The tidal trail of NGC 205?

Abstract

Using data taken as part of the Isaac Newton Telescope Wide Field Camera (INT WFC) survey of M31, we have identified an arc-like overdensity of blue, presumably metal-poor, red giant branch stars in the north-west quadrant of M31. This feature is ∼ 1° (15 kpc) in extent and has a surface brightness of Σv′ ≃ 28.5 ± 0.5 mag arcsec-2. The arc appears to emanate from the dwarf ell iptical galaxy NGC 205, and the colour of its red giant branch is significantly different from the M31 disc population but closely resembles that of NGC 205. Further, using data taken with the Deep Imaging Multi-Object Spectrograph (DEIMOS) on Keck II, we identify the radial velocity signature of this arc. Its velocity dispersion is measured to be ≃ 10 km s-1, similar to that of the central regions of NGC 205 and typical of stellar streams. Based upon the spatial coincidence of these objects, the surface brightness, the velocity dispersions and the similarity in colour of the red giant branches, we postulate that the arc is part of a stellar stream, the progenitor of which is NGC 205.