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

Abstract

Five stars in the extreme outskirts (from ∼ 5 to ∼ 12 elliptical half-light radii, r h ) of the Ursa Minor (UMi) dwarf galaxy have been identified as potential new members using a Bayesian algorithm applied to Gaia EDR3 data. These targets were observed with the GRACES spectrograph, resulting in precise radial velocities and metallicities that confirm their association with UMi. For the brightest and outermost star (Target 1, at ∼ 12 r h ), the chemical abundances of α-(Mg, Ca, Ti), odd-Z (Na, K, Sc), Fe-peak (Fe, Ni, Cr), and neutron-capture process (Ba) elements have also been determined. We also discuss data from the literature and from APOGEE DR17. We find the chemical patterns in UMi are consistent with a star formation history that includes yields from core collapse supernovae, asymptotic giant branch stars, and supernovae Ia. Evidence for a knee in the [α/Fe] ratios near [Fe/H] ∼ −2.1 indicates a low star formation efficiency similar to that in other dwarf galaxies. Given the distance of Target 1 from the centre of UMi (R∼4.5 kpc), we show that UMi has a more extended structure than previously thought. This "stellar halo" around UMi could be a secondary feature resulting from tidal stripping after multiple orbits around the Galaxy, or maybe a primary UMi feature due to early hierarchical accretion activity or to strong gravitational fluctuations prompted by feedback in the early star formation phase. Also consistent with observations is a late-time merger-free scenario where outside-in star formation is accompanied by gradual supernovae Ia enrichment.

Streams on FIRE: Populations of Detectable Stellar Streams in the Milky Way and FIRE

“…Crater II, a classical satellite just below the mass range considered here (M * = 10 5.55 M e ) at a distance of 117.5 kpc (Ji et al 2021), also has strong evidence of tidal disruption, including a low velocity dispersion and low surface brightness (Sanders et al 2018;Fu et al 2019;Ji et al 2021;Borukhovetskaya et al 2022). Several other Milky Way satellites, including Boötes I (Filion & Wyse 2021;Longeard et al 2022) and Ursa Minor (Sestito et al 2023), have recently been found to have extended stellar populations that may be evidence of tidal disruption. In addition, Pace et al (2022) examine the morphologies and orbits of the Milky Way dwarf spheroidals to explore the possibly that they are tidally disrupting.…”

Abundance Analysis of Stars at Large Radius in the Sextans Dwarf Spheroidal Galaxy*

“…Recent observations suggest that extended stellar halos may be a relatively common feature of dwarf galaxies (e.g., Chiti et al 2021;Stringer et al 2021;Yang et al 2022;Sestito et al 2023b), including Sextans (Qi et al 2022), even in the absence of tidal distortions. The stars in our sample are located at much larger radii than the stellar substructures in Sextans identified by previous work using stellar velocities and metallicities.…”

“…These efforts have been based on spectroscopic follow-up of wide-field photometric searches (e.g., Muñoz et al 2005Muñoz et al , 2006Westfall et al 2006;Hendricks et al 2014) or wide-field broadband photometry combined with proper-motion measurements from the Gaia mission (Prusti et al 2016). Studies by Chiti et al (2021Chiti et al ( , 2023, Filion & Wyse (2021), Longeard et al (2022Longeard et al ( , 2023, Qi et al (2022), Yang et al (2022), and Sestito et al (2023aand Sestito et al ( , 2023b have shown that several dSph and UFD galaxies contain stars near their tidal radii. These extended stellar halos may have formed through dwarf galaxy mergers (Rey et al 2019;Tarumi et al 2021), and multiple mergers may have occurred within individual dSph galaxies around the Milky Way (Griffen et al 2018;Deason et al 2023).…”