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 CFHT/MegaCam survey , overlapping the tidal streams , Stream ‘ Cp ’ and Stream ‘ Cr ’ .
These observations used the DEep Imaging Multi-Object Spectrograph ( DEIMOS ) mounted on the Keck II telescope to obtain spectra around the Ca II triplet region with \sim 1.3 \buildrel \circ \over { A } resolution .
Six stars lying on the red giant branch within 2 core-radii of the centre of EC4 are found to have an average v _ { r } = -287.9 ^ { +1.9 } _ { -2.4 } kms ^ { -1 } and \sigma _ { v,corr } = 2.7 ^ { +4.2 } _ { -2.7 } kms ^ { -1 } , taking instrumental errors into account .
The resulting mass-to-light ratio for EC4 is M/L = 6.7 ^ { +15 } _ { -6.7 } { M _ { \odot } } / { L _ { \odot } } , a value that is consistent with a globular cluster within the 1 \sigma errors we derive .
From the summed spectra of our member stars , we find EC4 to be metal-poor , with [ Fe/H ] =-1.6 \pm 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 Stream ‘ Cp ’ and Stream ‘ 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 ’ .