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The core–cusp problem: a matter of perspective

Genina, Anna; Benítez-Llambay, Alejandro; Frenk, Carlos S.; Cole, Shaun; Fattahi, Azadeh; Navarro, Julio F.; Oman, Kyle A.; Sawala, Till; Theuns, Tom

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Anna Genina

Alejandro Benítez-Llambay

Carlos S. Frenk

Julio F. Navarro

Kyle A. Oman

Till Sawala


The existence of two kinematically and chemically distinct stellar subpopulations in the Sculptor and Fornax dwarf galaxies offers the opportunity to constrain the density profile of their matter haloes by measuring the mass contained within the well-separated half-light radii of the two metallicity subpopulations. Walker and Peñarrubia have used this approach to argue that data for these galaxies are consistent with constant-density ‘cores’ in their inner regions and rule out ‘cuspy’ Navarro–Frenk–White (NFW) profiles with high statistical significance, particularly in the case of Sculptor. We test the validity of these claims using dwarf galaxies in the APOSTLE (A Project Of Simulating The Local Environment) Λ cold dark matter cosmological hydrodynamic simulations of analogues of the Local Group. These galaxies all have NFW dark matter density profiles and a subset of them develop two distinct metallicity subpopulations reminiscent of Sculptor and Fornax. We apply a method analogous to that of Walker and Peñarrubia to a sample of 50 simulated dwarfs and find that this procedure often leads to a statistically significant detection of a core in the profile when in reality there is a cusp. Although multiple factors contribute to these failures, the main cause is a violation of the assumption of spherical symmetry upon which the mass estimators are based. The stellar populations of the simulated dwarfs tend to be significantly elongated and, in several cases, the two metallicity populations have different asphericity and are misaligned. As a result, a wide range of slopes of the density profile are inferred depending on the angle from which the galaxy is viewed.


Genina, A., Benítez-Llambay, A., Frenk, C. S., Cole, S., Fattahi, A., Navarro, J. F., …Theuns, T. (2017). The core–cusp problem: a matter of perspective. Monthly Notices of the Royal Astronomical Society, 474(1), 1398-1411.

Journal Article Type Article
Acceptance Date Nov 1, 2017
Online Publication Date Nov 3, 2017
Publication Date Nov 3, 2017
Deposit Date Jan 5, 2018
Publicly Available Date Jan 10, 2018
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 474
Issue 1
Pages 1398-1411


Published Journal Article (2.7 Mb)

Copyright Statement
This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society © 2017. The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.

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