The discovery of numerous UV-bright globular clusters in the giant elliptical galaxy M87 is also remarkable (Sohn et al. 2006). Using HST/STIS UV filters Sohn et al. found 66 globular clusters from small fields of view most of which are bluer and hotter than the Milky Way counterparts. Kaviraj et al. (2007) found that the canonical population synthesis models with normal values of helium cannot reproduce their UV properties at all, as shown in Figure 5. Kaviraj et al. found that their UV brightness can be reproduced if a similar amount of EHB stars found in the Cen study by Lee et al. (2005b) are artificially added to the canonical population models as well. This is very interesting. The more massive M87 is believed to contain 2 orders of magnitude more star clusters than the Milky Way does and thus it is very natural for us to find many more UV-bright globular clusters from M87 than from Milky Way. This can be compatible with the enhanced helium hypothesis. If the enhanced helium is present, say in 10% of the star clusters, roughly 10 clusters in the Milky Way and up to 1000 clusters in M87 might be helium-enhanced and thus UV-bright. A part of them may have been found by Sohn et al. (2006).
Figure 5. Model (FUV - V) versus (V - I) grid for a range of metallicities and ages, generated from stellar models with the fiducial value of He enrichment (Y/Z = 2). The lowest age plotted is 1 Gyr and the largest age plotted is 15 Gyr. Ages 1, 5, 10 and 15 Gyr are shown using filled circles of increasing sizes. The globular cluster data of M87 (filled circles) and Milky Way (open triangles) with errors are overplotted. It is apparent that the M87 photometry lies outside the age range 1-14 Gyr for all metallicities. Excerpted from Kaviraj et al. (2007).