We study the UV and optical properties of 38 massive globular clusters ( GCs ) in the Virgo elliptical , M87 , imaged using the STIS and WFPC 2 instruments onboard the Hubble Space Telescope .
The majority of these GCs appear extremely bright in the far-ultraviolet ( 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 , \Delta Y / \Delta Z = 2 , are able to reproduce the observed FUV fluxes only if ages \sim 3–5 Gyr larger than the “ 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 ( \sim 3 Gyr ) is found between the mass-weighted and UV -luminosity weighted ages of the massive Galactic GC \omega  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 ( \Delta Y / \Delta Z \ga 90 ) stellar component .
By comparison to \omega  Cen , we propose that the majority of M87 GCs in our sample contain strong signatures of similarly minor super-He-rich sub-components .
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 of the prediction from canonical models .
Although we can not prove or disprove the extreme helium scenario at the moment , we show that the same phenomenon that causes the extended horizontal branch of \omega  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 .