Local Group Populations With the Hubble Space Telescope. I. The M31 Globular Cluster G1=Mayall II

Abstract

We present WFPC2 photometry of the M31 globular cluster G1 and its adjacent halo field population. The cluster is one of the most luminous in the Local Group (~10^6^ L_sun_) and is at a projected distance of 39.5 kpc from the nucleus near the major axis of M31. The color of the red giant branch is similar to that of 47 Tuc, confirming previous ground- based imaging of its brightest giants. G1 has a red horizontal branch at V = 25.32 +/- 0.05 mag (M_v_ = +0.82 mag) and a small number of blue horizontal branch stars at (V - I) = 0.0 mag. We find no evidence for an extended giant branch: a deep image at 1 micron finds no red, luminous AGB stars. We conclude that the properties of the color-magnitude diagram are most consistent with those of an old globular cluster with the metallicity of 47 Tuc. The surrounding halo field has a red horizontal branch at the same color and luminosity of the cluster. The luminosity function of the field in V and I is more steep (fewer bright stars) than that of G1. While G1 is well fit by published luminosity functions of 47 Tuc and M3, these templates fail for the field luminosity function. If this difference is real, then either the outer M31 halo largely consists of stars as metal rich as 47 Tuc, or the red horizontal branch and steep giant branch luminosity function may be the product of a field population younger than the oldest Galactic globular clusters. We present a surface brightness profile for G1. We find a high central surface brightness of V = 13.5 mag arcsec^-2^. The core radius of G1 is r_c_ = 0.54 pc and its tidal radius is r_t_ = 90.0 pc.