Skip to main content

Research Repository

Advanced Search

Cusp or core? Revisiting the globular cluster timing problem in Fornax

Meadows, Noah; Navarro, Julio F; Santos-Santos, Isabel; Benítez-Llambay, Alejandro; Frenk, Carlos

Cusp or core? Revisiting the globular cluster timing problem in Fornax Thumbnail


Noah Meadows

Julio F Navarro

Isabel Santos-Santos

Alejandro Benítez-Llambay


We use N-body simulations to revisit the globular cluster (GC) ‘timing problem’ in the Fornax dwarf spheroidal (dSph). In agreement with earlier work, we find that, due to dynamical friction, GCs sink to the centre of dark matter haloes with a cuspy inner density profile but ‘stall’ at roughly 1/3 of the core radius (rcore) in haloes with constant-density cores. The time-scales to sink or stall depend strongly on the mass of the GC and on the initial orbital radius, but are essentially the same for either cuspy (Navarro–Frenk–White) or cored haloes normalized to have the same total mass within rcore. Arguing against a cusp on the basis that GCs have not sunk to the centre is thus no different from arguing against a core, unless all clusters are today at ∼(1/3)rcore⁠. This would imply a core radius exceeding ∼3 kpc, much larger than seems plausible in any core-formation scenario. (The average projected distance of Fornax GCs is 〈RGC, Fnx〉 ∼ 1 kpc and its effective radius is ∼700 pc.) A simpler explanation is that Fornax GCs have only been modestly affected by dynamical friction, as expected if clusters started orbiting at initial radii of the order of ∼1–2 kpc, just outside Fornax’s present-day half-light radius but well within the tidal radius imprinted by Galactic tides. This is not entirely unexpected. Fornax GCs are significantly older and more metal-poor than most Fornax stars, and such populations in dSphs tend to be more spatially extended than their younger and more metal-rich counterparts. Contrary to some earlier claims, our simulations further suggest that GCs do not truly ‘stall’ at ∼0.3rcore⁠, but rather continue decaying towards the centre, albeit at reduced rates. We conclude that dismissing the presence of a cusp in Fornax based on the spatial distribution of its GC population is unwarranted.


Meadows, N., Navarro, J. F., Santos-Santos, I., Benítez-Llambay, A., & Frenk, C. (2020). Cusp or core? Revisiting the globular cluster timing problem in Fornax. Monthly Notices of the Royal Astronomical Society, 491(3), 3336-3342.

Journal Article Type Article
Acceptance Date Nov 21, 2019
Online Publication Date Nov 26, 2019
Publication Date Jan 31, 2020
Deposit Date Feb 26, 2020
Publicly Available Date Feb 26, 2020
Journal Monthly Notices of the Royal Astronomical Society
Print ISSN 0035-8711
Electronic ISSN 1365-2966
Publisher Royal Astronomical Society
Peer Reviewed Peer Reviewed
Volume 491
Issue 3
Pages 3336-3342


Published Journal Article (607 Kb)

Copyright Statement
This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2020 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.

You might also like

Downloadable Citations