The Far-away Blues: Exploring the Furthest Extents of the Boötes I Ultra-faint Dwarf Galaxy

Abstract

Establishing the spatial extents and the nature of the outer stellar populations of dwarf galaxies is necessary for the determination of their total masses, current dynamical states, and past evolution. We here describe our investigation of the outer stellar content of the Boötes I ultra-faint dwarf galaxy, a satellite of the the Milky Way. We identify candidate member blue horizontal branch and blue straggler stars of Boötes I, both tracers of the underlying ancient stellar population, using a combination of multiband Pan-STARRS photometry and Gaia astrometry. We find a total of twenty-four candidate blue horizontal branch member stars with apparent magnitudes and proper motions consistent with membership of Boötes I, nine of which reside at projected distances beyond the nominal King profile tidal radius derived from earlier fits to photometry. We also identify four blue straggler stars of appropriate apparent magnitude to be at the distance of Boötes I, but all four are too faint to have high-quality astrometry from Gaia. The outer blue horizontal branch stars that we have identified confirm that the spatial distribution of the stellar population of Boötes I is quite extended. The morphology on the sky of these outer envelope candidate member stars is evocative of tidal interactions, a possibility that we explore further with simple dynamical models.

Faint Stars in a Faint Galaxy. I. Ultradeep Photometry of the Boötes I Ultrafaint Dwarf Galaxy

“…This is more distant than was assumed in Paper I, but is a good fit to the blue horizontal branch population(Filion & Wyse 2021) and is consistent with RR Lyrae-based distance estimates(Siegel 2006;Dall'Ora et al 2006) …”

Proper Motions, Orbits, and Tidal Influences of Milky Way Dwarf Spheroidal Galaxies

“…There is a tentative velocity gradient in Cra II and the tidal radius is less than the half-light radius but the predicted tidal features are beyond the range of current spectroscopic samples (Ji et al 2021). For Boo I there is a potential velocity gradient (Longeard et al 2021b), and there are several blue horizontal branch star candidates at large distances, outside the King limiting radius (Filion & Wyse 2021), which are both consistent with tidal stripping models (Longeard et al 2021b;Filion & Wyse 2021). Tuc IV has had a recent, direct (Δd ∼ 4 kpc) collision with the LMC (Simon et al 2020).…”

Proper Motions, Orbits, and Tidal Influences of Milky Way Dwarf Spheroidal Galaxies

“…There is a tentative velocity gradient in Cra II and the tidal radius is less than the half-light radius but the predicted tidal features are beyond the range of current spectroscopic samples (Ji et al 2021). For Boo I there is a potential velocity gradient (Longeard et al 2021b) and there are several blue horizontal branch star candidates at large distances, outside the King limiting radius (Filion & Wyse 2021) which are both consistent with tidal stripping models (Longeard et al 2021b;Filion & Wyse 2021). Tuc IV has had a recent, direct (∆d ∼ 4 kpc) collision with the LMC (Simon et al 2020).…”

“…While this analysis was in preparation a new analysis measuring the distance to Ant II with RRL stars which slightly improved and updated the distance measurement (Vivas et al 2022). Boo I-This dSph is included in our potentially tidally disrupting sample due to literature analysis (Longeard et al 2021b;Filion & Wyse 2021) and a low value of ρ 1/2 /ρ M W (r = r peri ) ∼ 25. Boo III-This dSph is included in our potentially tidally disrupting based on its small pericenter and low average density ).…”