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

TL;DR

This study identifies and analyzes stars in the Ursa Minor dwarf galaxy's outskirts using Gaia data and high-resolution spectroscopy, revealing its extended stellar halo and signs of tidal interactions.

Contribution

It introduces a new Bayesian method for star candidate selection and provides detailed chemical and structural analysis of UMi's outer regions, highlighting tidal effects.

Findings

Confirmed membership of outer stars via spectroscopy

Detected chemical abundance patterns consistent with dwarf galaxy evolution

Evidence of tidal effects shaping UMi's outskirts

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.