2019/09/30 by Ivanna Escala, Karoline M. Gilbert, Evan N. Kirby +3 · 1 citation
Physics and Astronomy · #astro-ph.GA
paper · pdf · doi:10.3847/1538-4357/ab6659
32 pages, 19 figures, 6 tables, ApJ in press
arxiv created 2020/01/24 · arxiv updated 2020/02/12
We measured [Fe/H] and [α/Fe] using spectral synthesis of low-resolution stellar spectroscopy for 70 individual red giant branch stars across four fields spanning the outer disk, Giant Stellar Stream (GSS), and inner halo of M31. Fields at M31-centric projected distances of 23 kpc in the halo, 12 kpc in the halo, 22 kpc in the GSS, and 26 kpc in the outer disk are α-enhanced, with ⟨[α/Fe]⟩ = 0.43, 0.50, 0.41, and 0.58, respectively. The 23 kpc and 12 kpc halo fields are relatively metal-poor, with ⟨[Fe/H]⟩ = -1.54 and -1.30, whereas the 22 kpc GSS and 26 kpc outer disk fields are relatively metal-rich with ⟨[Fe/H]⟩ = -0.84 and -0.92, respectively. For fields with substructure, we separated the stellar populations into kinematically hot stellar halo components and kinematically cold components. We did not find any evidence of an [α/Fe] gradient along the high surface brightness core of the GSS between ∼17-22 kpc. However, we found tentative suggestions of a negative [α/Fe] gradient in the stellar halo, which may indicate that different progenitor(s) or formation mechanisms contributed to the build up of the inner versus outer halo. Additionally, the [α/Fe] distribution of the metal-rich ([Fe/H] > -1.5), smooth inner stellar halo (r_\rmproj \lesssim 26 kpc) is inconsistent with having formed from the disruption of progenitor(s) similar to present-day M31 satellite galaxies. The 26 kpc outer disk is most likely associated with the extended disk of M31, where its high α-enhancement provides support for an episode of rapid star formation in M31's disk, possibly induced by a major merger.