|
Description
We measured [Fe/H] and [{alpha}/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 23kpc in the halo, 12kpc in the halo, 22kpc in the GSS, and 26kpc in the outer disk are {alpha}-enhanced, with <[{alpha}/Fe]>=0.43, 0.50, 0.41, and 0.58, respectively. The 23 and 12kpc halo fields are relatively metal-poor, with <[Fe/H]>=-1.54 and -1.30, whereas the 22kpc GSS and 26kpc 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 a radial [{alpha}/Fe] gradient along the high surface brightness core of the GSS between ~17 and 22kpc. However, we found tentative suggestions of a negative radial [{alpha}/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 [{alpha}/Fe] distribution of the metal-rich ([Fe/H]>-1.5), smooth inner stellar halo (r_proj_<~26kpc) is inconsistent with having formed from the disruption of a progenitor(s) similar to present-day M31 satellite galaxies. The 26kpc outer disk is most likely associated with the extended disk of M31, where its high {alpha}-enhancement provides support for an episode of rapid star formation in M31's disk possibly induced by a major merger.
|
