UV bright globular clusters in M87: more evidence for super-He-rich stellar populations?

Abstract

We study the ultraviolet (UV) and optical properties of 38 massive globular clusters (GCs) in the Virgo elliptical, M87, imaged using the STIS and WFPC2 instruments onboard the Hubble Space Telescope. The majority of these GCs appear extremely bright in the far-UV (FUV) – roughly a magnitude brighter than their Galactic counterparts with similar metallicities. The observed FUV flux is several times larger than predictions of canonical old stellar population models. These canonical models, which assume a fiducial helium enrichment parameter, ΔY/ΔZ= 2, are able to reproduce the observed FUV fluxes only if ages ∼3–5 Gyr larger than the Wilkinson Microwave Anisotropy Probe (WMAP) age of the Universe are invoked, although the same models fit the UV photometry of Galactic and M31 GCs for ages less than the ‘WMAP age’. A similar discrepancy (∼3 Gyr) is found between the mass-weighted and UV-luminosity-weighted ages of the massive Galactic GC ω Cen, whose colour–magnitude diagram (including peculiar features on its well-populated horizontal branch) can be accurately reproduced by invoking a small super-He-rich (ΔY/ΔZ≳ 90) stellar component. By comparison to ω Cen, we propose that the majority of M87 GCs in our sample contain strong signatures of similarly minor super-He-rich subcomponents. This hypothesis is supported by simulations which suggest that, based on the UV detection limit of this survey, the number of GCs detected is several times that predicted by canonical models. Although we cannot prove or disprove the extreme helium scenario at the moment, we show that the same phenomenon that causes the extended horizontal branch of ω Cen explains the UV brightness of our sample. If this is indeed due to the extreme helium, this study would be the first to find its signatures in extragalactic objects.