Hot Populations in M87 Globular Clusters

We have obtained HST/STIS far- and near-UV photometry of globular clusters infour fields in the gE galaxy M87. To a limit of m(FUV) = 25 we detect a totalof 66 globular clusters (GCs) in common with the deep HST optical-band study ofKundu et al. (1999). Despite strong overlap in V- and I-band properties, theM87 GCs have UV/optical properties that are distinct from clusters in the MilkyWay and in M31. M87 clusters, especially metal-poor ones, produce larger hot HBpopulations than do Milky Way analogues. Cluster mass is probably not a factorin these distinctions. The most metal-rich M87 GCs in our sample are near Z_sunand overlap the local E galaxy sample in estimated Mg_2 line indices.Nonetheless, the clusters produce much more UV light at a given Mg_2, being upto 1 mag bluer than any gE galaxy in (FUV-V) color. The M87 GCs do not appearto represent a transition between Milky Way-type clusters and E galaxies. Thedifferences are in the correct sense if the clusters are significantly olderthan the E galaxies. Comparisons with Galactic open clusters indicate that thehot stars lie on the extreme horizontal branch, rather than being bluestragglers, and that the EHB becomes well populated for ages > 5 Gyr. We findthat 43 of our UV detections have no optical-band counterparts. Most appear tobe UV-bright background galaxies, seen through M87. Eleven NUV variable sourcesdetected at only one epoch in the central field are probably classical novae.[Abridged]