Kinematics and Chemistry of Recently Discovered Reticulum 2 and Horologium 1 Dwarf Galaxies

Abstract

We report on VLT/GIRAFFE spectra of stars in two recently discovered ultra-faint satellites, Reticulum 2 and Horologium 1, obtained as part of the Gaia-ESO Survey. We identify 18 members in Reticulum 2 and five in Horologium 1. We find Reticulum 2 to have a velocity dispersion of {3.22}<SUB>-0.49</SUB><SUP>+1.64</SUP> {km} {{{s}}}<SUP>-1</SUP>, implying a mass-to-light ratio (M/L) of ̃500. The mean metallicity of Reticulum 2 is [{Fe}/{{H}}]=-2.46, with an intrinsic dispersion of ̃0.3 dex and α-enhancement of ̃0.4 dex. We conclude that Reticulum 2 is a dwarf galaxy. We also report on the serendipitous discovery of four stars in a previously unknown stellar substructure near Reticulum 2 with [{Fe}/{{H}}]̃ -2 and {V}<SUB>{hel</SUB>}̃ 220 {km} {{{s}}}<SUP>-1</SUP>, far from the systemic velocity of Reticulum 2. For Horologium 1 we infer a velocity dispersion of σ ≤ft(V\right)={4.9}<SUB>-0.9</SUB><SUP>+2.8</SUP> {km} {{{s}}}<SUP>-1</SUP> and a M/L ratio of ̃600, leading us to conclude that Horologium 1 is also a dwarf galaxy. Horologium 1 is slightly more metal-poor than Reticulum 2 ([{Fe}/{{H}}]=-2.76) and is similarly α-enhanced: [α /{Fe}]̃ 0.3 {dex} with a significant spread of metallicities of 0.17 dex. The line-of-sight velocity of Reticulum 2 is offset by 100 km s<SUP>-1</SUP> from the prediction of the orbital velocity of the Large Magellanic Cloud (LMC), thus making its association with the Cloud uncertain. However, at the location of Horologium 1, both the backward-integrated orbit of the LMC and its halo are predicted to have radial velocities similar to that of the dwarf. Therefore, it is possible that Horologium 1 is or once was a member of the Magellanic family.