Resolving the timing problem of the globular clusters orbiting the Fornax dwarf galaxy

Journal article

Authors/Editors

Angus, Garry

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Publication Details

Author list: Angus GW, Diaferio A

Publisher: Oxford University Press

Place: MALDEN

Publication year: 2009

Journal: Monthly Notices of the Royal Astronomical Society (0035-8711)

Journal acronym: MON NOT R ASTRON SOC

Volume number: 396

Issue number: 2

Start page: 887

End page: 893

Number of pages: 7

ISSN: 0035-8711

eISSN: 1365-2966

Languages: English-Great Britain (EN-GB)

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Abstract

We re-investigate the old problem of the survival of the five globular clusters (GCs) orbiting the Fornax dwarf galaxy in both standard and modified Newtonian dynamics (MOND). For the first time in the history of the topic, we use accurate mass models for the Fornax dwarf, obtained through Jeans modelling of the recently published line-of-sight (LOS) velocity dispersion data, and we are also not resigned to circular orbits for the GCs. Previously conceived problems stem from fixing the starting distances of the globulars to be less than half the tidal radius. We relax this constraint since there is absolutely no evidence for it and show that the dark matter (DM) paradigm, with either cusped or cored DM profiles, has no trouble sustaining the orbits of the two least massive GCs for a Hubble time almost regardless of their initial distance from Fornax. The three most massive globulars can remain in orbit as long as their starting distances are marginally outside the tidal radius. The outlook for MOND is also not nearly as bleak as previously reported. Although dynamical friction (DF) inside the tidal radius is far stronger in MOND, outside DF is negligible due to the absence of stars. This allows highly radial orbits to survive, but more importantly circular orbits at distances more than 85 per cent of Fornax's tidal radius to survive indefinitely. The probability of the GCs being on circular orbits at this distance compared with their current projected distances is discussed and shown to be plausible. Finally, if we ignore the presence of the most massive globular (giving it a large LOS distance), we demonstrate that the remaining four globulars can survive within the tidal radius for the Hubble time with perfectly sensible orbits.

Keywords

galaxies: individual: Fornax dwarf spheroidal, globular clusters: general

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