Journal Article

The graininess of dark matter haloes

Marcel Zemp, Jürg Diemand, Michael Kuhlen, Piero Madau, Ben Moore, Doug Potter, Joachim Stadel and Lawrence Widrow

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 394, issue 2, pages 641-659
Published in print April 2009 | ISSN: 0035-8711
Published online March 2009 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2008.14361.x
The graininess of dark matter haloes

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We use the recently completed one billion particle Via Lactea IIΛ cold dark matter simulation to investigate local properties like density, mean velocity, velocity dispersion, anisotropy, orientation and shape of the velocity dispersion ellipsoid, as well as the structure in velocity space of dark matter haloes. We show that at the same radial distance from the halo centre, these properties can deviate by orders of magnitude from the canonical, spherically averaged values, a variation that can only be partly explained by triaxiality and the presence of subhaloes. The mass density appears smooth in the central relaxed regions but spans four orders of magnitude in the outskirts, both because of the presence of subhaloes as well as of underdense regions and holes in the matter distribution. In the inner regions, the local velocity dispersion ellipsoid is aligned with the shape ellipsoid of the halo. This is not true in the outer parts where the orientation becomes more isotropic. The clumpy structure in local velocity space of the outer halo cannot be well described by a smooth multivariate normal distribution. Via Lactea II also shows the presence of cold streams made visible by their high 6D phase space density. Generally, the structure of dark matter haloes shows a high degree of graininess in phase space that cannot be described by a smooth distribution function.

Keywords: methods: N-body simulations; methods: numerical; galaxies: haloes; galaxies: kinematics and dynamics; galaxies: structure; dark matter

Journal Article.  9428 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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