An unusually deep (V,I) imaging dataset for the Virgo supergiant M87 with the
Hubble Space Telescope ACS successfully resolves its brightest red-giant stars,
reaching M_I(lim) = -2.5. After assessing the photometric completeness and
biasses, we use this material to estimate the metallicity distribution for the
inner halo of M87, finding that the distribution is very broad and likely to
peak near [m/H] ~ -0.4 and perhaps higher. The shape of the MDF strongly
resembles that of the inner halo for the nearby giant E galaxy NGC 5128. As a
byproduct of our study, we also obtain a preliminary measurement of the
distance to M87 with the TRGB (red-giant branch tip) method; the result is
(m-M)_0 = 31.12 +- 0.14 (d = 16.7 +- 0.9 Mpc). Averaging this result with three
other recent techniques give a weighted mean d(M87) = (16.4 +- 0.5) Mpc.Comment: In press for Astronomy and Astrophysic
