| Abstract | 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 1.5mag 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+/-1.5Mpc. 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]~-0.7 and one has [Fe/H]~-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. |
