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

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.

12 pages, 10 figures. Accepted by MNRAS

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• The fall of the Northern Unicorn: Tangential motions in the Galactic Anti-centre with SDSS and Gaia
• Exposing Sgr tidal debris behind the Galactic disk with M giants selected in WISE$\cap$2MASS
• Identification of RR Lyrae stars in multiband, sparsely-sampled data from the Dark Energy Survey using template fitting and Random Forest classification
• What is the Milky Way outer halo made of? High resolution spectroscopy of distant red giants
• Cartography of Triangulum-Andromeda using SDSS stars
• Triangulum-Andromeda overdensity: a region with a complex stellar population