VLT spectroscopy of globular clusters in low surface brightness dwarf galaxies.
PUZIA T.H. and SHARINA M.E.
Abstract (from CDS):
We present VLT/FORS2 spectroscopic observations of globular clusters (GCs) in five low surface brightness dwarf galaxies: KK 211 and KK 221, both dwarf spheroidal satellites (dSph) of NGC 5128; dSph KK 84 located close to the isolated S0 galaxy NGC 3115; and two isolated dwarf irregular (dIrr) galaxies, UGC 3755 and ESO 490-17. Our sample is selected from the Sharina et al. database of HST WFPC2 photometry of GC candidates in dwarf galaxies. For objects with accurate radial velocity measurements we confirm 26 as genuine GCs out of the 27 selected candidates from our WFPC2 survey. One candidate appears to be a distant galaxy. Our measurements of the Lick absorption-line indices in the spectra of confirmed GCs and the subsequent comparison with SSP model predictions show that all confirmed GCs in dSphs are old, except GC KK 211-3-149 (6±2 Gyr), which we consider to be the nucleus of KK 211. GCs in UGC 3755 and ESO 490-17 show a spread in ages ranging from t>10 Gyr to around 1 Gyr. Most of our sample GCs have low metallicities [Z/H]≤-1. Two relatively metal-rich clusters with [Z/H]~-0.3 are likely to be associated with NGC 3115. Our sample GCs show in general a complex distribution of α-element enhancement with a mean <[α/Fe]≥0.19±0.04 derived with the χ2 minimization technique and 0.18±0.12 dex computed with the iterative approach. These values are slightly lower than the mean <[α/Fe]≥0.29±0.01 for typical Milky Way GCs. We compare other abundance ratios with those of Local Group GCs and find indications for systematic differences in N and Ca abundance. The specific frequencies, SN, of our sample galaxies are in line with the predictions of a simple mass-loss model for dwarf galaxies and compare well with SN values found for dwarf galaxies in nearby galaxy clusters.
Galaxies: Dwarf - Galaxies: Star Clusters - Galaxy: Globular Clusters: General