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 vHerc=46.4±1.3 kms−1 and a velocity dispersion σv,Herc=4.4−1.2+1.4 kms−1. From the strength of the Ca II triplet, we obtain a metallicity of [Fe/H]= −2.48±0.19 dex and dispersion of σ[Fe/H]=0.63−0.13+0.18 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 μα∗=μαcos(δ)=−0.153±0.074 mas yr−1, μδ=−0.397±0.063 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.
@article{arxiv.1912.00156,
title = {Uncovering the Orbit of the Hercules Dwarf Galaxy},
author = {Alexandra L. Gregory and Michelle L. M. Collins and Denis Erkal and Erik Tollerud and Maxime Delorme and Lewis Hill and David J. Sand and Jay Strader and Beth Willman},
journal= {arXiv preprint arXiv:1912.00156},
year = {2020}
}