Stellar Populations in the Outer Halo of the Massive Elliptical M49

TL;DR

This study uses deep photometry to analyze the stellar populations in the outer halo of elliptical galaxy M49, revealing extremely metal-poor stars and signs of ongoing accretion that challenge existing galaxy formation models.

Contribution

It provides detailed observations of M49's outer halo, highlighting its extremely metal-poor populations and the impact of accretion, which contrasts with predictions from hierarchical galaxy formation models.

Findings

Outer halo extends to ~100 kpc with steep color gradient.

Outer regions are very blue, indicating extremely metal-poor stars.

Accretion shells are redder, suggesting buildup of mass and metallicity.

Abstract

We use deep surface photometry of the giant elliptical M49 (NGC 4472), obtained as part of our survey for diffuse light in the Virgo Cluster, to study the stellar populations in its outer halo. Our data trace M49's stellar halo out to ~ 100 kpc (7 Re), where we find that the shallow color gradient seen in the inner regions becomes dramatically steeper. The outer regions of the galaxy are quite blue (B-V ~ 0.7); if this is purely a metallicity effect, it argues for extremely metal poor stellar populations with [Fe/H] < -1. We also find that the extended accretion shells around M49 are distinctly redder than the galaxy's surrounding halo, suggesting that we are likely witnessing the buildup of both the stellar mass and metallicity in M49's outer halo due to late time accretion. While such growth of galaxy halos is predicted by models of hierarchical accretion, this growth is thought to be driven by more massive accretion events which have correspondingly higher mean metallicity than inferred for M49's halo. Thus the extremely metal-poor nature of M49's extended halo provides some tension against current models for elliptical galaxy formation.