| Abstract | We have used the ACS camera on HST to obtain (V, I) photometry for 5300 red giant stars in the halo of the dominant Leo group member NGC 3379, a galaxy usually regarded as a classic normal giant elliptical. We use this data to derive the metallicity distribution function (MDF) for its outer-halo field stars at a location centered 33 kpc from the galaxy center. In at least two ways the MDF is distinctly unlike all the other E galaxies for which similar data exist. First, the NGC 3379 MDF is extremely broad and flat, with many stars at every interval in [m/H]. Second, we see a metallicity gradient such that in ithe outermost parts of the field the low-metallicity stars ([m/H]<-0.7) begin to dominate and the higher metallicity stars are rapidly diminishing. We find that a distinct two-stage chemical evolution model is necessary to explain the MDF shape. Our target field is centered at a projected distance of 12Re, twice as far out in units of effective radius as in any of the other galaxies yet surveyed. If NGC 3379 is indeed representative of large E/S0 galaxies, we predict that other such galaxies will reveal diffuse low-metallicity subpopulations, but that photometry at radii r~=(10-15)Re will be necessary to get beyond the edge of the dominant metal-rich component. 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 number HST-GO-09811.01-A 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. |
