UV Insights into the Complex Populations of M 87 Globular Clusters
  We have imaged with HST's WFC3/UVIS the central 2.7$\times$2.7 arcmin$^2$
region of the giant elliptical galaxy M 87, using the ultraviolet filter F275W.
In combination with archival ACS/WFC data taken through the F606W and F814W
filters, covering the same field, we have constructed integrated-light
UV-optical colors and magnitudes for 1460 objects, most of which are believed
to be globular clusters belonging to M 87. The purpose was to ascertain whether
the multiple-populations syndrome, ubiquitous among Galactic globular clusters
(GCs), exists also among the M 87 family of clusters. To achieve this goal, we
sought those GCs with exceptionally blue UV-to-optical colors, because
helium-enriched sub-populations produce a horizontal-branch morphology that is
well populated at high effective temperature. For comparison, integrated,
synthetic UV$-$optical and purely optical colors and magnitudes have been
constructed for 45 Galactic GCs, starting from individual-star photometry
obtained with the same instruments and the same filters. We identify a small
group of M 87 clusters exhibiting a radial UV$-$optical color gradient,
representing our best candidate GCs hosting multiple populations with extreme
helium content. We also find that the central spatial distribution of the bluer
GCs is flattened in a direction parallel to the jet, while the distribution of
redder GCs is more spherical. We release to the astronomical community our
photometric catalog in F275W, F606W and F814W bands and the high-quality image
stacks in the same bands.
