The inner halo of M 87: a first direct view of the red-giant population

An unusually deep (V, I) imaging dataset for the Virgo supergiant M 87 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] similar or equal to -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 M 87 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(M 87) = (16.4+/-0.5) Mpc.