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Exploring Halo Substructure with Giant Stars: The Nature of the Triangulum-Andromeda Stellar Features

As large-scale stellar surveys have become available over the past decade, the ability to detect and characterize substructures in the Galaxy has increased dramatically. These surveys have revealed the Triangulum-Andromeda (TriAnd) region to be rich with substructure in the distance range 20-30 kpc, and the relation of these features to each other -- if any -- remains unclear. This complex situation motivates this re-examination of the TriAnd region with a photometric and spectroscopic survey of M giants. An exploration using 2MASS photometry reveals not only the faint sequence in M giants detected by Rocha-Pinto et al. (2004) spanning the range $100^{\circ}<l<160^{\circ}$ and $-50^{\circ}<b<-15^{\circ}$ but, in addition, a second, brighter and more densely populated M giant sequence. These two sequences are likely associated with the two distinct main-sequences discovered (and labeled TriAnd1 and TriAnd2) by Martin et al. (2007) in an optical survey in the direction of M31, where TriAnd2 is the optical counterpart of the fainter RGB/AGB sequence of Rocha-Pinto et al. (2004). Here, the age, distance, and metallicity ranges for TriAnd1 and TriAnd2 are estimated by simultaneously fitting isochrones to the 2MASS RGB tracks and the optical MS/MSTO features. The two populations are clearly distinct in age and distance: the brighter sequence (TriAnd1) is younger (6-10 Gyr) and closer (distance of $\sim$ 15-21 kpc), while the fainter sequence (TriAnd2) is older (10-12 Gyr) and is at an estimated distance of $\sim$ 24-32 kpc. A comparison with simulations demonstrates that the differences and similarities between TriAnd1 and TriAnd2 can simultaneously be explained if they represent debris originating from the disruption of the same dwarf galaxy, but torn off during two distinct pericentric passages. [Abridged]