The Pristine Dwarf-Galaxy survey -- V. The edges of the dwarf galaxy Hercules

TL;DR

This study uses spectroscopy, Gaia proper motions, and photometry to identify Hercules dwarf galaxy members and finds no evidence of tidal disruption, challenging previous assumptions about its dynamical state.

Contribution

It introduces a combined method for contamination removal and provides new spectroscopic data showing Hercules is not tidally disrupting.

Findings

Identified three new member stars, including one far from the center.

Found no velocity gradient across Hercules, indicating no tidal disruption.

Confirmed Hercules's velocity and metallicity are consistent throughout the galaxy.

Abstract

We present a new spectroscopic study of the dwarf galaxy Hercules (d ~ 132 kpc) with data from the Anglo-Australian Telescope and its AAOmega spectrograph together with the Two Degree Field multi-object system to solve the conundrum that whether Hercules is tidally disrupting. We combine broadband photometry, proper motions from Gaia, and our Pristine narrow-band and metallicity-sensitive photometry to efficiently weed out the Milky Way contamination. Such cleaning is particularly critical in this kinematic regime, as both the transverse and heliocentric velocities of Milky Way populations overlap with Hercules. Thanks to this method, three new member stars are identified, including one at almost 10rh of the satellite. All three have velocities and metallicities consistent with that of the main body. Combining this new dataset with the entire literature cleaned out from contamination shows that Hercules does not exhibit a velocity gradient (d<v>/dX = 0.1+0.4/-0.2 km s-1 arcmin-1) and, as such, does not show evidence to undergo tidal disruption.