The Elusive Distance Gradient in the Ultra-Faint Dwarf Galaxy Hercules: A Combined Hubble Space Telescope and Gaia View

Understand

The ultra-faint dwarf galaxy Hercules has an extremely elongated morphology with both photometric overdensities and kinematic members at large radii, suggesting that it may be tidally disrupting due to a previous close encounter with the Milky Way.

• To explain its observational peculiarities, we present a deep Hubble Space Telescope (HST) imaging study of Hercules and its surrounding regions and investigate its tidal history through a careful search for a distance gradient along its stretched body.
• Our off-center HST data clearly resolve a main sequence, showing that the stellar extension seen along the major-axis of Hercules is genuine, not a clump of background galaxies.
• Utilizing Gaia DR2 data, we clean the region around Hercules of field contamination, and find four new plausible member stars, all of which are located at the outskirts of the dwarf galaxy.