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R.M. Rich, D. Reitzel (Department of Physics and Astronomy, UCLA), K. Gebhardt (Department of Astronomy, U.T. Austin), L.C. Ho (Carnegie Observatories), P. Guhathakurta (UCO/Lick Observatories, University of California)
We report photometry from WFPC2 parallel imagery obtained for a field in the M31 halo near the luminous globular cluster G1. The field lies on the SW major axis at a projected distance of 34 kpc from the nucleus of M31. The F606W (V) and F814W (I) images were obtained in parallel with STIS spectroscopy of G1 (GO-9099) and total 7.06 hours of integration time--the deepest HST image of the M31 halo to date. The color-magnitude diagram (CMD) of the field shows a clearly defined red clump at V=24.5 and a metal rich red giant branch. While the field CMD is similar to that of G1, the field red giant branch luminosity function is steeper than that of G1. We construct the luminosity function to V=28 using stars with 5-sigma detections in F606W only. We compare our luminosity function to those of Bergbusch and VandenBerg (2001 ApJ 556, 322); we find a clear excess of stars at V>27 relative to that expected for a 13 Gyr old stellar population. At V=28 we find nearly a factor of 5 more stars than would be expected for a pure stellar population of 13 Gyr, even restricting our sample to stars with 5-sigma detections and accounting for extragalactic sources. Artificial star experiments show that the excess is not due to blended images. We conclude that most of the stellar population in this field is younger than 8 Gyr. As this field lies near the major axis of M31, the field stars might belong to the extreme outer disk as opposed to the true halo.
Based on observations made with the the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA Contract NAS-5-26555. Support for proposal GO-9099 was provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.
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Bulletin of the American Astronomical Society, 34, #4
© 2002. The American Astronomical Soceity.