A LARGE GROUP OF ASYMPTOTIC GIANT BRANCH STARS IN THE DISK OF M31: A MISSING PIECE OF THE PUZZLE?

We examine the properties of a stellar grouping that is ~3.5 kpc to the northeast of the center of M31. This structure has (1) a surface brightness that is lower than the surrounding disk, (2) a more-or-less round appearance, (3) a size of ~300 arcsec (~1 kpc), and (4) an integrated brightness K = 6.5. It is populated by stars with ages ≥100 Myr and J – K colors that tend to be bluer than those of stars in the surrounding disk. Comparisons with model luminosity functions suggest that the star formation rate in this object has changed twice in the past few hundred Myr. Fitting a Sersic function to the light profile reveals a power-law index and effective surface brightness that are similar to those of dwarf galaxies with the same integrated brightness. Two possible origins for this object are considered: (1) it is a heretofore undiscovered satellite of M31 that is seen against/in/through the M31 disk or (2) it is a fossil star-forming region in the M31 disk.