Mon. Not. R. Astron. Soc., 429, 281-293 (2013/February-2)
Young accreted globular clusters in the outer halo of M31.
MacKEY A.D., HUXOR A.P., FERGUSON A.M.N., IRWIN M.J., VELJANOSKI J., McCONNACHIE A.W., IBATA R.A., LEWIS G.F. and TANVIR N.R.
Abstract (from CDS):
We report on observations of two newly discovered globular clusters in the outskirts of M31 made using the Gemini Multi-Object Spectrograph (GMOS) instrument on Gemini North. These objects, PAndAS-7 (PA-7) and PAndAS-8 (PA-8), lie at a galactocentric radius of ~87kpc and are projected, with separation ~19kpc, on to a field halo substructure known as the South-West Cloud. We measure radial velocities for the two clusters which confirm that they are almost certainly physically associated with this feature. Colour-magnitude diagrams reveal strikingly short, exclusively red horizontal branches in both PA-7 and PA-8; both also have photometric [Fe/H] = -1.35±0.15. At this metallicity, the morphology of the horizontal branch is maximally sensitive to age, and we use the distinctive configurations seen in PA-7 and PA-8 to demonstrate that both objects are very likely to be at least 2Gyr younger than the oldest Milky Way globular clusters. Our observations provide strong evidence for young globular clusters being accreted into the remote outer regions of M31 in a manner entirely consistent with the established picture for the Milky Way, and add credence to the idea that similar processes play a central role in determining the composition of globular cluster systems in large spiral galaxies in general.
© 2012 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society (2012)
globular clusters: general - galaxies: individual: M31 - galaxies: haloes - Local Group
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