The Missing Bulge Globular Clusters in M31: New Optical Candidates
A. Wirth, L. Smarr, and T. L. Bruno, Astrophysical Journal, 290, 140-153. (1985).
Abstract
We use a new method to attack the question of the “missing” globular clusters in the bulge of M31. Image processing techniques were used on 13 video camera fields to obtain an accurate photometric census of stellar
objects in M31’s bulge down to a limiting B magnitude of 21. This luminosity distribution is compared with the Bahcall-Soneira model of galactic foreground stars. We find a statistically significant excess of bright images in the luminosity range of globular clusters at M31’s distance. If our optical candidates prove to be globular clusters, they would double the number of known globular clusters in the surveyed region. The colors of a subsample of our candidates are the same as those of the known globular clusters. We conclude that the previously observed flattening away from a de Vaucouleurs law in the radial distribution of M31 may be an observational selection effect. As an offshoot of this analysis, we find no evidence for very luminous stars in the inner bulge of M31. The lack of such stars indicates that there has not been active star formation (with a normal IMF) in the recent past. Coupled with the existence of many planetary nebulae in the bulge, this may strengthen the case for a galactic wind in M31’s bulge.