Astronomy and Astrophysics – Astrophysics
Scientific paper
1996-12-04
Astronomy and Astrophysics
Astrophysics
15 pages, 4 figures. MNRAS, in press
Scientific paper
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.
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