# Dynamical Histories of the Crater II and Hercules Dwarf Galaxies

## TL;DR

This study uses spectroscopic and astrometric data to assess whether the dwarf galaxies Crater II and Hercules have undergone tidal stripping by the Milky Way, providing insights into their dark matter content and orbital histories.

## Contribution

It presents new spectroscopic measurements and proper motions for Crater II and Hercules, evaluating their likelihood of having experienced tidal stripping.

## Key findings

- Crater II has a velocity dispersion of 2.7 km/s, indicating a dark matter dominated halo.
- Hercules has a ~40% probability of having been tidally stripped.
- Crater II is almost certainly tidally stripped based on its proper motion.

## Abstract

We investigate the possibility that the dwarf galaxies Crater II and Hercules have previously been tidally stripped by the Milky Way. We present Magellan/IMACS spectra of candidate member stars in both objects. We identify 37 members of Crater II, 25 of which have velocity measurements in the literature, and we classify 3 stars within that subset as possible binaries. We find that including or removing these binary candidates does not change the derived velocity dispersion of Crater II. Excluding the binary candidates, we measure a velocity dispersion of $\sigma_{V_{los}} = 2.7^{+0.5}_{-0.4}$ \kms, corresponding to $M/L = 47^{+17}_{-13}$ M$_{\odot}/$L$_{\odot}$. We measure a mean metallicity of [Fe/H] = $-1.95^{+0.06}_{-0.05}$, with a dispersion of $\sigma_{\mbox{[Fe/H]}} = 0.18^{+0.06}_{-0.08}$. Our velocity dispersion and metallicity measurements agree with previous measurements for Crater II, and confirm that the galaxy resides in a kinematically cold dark matter halo. We also search for spectroscopic members stripped from Hercules in the possible extratidal stellar overdensities surrounding the dwarf. For both galaxies, we calculate proper motions using \textit{Gaia} DR2 astrometry, and use their full 6D phase space information to evaluate the probability that their orbits approach sufficiently close to the Milky Way to experience tidal stripping. Given the available kinematic data, we find a probability of $\sim$40% that Hercules has suffered tidal stripping. The proper motion of Crater II makes it almost certain to be stripped.