Rediscovery of the Sixth Star Cluster in the Fornax Dwarf Spheroidal Galaxy
Bacon, D.; Bechtol, K.; Wang, M. Y.; Smith, M.; Abbott, T. M. C.; Annis, J.; Bertin, E.; Brooks, D.; Burke, D. L.; Carnero Rosell, A.; Carrasco Kind, M.; Carretero, J.; da Costa, L. N.; Desai, S.; Diehl, H. T.; Doel, P.; Flaugher, B.; Fosalba, P.; Frieman, J.; García-Bellido, J.; Gerdes, D. W.; Gruen, D.; Gruendl, R. A.; Gschwend, J.; Gutierrez, G.; Honscheid, K.; James, D. J.; Kuehn, K.; Kuropatkin, N.; Li, T. S.; Maia, M. A. G.; Marshall, J. L.; Menanteau, F.; Miquel, R.; Plazas, A. A.; Sanchez, E.; Scarpine, V.; Schindler, R.; Schubnell, M.; Sevilla-Noarbe, I.; Smith, R. C.; Sobreira, F.; Suchyta, E.; Swanson, M. E. C.; Tarle, G.; Walker, A. R.; DES Collaboration; Belokurov, V.; Hollowood, D. L.; Buckley-Geer, E.; De Vicente, J.; Serrano, S.; Thomas, D.; Drlica-Wagner, A.; Hoyle, B.; Pace, A. B.; Pieres, A.; Santiago, B.; Tucker, D. L.; Kent, S.; Koposov, S.; de Boer, T.; Estrada, J.
United States, United Kingdom, Brazil, Chile, France, Spain, India, Germany, Australia
Abstract
Since first noticed by Shapley in 1939, a faint object coincident with the Fornax dwarf spheroidal has long been discussed as a possible sixth globular cluster (GC) system. However, debate has continued over whether this overdensity is a statistical artifact or a blended galaxy group. In this Letter we demonstrate, using deep DECam imaging data, that this object is well resolved into stars and is a bona fide star cluster. The stellar overdensity of this cluster is statistically significant at the level of ∼6-6.7σ in several different photometric catalogs including Gaia. Therefore, it is highly unlikely to be caused by random fluctuation. We show that Fornax 6 is a star cluster with a peculiarly low surface brightness and irregular shape, which may indicate a strong tidal influence from its host galaxy. The Hess diagram of Fornax 6 is largely consistent with that of Fornax field stars, but it appears to be slightly bluer. However, it is still likely more metal-rich than most of the GCs in the system. Faint clusters like Fornax 6 that orbit and potentially get disrupted in the centers of dwarf galaxies can prove crucial for constraining the dark matter distribution in Milky Way satellites.