Ultra-Compact Stellar Systems in the Fornax Galaxy Cluster

Abstract

Observing the UCD/GC Interface [7]; or the remnant cores of tidallystripped nucleated dwarf galaxies [8].We completed spectroscopic observations in November 2004 of colourselected point source targets (18.00 <r ′ <22.75) in four 25 ′ diameter VLT fields surrounding NGC1399, the massive cD galaxy at the core of the Fornax galaxy cluster (Figure 1:left). Targets were selected from g ′ r ′ i ′ imaging with the CTIO Blanco 4m telescope [9]. We have discovered 30 new compact stellar systems at the cluster redshift, adding to 62 previously catalogued UCDs [1][2][3]. Our observations extend to the absolute magnitude range of globular clusters, enabling us to explore the UCD/GC interface (−12 <M r ′ <−9) in a forthcoming paper. UCD Radial Distribution and KinematicsIn Figure 1:right the radial distribution of 51 previously known Fornax UCDs for which we have u ′ g ′ r ′ i ′ z ′ photometry, and 30 new compact stellar systems from our VLT observations, is plotted against M r ′ magnitude. The new data show that compact stellar systems over a range of magnitudes are found extensively in intra-cluster space.The UCD system has a mean velocity of 1478 km s −1 and a dispersion (σ0) of 244 km s −1 . Our redshift data show weak evidence for a net rotation of the 92-member UCD system about NGC1399. The velocity gradient is 57 ± 42 km s −1 deg −1 in R.A. from NGC1399 (V/σ0 = 0.23 ± 0.17). This