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

TL;DR

This study uses HST photometry to analyze the metallicity distribution of stars in NGC 3379's outer halo, revealing a broad, flat MDF with a gradient indicating a dominance of low-metallicity stars at large radii.

Contribution

It provides the first detailed MDF of NGC 3379's outer halo, showing a broad, flat distribution and a metallicity gradient, suggesting diffuse low-metallicity populations in large elliptical galaxies.

Findings

The MDF is broad and flat with many stars at all metallicities.

A metallicity gradient shows low-metallicity stars dominate at larger radii.

Outer halo stars are predominantly low-metallicity at ~12 R_e.

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.