The data presented herein were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation.
A spectroscopic survey of EC4, an extended cluster in Andromeda's halo★
Article first published online: 20 MAY 2009
© 2009 The Authors. Journal compilation © 2009 RAS
Monthly Notices of the Royal Astronomical Society
Volume 396, Issue 3, pages 1619–1628, July 2009
How to Cite
Collins, M. L. M., Chapman, S. C., Irwin, M., Ibata, R., Martin, N. F., Ferguson, A. M. N., Huxor, A., Lewis, G. F., Mackey, A. D., McConnachie, A. W. and Tanvir, N. (2009), A spectroscopic survey of EC4, an extended cluster in Andromeda's halo. Monthly Notices of the Royal Astronomical Society, 396: 1619–1628. doi: 10.1111/j.1365-2966.2009.14830.x
- Issue published online: 16 JUN 2009
- Article first published online: 20 MAY 2009
- Accepted 2009 March 25. Received 2009 March 25; in original form 2009 March 3
- stars: kinematics;
- galaxies: individual: M31;
- galaxies: star clusters
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 vr=−287.9+1.9−2.4 km s−1 and σv,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’.