The Leo Elliptical NGC 3379: A Metal‐Poor Halo Emerges
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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 sample of
stars to derive the metallicity distribution function (MDF) for its outer-halo
field stars 33 kpc from the galaxy center. The MDF is distinctly unlike all the
other E galaxies for which we have similar data (including the Local Group
dwarf ellipticals, the intermediate-luminosity NGC 3377, and the giant NGC
5128). First, the MDF for the NGC 3379 outer halo is broad and flat, with many
stars at every interval in [m/H]. Second, we see a metallicity gradient across
our ACS field such that in its outermost region the blue, low-metallicity stars
([m/H] < -0.7) are beginning to dominate and the higher-metallicity stars are
rapidly diminishing. Our target field is centered at a projected distance about
equal to 12 R_e, twice as far out in units of effective radius as in any of the
other galaxies that we have surveyed. If NGC 3379 is indeed representative of
large E/S0 galaxies, we predict that such galaxies in general will reveal
diffuse low-metallicity subpopulations, but that photometry at radii r ~ 10 -
15 R_e will be necessary to see the faint low-metallicity component clearly.
