Introduction

In cold dark matter (CDM) galaxy formation, small dense halos of dark matter collapse at high redshift and eventually merge to form the large virialised galaxy clusters seen today. The CDM model is very good at reproducing large-scale structure, but only very recently have the best numerical simulations (Moore et al. 1998) had the resolution to trace the formation of small halos within galaxy clusters, with masses M. We do not yet know what the lower mass limit is for the formation of halos in the cluster environment: determining the lower limit of galaxy mass in clusters will provide an important constraint on these models. Most of the smallest cluster galaxies are low surface brightness dwarfs for which mass estimates are very difficult, though comparison with field low surface brightness dwarfs would suggest that they may be dark matter dominated (e.g. Carignan & Freeman 1988).

In this paper we describe a population of small objects we have found in the Fornax Cluster (see also Minniti et al. 1998 and Hilker et al. 1999) which have high surface brightness. The origin and nature of these objects is not yet clear, but if they are a product of the galaxy formation process in clusters, their high surface brightness will make it possible to probe the low-mass limit discussed above. They may represent extreme examples of compact low luminosity (``M32-type'') dwarf ellipticals. Alternatively, these objects may be super-massive star clusters--there is a very large population of globular clusters associated with the central galaxy of the Fornax Cluster, NGC 1399 (Grillmair et al. 1994). These objects are generally similar to Galactic globular clusters with similar colours and luminosities (Forbes et al. 1998). There is evidence that they are not all bound to the NGC 1399 system. Kissler-Patig et al. (1999) show that the kinematics of 74 of the globular clusters indicate that they are associated with the cluster gravitational potential rather than that of NGC 1399. They infer that the most likely origin of these globular clusters is that they have been tidally stripped from neighbouring galaxies. This has also been suggested by West et al. (1995), although the effect would be diluted by the large number of halo stars that would presumably be stripped at the same time.

Bassino, Muzzio & Rabolli (1994) suggest that the NGC 1399 globular clusters are remnants of the nuclei of dwarf nucleated galaxies that have survived the disrupture of being captured by the central cluster galaxy. A related suggestion is a second model proposed by West et al. (1995) that intra-cluster globular clusters could have formed in situ in the cluster environment. Bassino et al. (1994) conclude their discussion by noting that remnant nuclei an order of magnitude larger (and more luminous) than standard globular clusters would also be formed in significant numbers, but that existing globular cluster searches would not have included them. In Section 2 of this paper we describe how the observations of our Fornax Spectroscopic Survey have sampled this part of the cluster population by measuring optical spectra of all objects brighter than B_J=19.7 in the centre of the Fornax Cluster. In Section 3 we describe the properties of a new population of compact objects found in the cluster that appear to be intermediate in size between globular clusters and the smallest compact dwarf galaxies. We discuss the nature of these objects in Section 4 and show that higher resolution observations will enable us to determine if they are more like globular clusters or dwarf galaxies.

© Copyright Astronomical Society of Australia 1997