Hubble Space Telescope Photometry for the Halo Stars in the Leo Elliptical NGC 3377**Based on observations made with 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. These observations are associated with program 9811. Support for this work was provided in part by NASA through grant HST-GO-09811.01-A from the Space Telescope Science Institute, under NASA contract NAS 5-26555.

We have used the ACS camera on HST to obtain (V, I) photometry for 57,000 red giant stars in the halo of the Leo elliptical NGC 3377, an intermediate-luminosity elliptical. We use this sample of stars to derive the metallicity distribution function (MDF) for its halo field stars and comment on its chemical evolution history compared with both larger and smaller E galaxies. Our ACS WFC field spans a radial range extending from 4 to 18 kpc projected distance from the center of NGC 3377 and thus covers a significant portion of this galaxy's halo. We find that the MDF is broad, reaching a peak at log(Z/Z⊙) ≃ -0.6, but containing virtually no stars more metal-poor than log(Z/Z⊙) = -1.5. It may, in addition, have relatively few stars more metal-rich than log(Z/Z⊙) ≃ -0.3, although interpretation of the high-metallicity end of the MDF is limited by photometric completeness that affects the detection of the reddest, most metal-rich stars. NGC 3377 appears to have an enrichment history intermediate between those of normal dwarf ellipticals and the much larger giants. As yet, we find no clear evidence that the halo of NGC 3377 contains a significant population of "young" (<3 Gyr) stars.