Discovery of tidal debris stars from G1/Mayall-II in M31

Abstract

Abstract The object Mayall II or G1 is the brightest globular cluster belonging to M31. Because of its extreme properties for a globular cluster, it has been speculated that G1 is the remnant nucleus of a dwarf galaxy that has been stripped by the tidal field of M31. Using the Keck DEIMOS spectrograph, we have conducted a survey for tidally stripped stars from G1, obtaining a sample of 351 stellar velocities over ∼ 320 sq. arcminutes of sky centered on G1. Thirteen are within $25~{\, \rm km\, s^{-1}\, }$ of the systemic velocity of G1, and exhibit spatial and velocity correlations consistent with being dynamically associated with G1, and all thirteen are well outside the tidal radius of the cluster. These thirteen stars could be either (i) the remnants of an almost completely evaporated stellar envelope, or (ii) G1 member stars lost through tidal interaction with M31. Estimates of the implied mass loss rate based on our data suggest a short dissolution time-scale for G1, thus favouring the stellar envelope hypothesis for the origin of the tidal tail stars, or, at the very least, an advanced stage of cluster dissolution. In either case, G1, and by extension compact stellar systems in general, have likely played a significant role in building the halo of M31.