Red giants in the halo of the SO galaxy NGC 3115: A distance and a bimodal metallicity distribution

Using the Hubble Space Telescope, we resolve the red giant branch in the halo of the S0 galaxy NGC 3115. We measure magnitudes and (V - I) colours for stars down to 1.5 mag below the tip of the red giant branch. From the brightest stars we estimate a distance modulus (m - M)(0) = 30.21 +/- 0.30, corresponding to a distance of 11.0 +/- 1.5 Mpc. This is in excellent agreement with the value (m - M)(0) = 30.17 +/- 0.13 determined from the planetary nebula luminosity function. Our results rule out the shorter distance modulus (m - M)(0) = 29.65 determined from surface brightness fluctuations. A histogram of (V - I) colours shows a clear bimodality, indicating the presence of two distinct halo populations of roughly equal size. One has [Fe/H] similar to -0.7 and one has [Fe/H] similar to -1.3. This is the most distant galaxy in which a Population II halo has been resolved, and it is the first time a colour bimodality has been observed among the halo stars of any early-type galaxy.