HST photometry of Globular Clusters in the Sombrero Galaxy

Abstract

We explore the rich globular cluster (GC) system of the nearby Sa galaxy M104, the "Sombrero" (NGC 4594), using archive WFPC2 data. The GC colour distribution is found to be bimodal at the > 99% level, with peaks at V-I = 0.96+/-0.03 and 1.21+/-0.03 (corrected for foreground reddening). The inferred metallicities are very similar to those of globular clusters in our Galaxy and M31. However the Sombrero reveals a much enhanced number of red (metal-rich) GCs compared to other well-studied spirals. Because the Sombrero is dominated by a huge bulge and only has a modest disk, we associate the two sub-populations with the halo and bulge components respectively. Thus our analysis supports the view that the metal-rich GCs in spirals are associated with the bulge rather than with the disk. The Sombrero GCs have typical effective (half-light) radii of about 2 pc with the red ones being about 30% smaller than the blue ones. We identify many similarities between the Sombrero's GC system and those of both late type spirals and early-type galaxies. Thus both the GC system and the Hubble type of the Sombrero galaxy appear to be intermediate in their nature.