Satellite group infall into the Milky Way: exploring the Crater-Leo case with new HST proper motions (2404.16110v1)

Abstract: Context. Within $\Lambda$ Cold Dark Matter ($\Lambda$CDM) simulations, Milky Way-like galaxies accrete some of their satellite galaxies in groups of 3-5 members rather than individually. It was also suggested that this might be the reason behind the origin of satellite planes. Objects accreted in groups are expected to share similar specific total energy and angular momentum, and also identical orbital planes and directions. Aims. Looking at observations of Milky Way satellites, the dwarf galaxies Leo II, IV, V, Crater II, and the star cluster Crater 1 were proposed to be a vestige of group infall. The suggested "Crater-Leo group" shows a coherent distance gradient and all these objects align along a great circle on the sky. We use proper motion data to investigate whether the phase-space distribution of the members of the proposed group are indeed consistent with group infall. Methods. To further investigate this possibility, we use Gaia Data Release 3 (DR3) and new Hubble Space Telescope (HST) proper motions, $(\mu_{\alpha*}, \mu_\delta) = (-0.1921 \pm 0.0514, -0.0686 \pm 0.0523)$ mas yr$^{-1}$ for Leo IV and $(\mu_{\alpha*}, \mu_\delta) = (0.1186 \pm 0.1943, -0.1183 \pm 0.1704)$ mas yr$^{-1}$ for Leo V, to derive accurate orbital properties for the proposed group objects. In addition, we explore other possible members of this putative association. Results. Leo II, Leo IV, and Crater 1 show orbital properties consistent with those we predict from assuming group infall. However, our results suggest that Crater II was not accreted with the rest of the objects. If confirmed with increasingly accurate proper motions in the future, the Crater-Leo objects appear to constitute the first identified case of a cosmologically expected, typical group infall event, as opposed to the highly hierarchical Magellanic Cloud system.