The Leo Elliptical NGC 3379: A Metal‐Poor Halo Emerges

Abstract

We have used the ACS camera on HST to obtain (V,I) photometry for 5300\nred-giant stars in the halo of the dominant Leo-group member NGC 3379, a galaxy\nusually regarded as a classic normal giant elliptical. We use this sample of\nstars to derive the metallicity distribution function (MDF) for its outer-halo\nfield stars 33 kpc from the galaxy center. The MDF is distinctly unlike all the\nother E galaxies for which we have similar data (including the Local Group\ndwarf ellipticals, the intermediate-luminosity NGC 3377, and the giant NGC\n5128). First, the MDF for the NGC 3379 outer halo is broad and flat, with many\nstars at every interval in [m/H]. Second, we see a metallicity gradient across\nour ACS field such that in its outermost region the blue, low-metallicity stars\n([m/H] < -0.7) are beginning to dominate and the higher-metallicity stars are\nrapidly diminishing. Our target field is centered at a projected distance about\nequal to 12 R_e, twice as far out in units of effective radius as in any of the\nother galaxies that we have surveyed. If NGC 3379 is indeed representative of\nlarge E/S0 galaxies, we predict that such galaxies in general will reveal\ndiffuse low-metallicity subpopulations, but that photometry at radii r ~ 10 -\n15 R_e will be necessary to see the faint low-metallicity component clearly.\n