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

TL;DR

This study combines Hubble and Gaia data to investigate the structure and potential tidal disruption of the ultra-faint dwarf galaxy Hercules, finding no conclusive distance gradient but highlighting observational challenges.

Contribution

It provides a detailed analysis of Hercules' morphology and distance, demonstrating the difficulty in detecting a distance gradient in ultra-faint galaxies with current data.

Findings

Hercules has an elongated morphology consistent with tidal disruption.

No conclusive distance gradient was detected along Hercules.

The updated distance to Hercules is approximately 130.6 kpc.

Abstract

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. We update the distance to Hercules, and find $130.6\pm1.2$ kpc ($m-M=20.58\pm0.02$) for the main body, which is consistent with earlier estimates in the literature. While we find no conclusive evidence for a distance gradient, our work demonstrates that constraining a distance gradient in such a faint system is not trivial, and the possible thickness of the dwarf along the line of sight and field contamination make it harder to make decisive conclusions even with these high-precision data. Future studies coupled with tailored theoretical models are needed to understand the true nature of Hercules and of tidal distortion observables in ultra-faint galaxies in general.