037-B327 in M31: Luminous Globular Cluster or Core of a Former Dwarf Spheroidal Companion to M31?

Abstract

The cluster 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 Telescope provide photometric data in the F606W band and also show that this cluster is crossed by a dust lane. We have determined the structural parameters of 037-B327 by fitting the observed surface brightness distribution to a King model with r_c=0.72" (=2.69 pc), r_t=5.87" (=21.93 pc), and a concentration index c=log(r_t/r_c)=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, r_h=1.11" (=4.15 pc). Combined with previous photometry, we find that this object falls in the same region of the M_V versus logR_h 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.

Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with proposal 9453.