Uncovering the Orbit of the Hercules Dwarf Galaxy
  We present new chemo--kinematics of the Hercules dwarf galaxy based on Keck
II-- DEIMOS spectroscopy. Our 21 confirmed members have a systemic velocity of
$v_{\mathrm{Herc}}=46.4\pm1.3$ kms$^{-1}$ and a velocity dispersion
$\sigma_{v,\mathrm{Herc}}=4.4^{+1.4}_{-1.2}$ kms$^{-1}$. From the strength of
the Ca II triplet, we obtain a metallicity of [Fe/H]= $-2.48\pm0.19$ dex and
dispersion of $\sigma_{\rm{[Fe/H]}}= 0.63^{+0.18}_{-0.13}$ dex. This makes
Hercules a particularly metal--poor galaxy, placing it slightly below the
standard mass--metallicity relation. Previous photometric and spectroscopic
evidence suggests that Hercules is tidally disrupting and may be on a highly
radial orbit. From our identified members, we measure no significant velocity
gradient. By cross--matching with the second \textit{Gaia} data release, we
determine an uncertainty--weighted mean proper motion of
$\mu_{\alpha}^*=\mu_{\alpha}\cos(\delta)=-0.153\pm{0.074}$ mas yr$^{-1}$,
$\mu_{\delta}=-0.397\pm0.063$ mas yr$^{-1}$. This proper motion is slightly
misaligned with the elongation of Hercules, in contrast to models which suggest
that any tidal debris should be well aligned with the orbital path. Future
observations may resolve this tension.
