Detection of the Main-Sequence Turnoff of a Newly Discovered Milky Way Halo Structure in the Triangulum-Andromeda Region

An upper main sequence (MS) and main-sequence turnoff (MSTO) feature appears in the color-magnitude diagram (CMD) of a large-area photometric survey of the southern half of M31 stretching to M33. Imaging in the Washington M, T2, DDO51 system allows us to remove the background M31/M33 giants from our CMD and more clearly see the dwarf star feature, which has an MSTO near M ~ 20.5. The corresponding stellar population shows little density variation over the 12° × 6° area of the sky sampled and is of very low surface brightness, Σ > 32 mag arcsec-2. We show that this feature is not the same as a previously identified MS+MSTO in the foreground of the Andromeda galaxy that has been associated with the tidal stream ringing the Milky Way disk at less than half the distance. Thus, the new stellar system is a separate, more distant entity, perhaps a segment of tidal debris from a disrupted satellite galaxy. It is most likely related to the structure with similar distance, location, and density uniformity seen as an excess of K and M giants in the Two Micron All Sky Survey reported in the companion paper by Rocha-Pinto and coworkers.