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 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 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 extremely broad and flat , with many stars at every interval in [ m/H ] and only a gradual rise towards higher metallicity .
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 .
In order to successfully match this extremely broad MDF , we find that a distinct two-stage chemical evolution model is necessary .
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 \simeq 10 - 15 R _ { e } will be necessary to get beyond the edge of the dominant metal-rich component and to see the faint low-metallicity component clearly .
Finally , we discuss possible connections of these outer-halo stars with the metallicity distributions that are beginning to be observed in the intracluster medium of nearby rich galaxy clusters , which also show flat MDFs .
These outermost-halo observations are challenging , but ones which may give an unusually direct window into the earliest star-forming stages of these galaxies .