Deep HST V- and I-band observations of the halo of M31: Evidence for multiple stellar populations

We present deep (V similar or equal to 27) V- and I-band stellar photometry obtained using the Hubble Space Telescope's WFPC2 in two fields in the M31 halo. These fields are located 32' and 50' from the center of M31 approximately along the SE minor axis at the locations of the M31 globular star clusters G302 and G312, respectively. The M31 halo luminosity functions are not consistent with a single high-metallicity population but are consistent with a mix of 50% to 75% metal-rich stars and 25% to 50% metal-poor stars. This is consistent with the observed red giant branch morphology, the luminosity of the horizontal branch, and the presence of RR Lyrae stars in the halo of M31. The morphology of the red giant branch indicates a spread in metallicity of -2 less than or similar to[m/H]less than or similar to-0.2 with the majority of stars having [m/H]less than or similar to-0.6, making the halo of M31 significantly more metal-rich than either the Galactic halo or the M31 globular cluster system. The horizontal branch is dominated by a red clump similar to the horizontal branch of the metal-rich Galactic globular cluster 47 Tuc but a small number of blue horizontal branch stars are visible, supporting the conclusion that there is a metal-poor component to the stellar population of the halo of M31. The number of blue horizontal branch stars is smaller than would be expected from the observed metallicity distribution but it is not clear if this is due to the photometric limits of our data or a second parameter effect in the halo of M31. We find a lower limit to the helium abundance of Y similar or equal to 0.20 to 0.27, comparable with the Galactic value. Luminosity functions show weak evidence that the R(M31)=50' field contains a higher fraction of metal-poor stars than the R(M31)=32' field but the metallicity distributions of the RGB stars in each field strongly suggest that both fields have the same mix of stellar populations. (C) 1996 American Astronomical Society.