A spectroscopic survey of EC4, an extended cluster in Andromeda's halo

We present a spectroscopic survey of candidate red giant branch stars in the extended star cluster, EC4, discovered in the halo of M31 from our Canada - France - Hawaii Telescope/MegaCam survey, overlapping the tidal streams, Streams 'Cp' and 'Cr'. These observations used the DEep Imaging Multi-Object Spectrograph mounted on the Keck II telescope to obtain spectra around the Caii triplet region with ∼1.3 Å resolution. Six stars lying on the red giant branch within two core radii of the centre of EC4 are found to have an average v _r = -287.9 ^+1.9 _-2.4 km s ^-1 and σ _v,corr = 2.7 ^+4.2 _-2.7 km s ^-1, taking instrumental errors into account. The resulting mass-to-light ratio for EC4 is M/L = 6.7 ⁺¹⁵ _-6.7 M _⊙/L _⊙, a value that is consistent with a globular cluster within the 1σ errors we derive. From the summed spectra of our member stars, we find EC4 to be metal-poor, with [Fe/H] = -1.6 ± 0.15. We discuss several formation and evolution scenarios which could account for our kinematic and metallicity constraints on EC4, and conclude that EC4 is most comparable with an extended globular cluster. We also compare the kinematics and metallicity of EC4 with Streams 'Cp' and 'Cr', and find that EC4 bears a striking resemblance to Stream 'Cp' in terms of velocity, and that the two structures are identical in terms of both their spectroscopic and photometric metallicities. From this, we conclude that EC4 is likely related to Stream 'Cp'.