LOCAL GROUP POPULATIONS WITH THE HUBBLE-SPACE-TELESCOPE .1. THE M31 GLOBULAR-CLUSTER G1=MAYALL-II

We present WFPC2 photometry of the M31 globular cluster G1 and its adj acent halo field population. The cluster is one of the most luminous i n the Local Group (approximate to 10(6) L.) and is at a projected dist ance of 39.5 kpc from the nucleus near the major axis of M31. The colo r of the red giant branch is similar to that of 47 Tuc, confirming pre vious ground-based imaging of its brightest giants. G1 has a red horiz ontal branch at V=25.32+/-0.05 mag (M(V)=+0.82 mag) and a small number of blue horizontal branch stars at (V-l)=0.0 mag. We find no evidence for an extended giant branch: a deep image at 1 mu m finds no red, lu minous AGE stars. We conclude that the properties of the color-magnitu de diagram are most consistent with those of an old globular cluster w ith the metallicity of 47 Tuc. The surrounding halo field has a red ho rizontal branch at the same color and luminosity of the cluster. The l uminosity 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 luminos ity function. If this difference is real, then either the outer M31 ha lo largely consists of stars as metal rich as 47 Tuc, or the red horiz ontal branch and steep giant branch luminosity function may be the pro duct of a field population younger than the oldest Galactic globular c lusters. We present a surface brightness profile for G1. We find a hig h 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. (C) 1996 American Astronomical Society.