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

Journal Article · Mon Dec 13 00:00:00 EST 2021 · The Astrophysical Journal

DOI:https://doi.org/10.3847/1538-4357/ac2cd2· OSTI ID:1983200

Pace, Andrew B.  ^[1]; Walker, Matthew G.  ^[1]; Koposov, Sergey E.  ^[2]; Caldwell, Nelson  ^[3]; Mateo, Mario ^[4]; Olszewski, Edward W.  ^[5]; Bailey, III, John I.  ^[6]; Wang, Mei-Yu  ^[1]

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1. Carnegie Mellon Univ., Pittsburgh, PA (United States)
2. Univ. of Edinburgh, Scotland (United Kingdom). The Royal Observatory; Univ. of Cambridge (United Kingdom); Carnegie Mellon Univ., Pittsburgh, PA (United States)
3. Harvard-Smithsonian Center for Astrophysics, Cambridge, MA (United States)
4. Univ. of Michigan, Ann Arbor, MI (United States)
5. Univ. of Arizona, Tucson, AZ (United States). Steward Observatory
6. Univ. of California, Santa Barbara, CA (United States)

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 $$\overline{[\mathrm{Fe}/{\rm{H}}]}$$ = –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 $${5.6}_{-1.6}^{+2.0}\mathrm{km}\,{{\rm{s}}}^{-1}$$ 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.

• Research Organization: Oak Ridge Institute for Science and Education (ORISE), Oak Ridge, TN (United States)
• Sponsoring Organization: USDOE Office of Science (SC); National Science Foundation (NSF)
• Grant/Contract Number: SC0014664; AST-1813881; AST-1909584; AST-1815767; AST-1812461; AST-1312997; AST-1726457; AST-1815403
• OSTI ID: 1983200
• Journal Information: The Astrophysical Journal, Vol. 923, Issue 1; ISSN 0004-637X
• Publisher: IOP Publishing
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
Astronomy &amp; Astrophysics
Globular star clusters
Dwarf spheroidal galaxies