The Inner Halo of M87: A First Direct View of the Red-Giant Population

TL;DR

This study provides the first direct observation of the red-giant star population in M87's inner halo, estimating its metallicity distribution and distance using deep Hubble imaging.

Contribution

It offers the first direct measurement of the red-giant population in M87's inner halo and refines its distance using multiple methods.

Findings

Metallicity distribution peaks near [m/H] ~ -0.4

Distance to M87 estimated at approximately 16.4 Mpc

Red-giant population shape resembles that of NGC 5128

Abstract

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.