Dynamical Histories of the Crater II and Hercules Dwarf Galaxies

Abstract

We investigate the possibility that the dwarf galaxies Crater II and Hercules have previously been tidally stripped by the Milky Way. We present Magellan/IMACS spectra of candidate member stars in both objects. We identify 37 members of Crater II, 25 of which have velocity measurements in the literature, and we classify 3 stars within that subset as possible binaries. We find that including or removing these binary candidates does not change the derived velocity dispersion of Crater II. Excluding the binary candidates, we measure a velocity dispersion of σ V los = 2.7 +0.5 −0.4 km s −1 , corresponding to M/L = 47 +17 −13 M /L . We measure a mean metallicity of [Fe/H] = −1.95 +0.06 −0.05 , with a dispersion of σ [Fe/H] = 0.18 +0.06 −0.08 . Our velocity dispersion and metallicity measurements agree with previous measurements for Crater II, and confirm that the galaxy resides in a kinematically cold dark matter halo. We also search for spectroscopic members stripped from Hercules in the possible extratidal stellar overdensities surrounding the dwarf. For both galaxies, we calculate proper motions using Gaia DR2 astrometry, and use their full 6D phase space information to evaluate the probability that their orbits approach sufficiently close to the Milky Way to experience tidal stripping. Given the available kinematic data, we find a probability of ∼ 40% that Hercules has suffered tidal stripping. The proper motion of Crater II makes it almost certain to be stripped.

The Orbital Histories of Magellanic Satellites Using Gaia DR2 Proper Motions

“…For Cra2, the impact of the LMC is different, such that it decreases the distance achieved at pericenter from ∼ 30 kpc to ∼10 kpc, making it well-aligned with previous conclusions that Cra2 may have suffered from extreme tidal stripping (Torrealba et al 2016a;Sanders et al 2018;Fattahi et al 2018;Fu et al 2019;Erkal & Belokurov 2019). Hyi1, Car3, Car2, Phx2, and Hor1 also exhibit noticeable perturbations when the LMC potential is included.…”

Andromeda XXI – a dwarf galaxy in a low-density dark matter halo

“…For Antlia 2 and Crater II, proper motions can be measured from Gaia DR2 (Fritz et al 2018;Fu et al 2019;Torrealba et al 2019), and both are consistent with being on radial orbits that bring them within a few 10s of kpc from the Galactic centre -the regime in which tidal shocks are likely important (Read et al 2006a). As such, it has been argued that tidal processes govern the evolution of these galaxies (Sanders et al 2018;Amorisco 2019).…”

“…All three of these "feeble giants" also appear to reside in surprisingly low mass halos. Through measured velocity dispersions, their mass within 𝑟 half are far lower than expected for systems of their size or brightness (Caldwell et al 2017;Fu et al 2019;Torrealba et al 2019;Collins et al 2020), raising questions about whether they can be understood in the context of the Λ Cold Dark Matter (ΛCDM) framework. For these systems, it is unlikely that star formation feedback alone can explain their low densities.…”

Birds of a Feather? Magellan/IMACS Spectroscopy of the Ultra-faint Satellites Grus II, Tucana IV, and Tucana V*

“…For individual systems, the minimum observed velocity dispersion for dwarf galaxies has been decreasing with new discoveries and improved kinematics (e.g., Collins et al 2017;Koposov et al 2018), and the previous examples of systems with unusually cold internal kinematics for their luminosities (Kirby et al 2013a;Caldwell et al 2017;Simon et al 2017) Caldwell et al 2017) implies significant tidal mass loss on its derived orbit (Sanders et al 2018;Fu et al 2019). GruII does not obviously fit this trend, with an upper limit to its velocity dispersion comparable to that for TucIII and Segue2 but no detected tidal tails, 50 as well as a metallicity that is compatible with the dwarf galaxy luminosity-metallicity relationship at its present luminosity.…”