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

Abstract

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 MI(lim) = −2.5. After assessing the photometric completeness and biasses, we use this material to estimate the metallicity distribution for the inner halo of M 87, 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 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(M87) = (16.4 ± 0.5) Mpc.