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We present a spectroscopic survey of candidate red giant branch stars in theextended star cluster, EC4, discovered in the halo of M31 from our CFHT/MegaCamsurvey, overlapping the tidal streams, Stream Cp and Stream Cr. Theseobservations used the DEep Imaging Multi-Object Spectrograph (DEIMOS) mountedon the Keck II telescope to obtain spectra around the CaII triplet region with~1.3 Angstroms resolution. Six stars lying on the red giant branch within 2core-radii of the centre of EC4 are found to have an averagevr=-287.9^{+1.9}_{-2.4}km/s and velocity dispersion of 2.7^{+4.2}_{-2.7}km/s,taking instrumental errors into account. The resulting mass-to-light ratio forEC4 is M/L=6.7^{+15}_{-6.7}Msun/Lsun, a value that is consistent with aglobular cluster within the 1 sigma errors we derive. From the summed spectraof our member stars, we find EC4 to be metal-poor, with [Fe/H]=-1.6+/-0.15. Wediscuss several formation and evolution scenarios which could account for ourkinematic and metallicity constraints on EC4, and conclude that EC4 is mostcomparable with an extended globular cluster. We also compare the kinematicsand metallicity of EC4 with Stream Cp and Stream Cr, and find that EC4 bears astriking resemblance to Stream Cp in terms of velocity, and that the twostructures are identical in terms of both their spectroscopic and photometricmetallicities. From this we conclude that EC4 is likely related to Stream Cp.

A spectroscopic survey of EC4, an Extended Cluster in Andromeda's halo