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.

中文翻译：

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发现大力神矮星系的轨道

我们展示了基于 Keck II--DEIMOS 光谱学的 Hercules 矮星系的新化学运动学。我们确认的 21 名成员的系统速度 $v_{\mathrm{Herc}}=46.4\pm1.3$ kms$^{-1}$ 和速度色散 $\sigma_{v,\mathrm{Herc}}= 4.4^{+1.4}_{-1.2}$ kms$^{-1}$。从 Ca II 三元组的强度，我们获得 [Fe/H]= $-2.48\pm0.19$ dex 的金属丰度和 $\sigma_{\rm{[Fe/H]}}= 0.63^{ +0.18}_{-0.13}$ dex。这使得赫拉克勒斯成为一个特别金属——贫乏的星系，使其略低于标准质量——金属度关系。先前的光度学和光谱学证据表明，赫拉克勒斯正在受到潮汐干扰，并且可能处于高度径向的轨道上。从我们确定​​的成员中，我们没有测量到显着的速度梯度。通过与第二个 \textit{Gaia} 数据发布交叉匹配，我们确定了一个不确定性——加权平均自行 $\mu_{\alpha}^*=\mu_{\alpha}\cos(\delta)=-0.153\pm{0.074}$ mas yr$^{-1} $, $\mu_{\delta}=-0.397\pm0.063$ mas yr$^{-1}$。这种自行运动与赫拉克勒斯的伸长率略有偏差，这与表明任何潮汐碎片都应与轨道路径对齐的模型形成对比。未来的观察可能会解决这种紧张局势。