The Far-Away Blues: Exploring the Furthest Extents of the Bo\"otes I Ultra Faint Dwarf Galaxy

TL;DR

This study investigates the outer stellar populations of the Bo"otes I ultra-faint dwarf galaxy, revealing an extended distribution of ancient stars and potential signs of tidal interactions, using combined photometry and astrometry data.

Contribution

It identifies and characterizes the extended outer stellar populations of Bo"otes I, providing new insights into its spatial extent and possible tidal features.

Findings

24 candidate blue horizontal branch stars consistent with galaxy membership

9 stars located beyond the nominal tidal radius

Outer stellar distribution suggests possible tidal interactions

Abstract

Establishing the spatial extents and the nature of the outer stellar populations of dwarf galaxies are necessary ingredients 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 Bo\"otes I ultra-faint dwarf galaxy, a satellite of the Milky Way Galaxy. We identify candidate member blue horizontal branch and blue straggler stars of Bo\"otes I, both tracers of the underlying ancient stellar population, using a combination of multi-band 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 Bo\"otes 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 Bo\"otes 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 Bo\"otes 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.