Deep SOAR follow-up photometry of two Milky Way outer-halo companions discovered with Dark Energy Survey

E. Luque, B. Santiago, A. Pieres, J. L. Marshall, A. B. Pace, R. Kron, A. Drlica-Wagner, A. Queiroz, E. Balbinot, M. Dal Ponte, A. Fausti Neto, L. N. Da Costa, M. A. G. Maia, A. R. Walker, F. B. Abdalla, S. Allam, J. Annis, K. Bechtol, A. Benoit-lévy, E. BertinD. Brooks, A. Carnero Rosell, M. Carrasco Kind, J. Carretero, M. Crocce, C. Davis, P. Doel, T. F. Eifler, B. Flaugher, J. García-Bellido, D. W. Gerdes, D. Gruen, R. A. Gruendl, G. Gutierrez, K. Honscheid, D. J. James, K. Kuehn, N. Kuropatkin, R. Miquel, R. C. Nichol, A. A. Plazas, E. Sanchez, V. Scarpine, R. Schindler, I. Sevilla-Noarbe, M. Smith, M. Soares-Santos, F. Sobreira, E. Suchyta, D. Thomas

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We report the discovery of a new star cluster, DES 3, in the constellation of Indus, and deeper observations of the previously identified satellite DES J0222.7−5217 (Eridanus III). DES 3 was detected as a stellar overdensity in first-year Dark Energy Survey data, and confirmed with deeper photometry from the 4.1-m Southern Astrophysical Research (SOAR) telescope. The new system was detected with a relatively high significance and appears in the DES images as a compact concentration of faint blue point sources. We determine that DES 3 is located at a heliocentric distance of ≃76.2 kpc and it is dominated by an old (≃9.8 Gyr) and metal-poor ([Fe/H] ≃ −1.84) population. While the age and metallicity values of DES 3 are comparable to typical globular clusters (objects with a high stellar density, stellar mass of ∼105 M⊙ and luminosity MV ∼ −7.3), its half-light radius (rh ∼ 6.87 pc) and luminosity (MV ∼ −1.7) are more indicative of faint star cluster. Based on the angular size, DES 3, with a value of rh ∼ 0′.31, is among the smallest faint star clusters known to date. Furthermore, using deeper imaging of DES J0222.7−5217 taken with the SOAR telescope, we update structural parameters and perform the first isochrone modelling. Our analysis yields the first age (≃12.6 Gyr) and metallicity ([Fe/H] ≃ −2.01) estimates for this object. The half-light radius (rh ≃ 11.24 pc) and luminosity (MV ≃ −2.4) of DES J0222.7−5217 suggest that it is likely a faint star cluster. The discovery of DES 3 indicates that the census of stellar systems in the Milky Way is still far from complete, and demonstrates the power of modern wide-field imaging surveys to improve our knowledge of the Galaxy’s satellite population.
Original languageEnglish
Pages (from-to)2006-2018
JournalMonthly Notices of the Royal Astronomical Society
Issue number2
Early online date25 Apr 2018
Publication statusPublished - 1 Aug 2018


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