We employ abundances from the Sloan Digital Sky Survey ( SDSS ) and the Sloan Extension for Galactic Understanding and Exploration ( SEGUE ) to study the \alpha -element distribution of the stellar members of the Sagittarius stream .
To test the reliability of SDSS/SEGUE abundances for the study of Sagittarius , we select high-likelihood samples tracing the different components of the Milky Way , and recover known literature \alpha -element distributions . Using selection criteria based on the spatial position , radial velocity , distance and colours of individual stars , we obtain a robust sample of Sagittarius-stream stars .
The \alpha -element distribution of the Sagittarius stream forms a narrow sequence at intermediate metallicities with a clear turn-down , consistent with the presence of an \alpha -element ” knee ” .
This is the first time that the \alpha -element knee of the Sagittarius dwarf galaxy has been detected .
Fitting a toy model to our data , we determine that the \alpha -knee in Sagittarius takes place at [ Fe/H ] = - 1.27 \pm 0.05 , only slightly less metal-poor than the knee in the Milky Way .
This indicates that a small number of Sagittarius-like galaxies could have contributed significantly to the build-up of the Milky Way ’ s stellar halo system at ancient times .