Completing an Accurate Map of M31 Microlensing
The halo microlensing masses detected in the MACHO survey (claimed to compose about 20% of the Galaxy's mass) represent a major enigma in astrophysics, one that must be effectively cross-examined by an independent test. We have completed a large, densely-sampled survey of M31 that can reveal in another galaxy such a halo microlensing signal if it exists. In a previous HST/ACS+WFPC2 program (GO 10273, Cycle 13, 16 orbits) we were able to learn considerably more about a subsample of these M31 microlensing events. We were pleased to find that in most cases we could isolate the source star for each event, find its baseline flux and colors (essential for ruling out classes of confusing variable stars), test for misidentification of background supernovae, and measure the Einstein parameters, which constrain the range of most likely lens mass. (These Cycle 13 results are published in The Astrophysical Journal Letters.) We propose to finish the job, taking a similar series of exposures to more than double the sample of well-constrained microlensing events, which together with the larger ground-based sample for which we are completing our analyses will provide 20-30 M31 bona fide microlensing events observed by HST. This will be done via a series of targetted PC exposures, meant to maximize the number of candidates studied, one (or two) at a time. A sample of this size and quality should be sufficient to settle the issue of a significant contribution to the halos of galaxies by stellar-mass lenses. Furthermore, if there is a surplus of such microlensing events above what might be expected from stars alone, the higher quality of information will allow us to more accurately describe the spatial distribution of these lenses. We will also complete several unique studies of M31 stellar populations, both in support of the microlensing measurement and in their own right.
These consist of ten visits of one orbot apiece consisting of WPFC2 prime observations on two targets, some separated by less than 2arcmin and most by more than 2arcmin. There are long parallel integrations on NIC2 as proposed to obtain images for faint stellar population studies.
RESOLVED STELLAR POPULATIONS
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