The extended "stellar halo" of the Ursa Minor dwarf galaxy

Abstract

Stellar candidates in the Ursa Minor (UMi) dwarf galaxy have been found using a new Bayesian algorithm applied to \textit{Gaia} EDR3 data. Five of these targets are located in the extreme outskirts of UMi, from $\sim5$ to 12 elliptical half-light radii (r$_h$), where r$_h$(UMi) $= 17.32 \pm 0.11$ arcmin, and have been observed with the GRACES high resolution spectrograph at the Gemini-Northern telescope. Precise radial velocities ($\sigma_{\rm{RV}} < 2$ km s$^{-1}$) and metallicities ($\sigma_{\rm{[Fe/H]}} < 0.2$ dex) confirm their memberships of UMi. Detailed analysis of the brightest and outermost star (Target~1, at $\sim12$ r$_h$), yields precision chemical abundances for the $\alpha$- (Mg, Ca, Ti), odd-Z (Na, K, Sc), Fe-peak (Fe, Ni, Cr), and neutron-capture (Ba) elements. With data from the literature and APOGEE DR17, we find the chemical patterns in UMi are consistent with an outside-in star formation history that includes yields from core collapse supernovae, asymptotic giant branch stars, and supernovae Ia. Evidence for a knee in the [$\alpha$/Fe] ratios near [Fe/H] $\sim-2.1$ indicates a low star formation efficiency similar to that in other dwarf galaxies. Detailed analysis of the surface number density profile shows evidence that UMi's outskirts have been populated by tidal effects, likely as a result of completing multiple orbits around the Galaxy.