We present a Bayesian method to identify multiple ( chemodynamic ) stellar populations in dwarf spheroidal galaxies ( dSphs ) using velocity , metallicity , and positional stellar data without the assumption of spherical symmetry , building on the work of [ ] .
We apply this method to a new Keck/DEIMOS spectroscopic survey of the Ursa Minor ( UMi ) dSph .
We identify 892 likely members , making this the largest UMi sample with line-of-sight velocity and metallicity measurements .
Our Bayesian method detects two distinct chemodynamic populations with high significance ( in logarithmic Bayes ’ factor , \ln { B } \sim 32 ) .
The metal-rich ( [ { Fe / H } ] = -2.05 _ { -0.03 } ^ { +0.03 } ) population is kinematically colder ( radial velocity dispersion of \sigma _ { v } = 4.7 _ { -1.1 } ^ { +0.8 } \mathrm { km } \mathrm { s } ^ { -1 } ) and more centrally concentrated than the metal-poor ( [ { Fe / H } ] = -2.27 _ { -0.06 } ^ { +0.05 } ) and kinematically hotter population ( \sigma _ { v } = 11.4 _ { -0.7 } ^ { +0.9 } \mathrm { km } \mathrm { s } ^ { -1 } ) .
Furthermore , we apply the same analysis to the independent MMT/Hectochelle data set with 413 members from \citet Spencer2018AJ….156..257S and confirm the existence of two chemodynamic populations in UMi .
In both data sets , the metal-rich population is significantly flattened ( \epsilon = 0.74 \pm 0.04 ) and the metal-poor population is closer to spherical ( \epsilon = 0.33 _ { -0.11 } ^ { +0.09 } ) .
Despite the presence of two populations , we are unable to robustly estimate the slope of the dynamical mass profile .
We found hints for prolate rotation of order \sim 2 \mathrm { km } \mathrm { s } ^ { -1 } in the MMT data set , but further observations are required to verify this .
The flattened metal-rich population invalidates assumptions built into simple dynamical mass estimators , so we computed new astrophysical dark matter annihilation ( J ) and decay profiles based on the rounder , hotter metal-poor population and inferred \log _ { 10 } { ( J ( 0.5 \degree ) / { GeV ^ { 2 } cm ^ { -5 } } ) } \approx 19.1 for the Keck data set .
Our results paint a more complex picture of the evolution of Ursa Minor than previously discussed .