TriAnd and its Siblings: Satellites of Satellites in the Milky Way Halo

TL;DR

This paper investigates the TriAnd overdensity and related structures in the Milky Way halo, suggesting they are remnants of a group infall, with satellites of satellites and shared kinematic properties.

Contribution

It provides evidence that TriAnd, PAndAS stream, and Segue 2 are interconnected, supporting the group-infall scenario in the Milky Way halo.

Findings

TriAnd shares distance and velocity with PAndAS stream.

TriAnd is prevalent near Segue 2 with similar kinematics.

Structures are likely remnants of a group infall event.

Abstract

We explore the Triangulum-Andromeda (TriAnd) overdensity in the SPLASH (Spectroscopic and Photometric Landscape of Andromeda's Stellar Halo) and SEGUE (the Sloan Extension for Galactic Understanding and Exploration) spectroscopic surveys. Milky Way main sequence turn-off stars in the SPLASH survey reveal that the TriAnd overdensity and the recently discovered PAndAS stream (Martin et al. 2014) share a common heliocentric distance (D ~ 20 kpc), position on the sky, and line-of-sight velocity (V_GSR ~ 50 km/s). Similarly, A-type, giant, and main sequence turn-off stars selected from the SEGUE survey in the vicinity of the Segue 2 satellite show that TriAnd is prevalent in these fields, with a velocity and distance similar to Segue 2. The coincidence of the PAndAS stream and Segue 2 satellite in positional and velocity space to TriAnd suggests that these substructures are all associated, and may be a fossil record of group-infall onto the Milky Way halo. In this scenario, the Segue 2 satellite and PAndAS stream are "satellites of satellites", and the large, metal-rich TriAnd overdensity is the remains of the group central.