A large population of ultra-compact dwarf galaxies in the Hydra I cluster

Abstract

Aims: We performed a large spectroscopic survey of compact, unresolved objects in the core of the Hydra I galaxy cluster (Abell 1060), with the aim of identifying ultra-compact dwarf galaxies (UCDs) and investigating the properties of the globular cluster (GC) system around the central cD galaxy NGC 3311.
Methods: We obtained VIMOS medium-resolution spectra of about 1200 candidate objects with apparent magnitudes 18.5 < V < 24.0 mag, covering both the bright end of the GC luminosity function and the luminosity range of all known UCDs.
Results: By means of spectroscopic redshift measurements, we identified 118 cluster members, from which 52 are brighter than M_V = -11.0 mag, and can therefore be termed UCDs. The brightest UCD in our sample has an absolute magnitude of M_V = -13.4 mag (corresponding to a mass of ≳ 5 × 10⁷ M_⊙) and a half-light radius of 25 pc. This places it among the brightest and most massive UCDs ever discovered. Most of the GCs/UCDs are both spatially and dynamically associated to the central cD galaxy. The overall velocity dispersion of the GCs/UCDs is comparable to what is found for the cluster galaxies. However, when splitting the sample into a bright and a faint part, we observe a lower velocity dispersion for the bright UCDs/GCs than for the fainter objects. At a dividing magnitude of M_V = -10.75 mag, the dispersions differ by more than 200 km s^-1 and up to 300 km s^-1 for objects within 5' around NGC 3311.
Conclusions: We interpret these results in the context of different UCD formation channels, and conclude that interaction-driven formation seems to play an important role in the centre of Hydra I.

Based on observations obtained at the European Southern Observatory, Chile (Observing Programmes 082.B-0680 and 076.B-0154).