Red Giants in the Halo of the S0 Galaxy NGC 3115: A Distance and a Bimodal Metallicity Distribution

Rebecca A. W. Elson

doi:10.1093/mnras/286.3.771

paper

Red Giants in the Halo of the S0 Galaxy NGC 3115: A Distance and a Bimodal Metallicity Distribution

arXiv:astro-ph/9612037 · doi:10.1093/mnras/286.3.771

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 1.5 magnitudes below the tip of the red giant branch. From the brightest 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 excellent agreement with the value $(m - M)_{0} = 30.17 \pm 0.13$ 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] $\sim - 0.7$ and one has [Fe/H] $\sim - 1.3$ . 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.

15 pages, 4 figures. MNRAS, in press

Red Giants in the Halo of the S0 Galaxy NGC 3115: A Distance and a Bimodal Metallicity Distribution

Abstract

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