037-b327 in M31: Luminous Globular Cluster or Core of a Former Dwarf Spheroidal Companion to M31?

037-B327 is of interest because it is both the most luminous and the most highly reddened cluster known in M31. Deep observations with the Advanced Camera for Surveys on the Hubble Space T elescope provide photometric data in the F606W band, and also show that this cluster is crossed by a dust lane. We determined the structural parameters of 037-B327 by fitting the observed surface brightness distribution to a King model with rc = 0.72 (= 2.69 pc), and rt = 5.87 (= 21.93 pc), and a concentration index c = log(rt/rc) = 0.91. The surface brightness profile appears to be essentially flat within 0.25 of the center and shows no signs of core collapse. Although the dust lane affects the photometry, the King model fits the surface brightness profile well except for the regions badly affected by the dust lane. We also calculate the half-light radius rh = 1.11 (= 4.15 pc). Combined with previous photometry, we find that this object falls in the same region of the MV versus log Rh diagram as do ω Centauri, M54 and NGC 2419 in the Milky Way and the massive cluster G1 in M31. All four of these objects have been claimed to be the stripped cores of former dwarf galaxies. This suggests that 037-B327 may also be the stripped core of a former dwarf companion to M31. Subject headings: galaxies: evolution – galaxies: individual (M31) – globular cluster: individual (037B327)