ABSTRACT We present new chemo-kinematics of the Hercules dwarf galaxy based on Keck II-DEIMOS spectroscopy. Our 21 confirmed members, including 9 newly confirmed members, have a systemic velocity of vHerc = 46.4 ± 1.3 km s−1 and a velocity dispersion $\sigma _{v,\mathrm{Herc}}=4.4^{+1.4}_{-1.2}$ km s−1, consistent with previous studies. From the strength of the Ca ii triplet, we obtain a metallicity of [Fe/H] = −2.48 ± 0.19 dex and dispersion of $\sigma _{\rm {[Fe/H]}}= 0.63^{+0.18}_{-0.13}$ dex. This result is within 1σ of previous measurements, and 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 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, 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.

We present new FLAMES+GIRAFFE spectroscopy of 36 member stars in the isolated Local Group dwarf spheroidal galaxy Tucana. We measure a systemic velocity for the system of v Tuc = 216.7 +2.9 −2.8 kms −1 , and a velocity dispersion of σ v,Tuc = 14.4 +2.8 −2.3 kms −1 . We also detect a rotation gradient of dv r dχ = 7.6 +4.2 −4.3 kms −1 kpc −1 , which reduces the systemic velocity to v Tuc = 215.2 +2.8 −2.7 kms −1 and the velocity dispersion to σ v,Tuc = 13.3 +2.7 −2.3 kms −1 . We perform Jeans modelling of the density profile of Tucana using the line-of-sight velocities of the member stars. We find that it favours a high central density consistent with 'pristine' subhalos in ΛCDM, and a massive dark matter halo (∼10 10 M ) consistent with expectations from abundance matching. Tucana appears to be significantly more centrally dense than other isolated Local Group dwarfs, making it an ideal laboratory for testing dark matter models.

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