Uncovering the Orbit of the Hercules Dwarf Galaxy

TL;DR

This study provides detailed chemo-kinematic data for Hercules dwarf galaxy, revealing its velocity, metallicity, and proper motion, and discusses its possible tidal disruption and orbital characteristics.

Contribution

It offers new spectroscopic measurements and proper motion data for Hercules, enhancing understanding of its dynamics and chemical properties, and challenges existing models of its tidal disruption.

Findings

Hercules has a systemic velocity of 46.4 km/s.

Metallicity is [Fe/H] = -2.48, below the mass-metallicity relation.

Proper motion measurements suggest a slight misalignment with the galaxy's elongation.

Abstract

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.