  The position of the giant elliptical galaxy M87 at the center of the Virgo
Cluster means that the inferred column density of dark matter associated with
both the cluster halo and the galaxy halo is quite large. This system is thus
an important laboratory for studying massive dark objects in elliptical
galaxies and galaxy clusters by gravitational microlensing, strongly
complementing the studies of spiral galaxy halos performed in the Local Group.
We have performed a microlensing survey of M87 with the WFPC2 instrument on the
Hubble Space Telescope. Over a period of thirty days, with images taken once
daily, we discover seven variable sources. Four are variable stars of some
sort, two are consistent with classical novae, and one exhibits an excellent
microlensing lightcurve, though with a very blue color implying the somewhat
disfavored possibility of a horizontal branch source being lensed. Based on
sensitivity calculations from artificial stars and from artificial lightcurves,
we estimate the expected microlensing rate. We find that the detection of one
event is consistent with a dark halo with a 20% contribution of microlensing
objects for both M87 and the Virgo Cluster, similar to the value found from
observations in the Local Group. Further work is required to test the
hypothesized microlensing component to the cluster.
