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 haloof the S0 galaxy NGC 3115. We measure magnitudes and $(V-I)$ colours for starsdown to 1.5 magnitudes below the tip of the red giant branch. From thebrightest stars we estimate a distance modulus $(m-M)_0=30.21 \pm 0.30$,corresponding to a distance of $11.0 \pm 1.5$ Mpc. This is in excellentagreement with the value $(m-M)_0=30.17 \pm 0.13$ determined from the planetarynebula 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 twodistinct halo populations of roughly equal size. One has [Fe/H]$\sim -0.7$ andone has [Fe/H]$\sim -1.3$. This is the most distant galaxy in which aPopulation II halo has been resolved, and it is the first time a colourbimodality has been observed among the halo stars of any early-type galaxy.Comment: 15 pages, 4 figures. MNRAS, in pres