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

¹ Tuorla Observatory, University of Turku, Väsiäläntie 20, 21500 Piikkiö, Finland
e-mail: sarbir@utu.fi; cflynn@utu.fi
² Department of Physics and Astronomy, McMaster University, Hamilton L8S 4M1, Canada
e-mail: harris@physics.mcmaster.ca
³ Herzberg Institute of Astrophysics, National Research Council of Canada, Victoria BC V9E 2E7, Canada
e-mail: john.blakeslee@nrc.ca
⁴ Finnish Centre for Astronomy with ESO (FINCA), University of Turku, Väsiäläntie 20, 21500 Piikkiö, Finland
⁵ Department of Physics and Astronomy, University of Sydney, Sydney, Australia

Received: 27 April 2010
Accepted: 15 September 2010

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 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 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.