A statistical analysis of the late-type stellar content in the Andromeda halo

TL;DR

This study statistically characterizes the ratio of carbon stars to M-giants in M31's halo, revealing a low C/M ratio consistent across different regions, challenging previous assumptions about the halo's stellar composition.

Contribution

It provides the first detailed statistical analysis of the C/M ratio in M31's halo using narrow-band photometry and spectroscopy, highlighting a low C/M ratio across the halo.

Findings

Low C/M ratio (~10%) in M31 halo and stream

No significant difference in C-star populations across regions

Halo's stellar content resembles dwarf elliptical galaxies

Abstract

We present a statistical characterization of the carbon-star to M-giant (C/M) ratio in the halo of M31. Based on application of pseudo-filter band passes to our Keck/DEIMOS spectra we measure the 81-77-color index of 1288 stars in the giant stellar stream and in halo fields out to large distances. From this well-established narrow-band system, supplemented by V-I colors, we find only a low number (five in total) of C-star candidates. The resulting low C/M ratio of 10% is consistent with the values in the M31 disk and inner halo from the literature. Although our analysis is challenged by small number statistics and our sample selection, there is an indication that the oxygen-rich M-giants occur in similar number throughout the entire halo. We also find no difference in the C-star population of the halo fields compared to the giant stream. The very low C/M ratio is at odds with the observed low metallicities and the presence of intermediate-age stars at large radii. Our observed absence of a substantial carbon star population in the these regions indicates that the (outer) M31 halo cannot be dominated by the debris of disk-like or SMC-type galaxies, but rather resemble the dwarf elliptical NGC 147.