Galactic Structure from Late-Type Giants at Mid-Latitudes I: Identifying Contributions to the Stellar Halo from Accreted and In Situ Populations

Details Galactic Structure from Late-Type Giants at Mid-Latitudes I: Identifying Contributions to the Stellar Halo from Accreted and In Situ Populations Galactic Structure from Late-Type Giants at Mid-Latitudes I: Identifying Contributions to the Stellar Halo from Accreted and In Situ Populations

2012-02-23

arxiv.org/abs/1202.5310v1

Astronomy and Astrophysics

Astrophysics

Galaxy Astrophysics

41 pages, 9 figures, and 4 tables; submitted to the Astrophysical Journal

Scientific paper

[Abridged] We present a medium-resolution spectroscopic survey of 1799 M giant stars at mid-Galactic latitudes designed to probe the properties of this population to distances of $\sim$9 kpc. The distribution of radial velocity (RV) as a function of $l$ for these stars shows (1) the expected thick disk population and (2) local metal-rich halo stars moving at high speeds relative to the disk, that in some cases form distinct sequences in $l$-RV. High-resolution echelle spectra taken for 34 of these "RV outliers" reveal the following patterns across the [Ti/Fe] and [Fe/H] plane: sixteen of the stars have abundances reminiscent of the populations present in dwarf satellites of the Milky Way; eight have abundances coincident with those of the Galactic disk and metal-rich halo; and ten of the stars fall on the locus defined by the majority of stars in the halo. The chemical abundance trends of the RV outliers suggest that our sample consists predominantly of stars accreted from infalling dwarf galaxies, with a more moderate fraction ($\sim$20%) of stars potentially formed in the Galactic disk and subsequently kicked to higher eccentricity orbits. These results support scenarios where the stellar halo arises from multiple formation mechanisms. We conclude that M giants with large RVs can provide particularly fruitful samples to mine for accreted structures and that some of the putative velocity sequences may indeed correspond to real physical associations resulting from recent accretion events.

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