Search for Intra-Cluster Machos by Pixel Lensing of M87

Abstract

Intra-Cluster Machos (ICMs) are a plausible candidate for at least some of the dark matter in clusters of galaxies. ICMs can be detected by searching toward M87 for ``pixel lensing'', gravitational microlensing of unresolved stars. Dedicated observations by the Wide Field and Planetary Camera on the {\it Hubble Space Telescope} would discover lenses at a rate $\Gamma \sim 7f\mathrm{da}{\mathrm{y}}^{-1}$ where $f$ is the fraction of the Virgo cluster composed of Machos (Massive Compact Objects). The only important background is lensing by stars in M87 itself. Such self-lensing should be detected at a rate $\Gamma \sim 1\mathrm{da}{\mathrm{y}}^{-1}$. However, most of these background events should be recognizable from their angular distribution and/or measurement of their proper motions (which are smaller by a factor $\sim 2$ than those of ICMs). The observations should be carried out principally in $I$, but with $V$ measurements made $\sim 20\%$ of the time in order to determine the colors of the lensed stars. It is also possible to detect ``Class II'' lensing events which are longer and hence for which lower signal to noise is balanced by a much larger number of observations per event. There are $\sim 5$ times more of these than the higher quality events. However, it is difficult to distinguish Class II events generated by ICMs from those generated by M87 stars.

16 pages, no figures, uuencoded postscript. Changes are mostly related to including a large group of newly-recognized `Class II' events.

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