Kinematics and chemistry of recently discovered Reticulum 2 and Horologium 1 dwarf galaxies

Sergey E. Koposov, Andrew R. Casey, Vasily Belokurov, James R. Lewis, Gerard Gilmore, Clare Worley, Anna Hourihane, T. Bensby, A. Bragaglia, M. Bergemann, G. Carraro, E. Flaccomio, U. Heiter, V. Hill, P. Jofre, P. de Laverny, L. Monaco, L. Sbordone, S. Mikolaitis, N. Ryde

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We report on VLT/GIRAFFE spectra of stars in two recently discovered ultra-faint satellites, Reticulum 2 andHorologium 1, obtained as part of theGaia-ESO Survey. We identify 18 members in Reticulum 2 andfive inHorologium 1. Wefind Reticulum 2 to have a velocity dispersion of3.22km s0.491.641-+-, implying a mass-to-lightratio(M/L)of∼500. The mean metallicity of Reticulum 2 isFe H2.46[]=-, with an intrinsic dispersion of∼0.3 dex andα-enhancement of∼0.4 dex. We conclude that Reticulum 2 is a dwarf galaxy. We also report on theserendipitous discovery of four stars in a previously unknown stellar substructure near Reticulum 2 withFe H2[]~-andV220 km shel1~-, far from the systemic velocity of Reticulum 2. For Horologium 1 we infer avelocity dispersion ofV4.9 km s0.92.81()s=-+-and aM/Lratio of∼600, leading us to conclude that Horologium 1 isalso a dwarf galaxy. Horologium 1 is slightly more metal-poor than Reticulum 2(Fe H2.76[]=-)and issimilarlyα-enhanced:Fe 0.3 dex[]a~with a significant spread of metallicities of 0.17 dex. The line-of-sightvelocity of Reticulum 2 is offset by 100 km s−1from the prediction of the orbital velocity of the Large MagellanicCloud(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 ofthe dwarf. Therefore, it is possible that Horologium 1 is or once was a member of the Magellanic family.
Original languageEnglish
JournalAstrophysical Journal
Publication statusPublished - 21 Sep 2015


  • astro-ph.GA

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