Discovery and analysis of three faint dwarf galaxies and a globular cluster in the outer halo of the Andromeda galaxy

Abstract

We present the discovery of three faint dwarf galaxies and a globular cluster in the halo of the Andromeda galaxy (M31), found in our MegaCam survey that spans the southern quadrant of M31, from a projected distance of ~50 to ~150 kpc. Though the survey covers 57 deg^2, the four satellites lie within 2° of one another. From the tip of the red giant branch (RGB), we estimate that the globular cluster lies at a distance of 631 ± 58 kpc from the Milky Way and along with a ~100 kpc projected distance from M31 we derive a total distance of 175 ± 55 kpc from its host, making it the farthest M31 globular cluster known. It also shows the typical characteristics of a bright globular cluster, with a half-light radius of 2.3 ± 0.2 pc and an absolute magnitude in the V band of M_(V,0)=−8.5 ± 0.3. Isochrone fitting reveals that it is dominated by a very old population with a metallicity of [Fe/H]~−1.3. The three dwarf galaxies are revealed as overdensities of stars that are aligned along the RGB tracks in their colour–magnitude diagrams. These satellites are all very faint, with absolute magnitudes in the range −7.3 ≲M_(V,0)≲−6.4, and show strikingly similar characteristics with metallicities of [Fe/H]∼−1.4 and half-light radii of ∼120 ± 45 pc, making these dwarf galaxies two to three times smaller than the smallest previously known satellites of M31. Given their faintness, their distance is difficult to constrain, but we estimate them to be between 740 and 955 kpc which places them well within the virial radius of the host galaxy. The panoramic view of the MegaCam survey can provide an unbiased view of the satellite distribution of the Andromeda galaxy and, extrapolating from its coverage of the halo, we estimate that up to 45 ± 20 satellites brighter than M_(V)~−6.5 should be orbiting M31. Hence faint dwarf galaxies cannot alone account for the missing satellites that are predicted by Λ cold dark matter models, unless they reside in dark matter minihaloes that are more massive than the typical masses of 10^7 M_⊙ currently inferred from their central radial velocity dispersion.