Journal Article

The angular momentum of cold dark matter haloes with and without baryons

Philip Bett, Vincent Eke, Carlos S. Frenk, Adrian Jenkins and Takashi Okamoto

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 404, issue 3, pages 1137-1156
Published in print May 2010 | ISSN: 0035-8711
Published online May 2010 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2010.16368.x
The angular momentum of cold dark matter haloes with and without baryons

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We investigate the magnitude and internal alignment of the angular momentum of cold dark matter haloes in simulations with and without baryons. We analyse the cumulative angular momentum profiles of hundreds of thousands of well-resolved haloes in the Millennium Simulation of Springel et al. and in a smaller, but higher resolution, simulation, in total spanning 5 orders of magnitude in mass. For haloes of a given mass, the median specific angular momentum increases with radius as j(≤r) ∝r. The direction of the vector varies considerably with radius: the median angle between the inner (≲0.25Rvir) and total (≤Rvir) angular momentum vectors is about 25°. To investigate how baryons affect halo spin, we use another high-resolution simulation, which includes gas cooling, star formation and feedback. This simulation produces a sample of galaxies with a realistic distribution of disc-to-total ratios, D/T: two-thirds of the galaxies have D/T > 0.5 in the B band. The formation of the galaxy spins up the dark matter within 0.1Rvir such that the specific halo angular momentum increases by ≈50 per cent in the median. The dark matter angular momentum becomes better aligned, but there remains a broad distribution of (mis-)alignments between the halo and the central galaxy, with a median angle between their angular momenta of ∼30°. Galaxies have a range of orientations relative to the shape of the halo: half of them have their minor axes misaligned by more than 45°, although only about 10 per cent of the galaxies lie within 30° of the plane perpendicular to the major axis of their halo. Finally, we align a sample of haloes according to the orientation of their galaxies and stack the projected mass distributions. Although the individual haloes are significantly aspherical, galaxy–halo misalignments produce a stacked mass distribution that cannot be distinguished from circular. If the lack of alignment found in our simulations is realistic, it will be extremely difficult for weak lensing studies to measure the ellipticity of cold dark matter haloes using this technique.

Keywords: methods: N-body simulations; galaxies: haloes; dark matter

Journal Article.  16071 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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