The Resolved Stellar Halo of NGC 253

TL;DR

This study uses deep imaging to analyze the structure, mass, and shape of NGC 253's stellar halo, revealing its flattened geometry, mass distribution, and substructures, and compares it to other galaxies within a cosmological framework.

Contribution

First detailed quantitative analysis of NGC 253's stellar halo outside the Local Group using combined ground and space-based imaging.

Findings

Halo is significantly flattened with axis ratio ~0.35

Stellar halo mass is approximately 2.5 billion solar masses, about 6% of galaxy mass

Halo profile follows a power law with index -2.8, indicating steep density decline

Abstract

We have obtained Magellan/IMACS and HST/ACS imaging data that resolve red giant branch stars in the stellar halo of the starburst galaxy NGC 253. The HST data cover a small area, and allow us to accurately interpret the ground-based data, which cover 30% of the halo to a distance of 30 kpc, allowing us to make detailed quantitative measurements of the global properties and structure of a stellar halo outside of the Local Group. The geometry of the halo is significantly flattened in the same sense as the disk, with a projected axis ratio of b/a ~ 0.35 +/- 0.1. The total stellar mass of the halo is estimated to be M_halo ~ 2.5 +/- 1.5 x 10^9 M_sun, or 6% of the total stellar mass of the galaxy, and has a projected radial dependence that follows a power law of index -2.8 +/- 0.6, corresponding to a three-dimensional power law index of ~ -4. The total luminosity and profile shape that we measure for NGC 253 are somewhat larger and steeper than the equivalent values for the Milky Way and M31, but are well within the scatter of model predictions for the properties of stellar halos built up in a cosmological context. Structure within the halo is seen at a variety of scales: there is small kpc-scale density variation and a large shelf-like feature near the middle of the field. The techniques that have been developed will be essential for quantitatively comparing our upcoming larger sample of observed stellar halos to models of halo formation.