Hot Populations in M87 Globular Clusters

Abstract

To explore the production of UV-bright stars in old, metal-rich populations like those in elliptical galaxies, we have obtained Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph far- and near-UV photometry of globular clusters (GCs) in four fields in the giant elliptical (gE) galaxy M87. To a limit of mFUV~25 we detect a total of 66 GCs in common with the deep HST optical-band study of Kundu et al. Despite strong overlap in V- and I-band properties, the M87 GCs have UV-optical properties that are distinct from clusters in the Milky Way and in M31. M87 clusters, especially metal-poor ones, produce larger hot horizontal-branch populations than do Milky Way analogs. In color plots including the near-UV band, the M87 clusters appear to represent an extension of the Milky Way sequence. Cluster mass is probably not a factor in these distinctions. The most metal-rich M87 GCs in our sample are near solar metallicity and overlap the local E galaxy sample in estimated Mg2 line indices. Nonetheless, the clusters produce much more UV light at a given Mg2, being up to 1 mag bluer than any gE galaxy in (FUV-V) color. The M87 GCs do not appear to represent a transition between Milky Way-type clusters and E galaxies. The differences are in the correct sense if the clusters are significantly older than the E galaxies. Comparisons with Galactic open clusters indicate that the hot stars lie on the extreme horizontal branch, rather than being blue stragglers, and that the extreme horizontal branch becomes well populated for ages >~5 Gyr. Existing model grids for clusters do not match the observations well, due to poorly understood giant branch mass loss or perhaps high helium abundances. We find that 41 of our UV detections have no optical-band counterparts. Most appear to be UV-bright background galaxies seen through M87. Eleven near-UV variable sources detected at only one epoch in the central field are probably classical novae. Two recurrent variable sources have no obvious explanation but could be related to activity in the relativistic jet.