A spectroscopic survey of Andromeda's Western Shelf

TL;DR

This study uses spectroscopic data from Keck to analyze Andromeda's Western Shelf, confirming it as a shell structure from a tidal stream, and links its properties to the Giant Southern Stream, supporting a common progenitor.

Contribution

It provides the first clear detection of a shell pattern in Andromeda's halo and links the W Shelf to the GSS through kinematic and metallicity analysis, supported by new modeling.

Findings

Detection of a wedge pattern consistent with a shell structure.

Strong metallicity correlation between W Shelf and GSS.

Confirmation of a common progenitor for the GSS and W Shelf.

Abstract

The Andromeda galaxy (M31) shows many tidal features in its halo, including the Giant Southern Stream (GSS) and a sharp ledge in surface density on its western side (the W Shelf). Using DEIMOS on the Keck telescope, we obtain radial velocities of M31's giant stars along its NW minor axis, in a radial range covering the W Shelf and extending beyond its edge. In the space of velocity versus radius, the sample shows the wedge pattern expected from a radial shell, which is detected clearly here for the first time. This confirms predictions from an earlier model of formation of the GSS, which proposed that the W Shelf is a shell from the third orbital wrap of the same tidal debris stream that produces the GSS, with the main body of the progenitor lying in the second wrap. We calculate the distortions in the shelf wedge pattern expected from its outward expansion and angular momentum, and show that these effects are echoed in the data. In addition, a hot, relatively smooth spheroid population is clearly present. We construct a bulge-disk-halo N-body model that agrees with surface brightness and kinematic constraints, and combine it with a simulation of the GSS. From the contrasting kinematic signatures of the hot spheroid and shelf components, we decompose the observed stellar metallicity distribution into contributions from each component using a non-parametric mixture model. The shelf component's metallicity distribution matches previous observations of the GSS superbly, further strengthening the evidence they are connected and bolstering the case for a massive progenitor of this stream.