Spectroscopic Confirmation of the Sixth Globular Cluster in the Fornax Dwarf Spheroidal Galaxy

Andrew B. Pace, Matthew G. Walker, Sergey E. Koposov, Nelson Caldwell, Mario Mateo, Edward W. Olszewski, John I. Bailey, Mei Yu Wang

Research output: Contribution to journalArticlepeer-review

21 Scopus citations

Abstract

The Fornax dwarf spheroidal galaxy has an anomalous number of globular clusters, five, for its stellar mass. There is a longstanding debate about a potential sixth globular cluster (Fornax 6) that has recently been "rediscovered"in DECam imaging. We present new Magellan/M2FS spectroscopy of the Fornax 6 cluster and Fornax dSph. Combined with literature data we identify ∼15-17 members of the Fornax 6 cluster, showing that this overdensity is indeed a star cluster and associated with the Fornax dSph. The cluster is significantly more metal-rich (mean metallicity of = -0.71± 0.05) than the other five Fornax globular clusters (-2.5 < [Fe/H] < -1.4) and more metal-rich than the bulk of Fornax. We measure a velocity dispersion of 5.6-1.6+2.0km s-1 corresponding to an anomalously high mass-to-light of 15 < M/L < 258 at 90% confidence when calculated assuming equilibrium. Two stars inflate this dispersion and may be either Fornax field stars or as yet unresolved binary stars. Alternatively, the Fornax 6 cluster may be undergoing tidal disruption. Based on its metal-rich nature, the Fornax 6 cluster is likely younger than the other Fornax clusters, with an estimated age of ∼2 Gyr when compared to stellar isochrones. The chemodynamics and star formation history of Fornax shows imprints of major events such as infall into the Milky Way, multiple pericenter passages, star formation bursts, and/or potential mergers or interactions. Any of these events may have triggered the formation of the Fornax 6 cluster.

Original languageEnglish (US)
Article number77
JournalAstrophysical Journal
Volume923
Issue number1
DOIs
StatePublished - Dec 10 2021

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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