Kavli Affiliate: Alexander Ji
| Summary:
We present updated systemic properties of the ultra-faint dwarf galaxy Boötes III from the Southern Stellar Stream Spectroscopic Survey (S$^5$). We identify 21 high-probability members and measure a velocity dispersion of $σ_v = 1.69^+1.03_-0.85$ km s$^-1$, about six times smaller than the previously reported $10.7 pm 3.5$ km s$^-1$, and a mean metallicity of [Fe/H] $= -2.34 pm 0.11$. The revised dispersion brings Boötes III in line with other tidally disrupting dwarfs such as Antlia II and Crater II. Orbit integrations in a Milky Way (MW) + Large Magellanic Cloud (LMC) potential confirm a highly eccentric ($e approx 0.8$), polar ($i approx 89.5^circ$) orbit with a recent pericentric passage $sim 0.14$ Gyr ago at $r_rm peri approx 9.5$ kpc. Boötes III is thus likely actively tidally disrupting, as its tidal radius at pericenter, $r_t approx 164$ pc, is only $sim 0.35$ of its half-light radius. The unusually low dispersion also implies that Boötes III has either lost most of its dark matter to tides or hosts a cored inner density profile, making it a probe of the nature of dark matter. Simulated tidal streams are broadly consistent with the Styx stellar stream, though the predicted track and kinematics are sensitive to the MW halo mass, LMC mass, and solar velocity. Boötes III overlaps the Typhon stream in integrals-of-motion space but has a much lower mean metallicity, suggesting the two are not the same system but may have had a common group infall origin. Sagittarius-stream contamination prevents a direct tidal-tail detection, so deep spectroscopic follow-up remains essential, both to confirm Styx as a genuine stream and to establish it as Boötes III’s tidal tail.
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