Hubble Space Telescope photometry of globular clusters in the Sombrero galaxy

We explore the rich globular cluster (GC) system of the nearby Sa galaxy M1 04, the 'Sombrero' (NGC 4594), using archive Wide Field Planetary Camera 2 data. The GC colour distribution is found to be bimodal at the >99 per cent confidence level, with peaks at (V-I)(0) = 0.96 +/-0.03 and 1.21 +/-0.03. The inferred metallicities are very similar to those of GCs in our Galaxy a nd M31. However, the Sombrero reveals a much enhanced number of red (metal- rich) GCs compared to other well-studied spirals. Because the Sombrero is d ominated by a huge bulge and only has a modest disc, we associate the two s ubpopulations with the halo and bulge components, respectively. Thus our an alysis supports the view that the metal-rich GCs in spirals are associated with the bulge rather than with the disc. The Sombrero GCs have typical eff ective (half-light) radii of similar to2 pc with the red ones being similar to 30 per cent smaller than the blue ones. We identify many similarities b etween the GC system of the Sombrero and those of both late-type spirals an d early-type galaxies. Thus both the GC system and the Hubble type of the S ombrero galaxy appear to be intermediate in their nature.