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

M L M Collins,N F Martin,R Ibata,G F Lewis,M Irwin,N Tanvir,A W Mcconnachie,A Huxor,S C Chapman,A M N Ferguson,A D Mackey

doi:10.1111/j.1365-2966.2009.14830.x

Abstract

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 CaII triplet region with ~1.3 Angstroms resolution. Six stars lying on the red giant branch within 2 core-radii of the centre of EC4 are found to have an average vr=-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 for EC4 is M/L=6.7^{+15}_{-6.7}Msun/Lsun, 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+/-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.

Highlights

Over the past few years, the number, depth and coverage of both photometric and spectroscopic surveys of the Local Group has increased dramatically, resulting in the discovery of a myriad of globular clusters (GCs) and dwarf spheroidal galaxies orbiting within its gravitational potential
SDSS has been instrumental in this process within the confines of our own Milky Way (MW), where in addition to a host of dwarf spheroidal galaxies (dSphs) a number of low-luminosity objects have been discovered out in the halo with properties that see them encroaching on the void in the size–luminosity-parameter space between GCs and dSphs (Willman et al 2005; Belokurov et al 2007; Simon & Geha 2007; Walsh, Jerjen & Willman 2007; Belokurov et al 2008)
Owing to the low density of Stream ‘Cp’ stars within the 83.5 arcmin2 DEep Imaging Multi-Object Spectrograph (DEIMOS) field [5–7 stars associated to Stream ‘Cp’ by Chapman et al (2008)], it is unlikely that a contaminating stream star would be found within the ∼0.8 arcmin tidal radius of the cluster and we can use the radial distance from the centre of the cluster as a discriminant between the two populations

Summary

Introduction

Over the past few years, the number, depth and coverage of both photometric and spectroscopic surveys of the Local Group has increased dramatically, resulting in the discovery of a myriad of globular clusters (GCs) and dwarf spheroidal galaxies (dSphs) orbiting within its gravitational potential. SDSS has been instrumental in this process within the confines of our own Milky Way (MW), where in addition to a host of dSphs a number of low-luminosity objects have been discovered out in the halo with properties that see them encroaching on the void in the size–luminosity-parameter space between GCs and dSphs (Willman et al 2005; Belokurov et al 2007; Simon & Geha 2007; Walsh, Jerjen & Willman 2007; Belokurov et al 2008). Wide-field photometric surveys of M31 and its extensive halo conducted with the Isaac Newton Telescope WideField Camera and Canada—France—Hawaii Telescope (CFHT)

Results

Discussion

Conclusion