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The unexpected diversity of dwarf galaxy rotation curves.

Oman, K. A. and Navarro, J. F. and Fattahi, A. and Frenk, C. S. and Sawala, T. and White, S. D. M. and Bower, R. and Crain, R. A. and Furlong, M. and Schaller, M. and Schaye, J. and Theuns, T. (2015) 'The unexpected diversity of dwarf galaxy rotation curves.', Monthly notices of the Royal Astronomical Society., 452 (4). pp. 3650-3665.


We examine the circular velocity profiles of galaxies in Λ cold dark matter (CDM) cosmological hydrodynamical simulations from the EAGLE and LOCAL GROUPS projects and compare them with a compilation of observed rotation curves of galaxies spanning a wide range in mass. The shape of the circular velocity profiles of simulated galaxies varies systematically as a function of galaxy mass, but shows remarkably little variation at fixed maximum circular velocity. This is especially true for low-mass dark-matter-dominated systems, reflecting the expected similarity of the underlying CDM haloes. This is at odds with observed dwarf galaxies, which show a large diversity of rotation curve shapes, even at fixed maximum rotation speed. Some dwarfs have rotation curves that agree well with simulations, others do not. The latter are systems where the inferred mass enclosed in the inner regions is much lower than expected for CDM haloes and include many galaxies where previous work claims the presence of a constant density ‘core’. The ‘cusp versus core’ issue is thus better characterized as an ‘inner mass deficit’ problem than as a density slope mismatch. For several galaxies, the magnitude of this inner mass deficit is well in excess of that reported in recent simulations where cores result from baryon-induced fluctuations in the gravitational potential. We conclude that one or more of the following statements must be true: (i) the dark matter is more complex than envisaged by any current model; (ii) current simulations fail to reproduce the diversity in the effects of baryons on the inner regions of dwarf galaxies; and/or (iii) the mass profiles of ‘inner mass deficit’ galaxies inferred from kinematic data are incorrect.

Item Type:Article
Keywords:Galaxies: haloes, Galaxies: structure, Dark matter.
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Publisher statement:This article has been accepted for publication in Monthly notices of the Royal Astronomical Society © 2015 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.
Date accepted:02 July 2015
Date deposited:19 February 2016
Date of first online publication:October 2015
Date first made open access:No date available

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