HST Photometry for the Halo Stars in the Leo Elliptical NGC 3377

TL;DR

This study uses HST photometry to analyze the metallicity distribution of halo stars in NGC 3377, revealing an intermediate chemical evolution history and a broad metallicity range with no significant young star population.

Contribution

First detailed metallicity distribution function for NGC 3377's halo stars, providing insights into its chemical evolution compared to other elliptical galaxies.

Findings

MDF peaks at [m/H] ~ -0.6

Virtually no stars more metal-poor than log [m/H] = -1.5

No clear evidence of young (<3 Gyr) stars in the halo

Abstract

We have used the ACS camera on HST to obtain (V,I) photometry for 57,000 red-giant stars in the halo of the Leo elliptical NGC 3377. We use this sample of stars to derive the metallicity distribution function (MDF) for its halo field stars, and comment on its chemical evolution history compared with both larger and smaller E galaxies. Our ACS/WFC field spans a radial range extending from 4 to 18 kpc projected distance from the center of NGC 3377 and thus covers a significant portion of this galaxy's halo. We find that the MDF is broad, reaching a peak at [m/H] ~ -0.6$, but containing virtually no stars more metal-poor than log [m/H] = -1.5$. It may, in addition, have relatively few stars more metal-rich than [m/H] = -0.3$, although interpretation of the high-metallicity end of the MDF is limited by photometric completeness that affects the detection of the reddest, most metal-rich stars. NGC 3377 appears to have an enrichment history intermediate between those of normal dwarf ellipticals and the much larger giants. As yet, we find no clear evidence that the halo of NGC 3377 contains a significant population of ``young'' (< 3 Gy) stars.