The Origin of Segue 1

TL;DR

This paper investigates the structure and origin of Segue 1, revealing it as a dissolving star cluster associated with the Sagittarius stream, with evidence of tidal distortions and kinematic features indicating its extragalactic origin.

Contribution

The study provides new evidence that Segue 1 is a dissolving star cluster linked to the Sagittarius stream, challenging previous notions of it being a dark matter-dominated dwarf galaxy.

Findings

Detection of tidal elongations extending from Segue 1.

Identification of Sagittarius stream stars near Segue 1.

Evidence that Sagittarius stars inflate Segue 1's velocity dispersion.

Abstract

We apply the optimal filter technique to Sloan Digital Sky Survey photometry around Segue 1 and find that the outer parts of the cluster are distorted. There is strong evidence for ~ 1 degree elongations of extra-tidal stars, extending both eastwards and southwestwards of the cluster. The extensions have similar differential Hess diagrams to Segue 1. A Kolmogorov-Smirnov test suggests a high probability that both come from the same parent distribution. The location of Segue 1 is close to crossings of the tidal wraps of the Sagittarius stream. By extracting blue horizontal branch stars from Sloan's spectral database, two kinematic features are isolated and identified with different wraps of the Sagittarius stream. We show that Segue 1 is moving with a velocity that is close to one of the wraps. At this location, we estimate that there are enough Sagittarius stars, indistinguishable from Segue 1 stars, to inflate the velocity dispersion and hence the mass-to-light ratio. All the available evidence is consistent with the interpretation that Segue 1 is a star cluster, originally from the Sagittarius galaxy, and now dissolving in the Milky Way.