2010AJ....139.2279K

We present a statistical characterization of the carbon-star to M-giant (C/M) ratio in the halo of M31. Based on the application of pseudo-filter bandpasses 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 these regions indicates that the (outer) M31 halo cannot be dominated by the debris of disk-like or Small-Magellanic-Cloud-type galaxies, but rather resemble the dwarf elliptical NGC 147.