to approximately 100 pc at the distance of the cluster), they are more compact and significantly l ess luminous than other known compact dwarf galaxies, yet much brighter tha n any globular cluster. In this paper we present new ground-based optical o bservations of these enigmatic objects. In addition to having extremely hig h central surface brightnesses, these objects show no evidence of any surro unding low surface brightness envelopes down to much fainter limits than is the case for, e.g., nucleated dwarf elliptical galaxies. Thus, if they are not merely the stripped remains of some other type of galaxy, then they ap pear to have properties unlike any previously known type of stellar system.