The shape of the gravitational potential in cold dark matter haloes

By E. Hayashi, J. Navarro, V. S. M. Garching, U. Victoria. 2006

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We use a set of cosmological N-body simulations to investigate the structural shape of galaxysized cold dark matter (CDM) haloes. Unlike most previous work on the subject ‐ which dealt with shapes as measured by the inertia tensor ‐ we focus here on the shape of the gravitational potential, a quantity more directly relevant to comparison with observational probes. A further advantage is that the potential is less sensitive to the effects of substructure and, as a consequence, the isopotential surfaces are typically smooth and well approximated by concentric ellipsoids. Our main result is that the asphericity of the potential increases rapidly towards the centre of the halo. The radial trend is more pronounced than expected from constant flattening in the mass distribution, and reflects a strong tendency for dark matter haloes to become increasingly aspherical inwards. Near the centre the halo potential is approximately prolate on average [(c/a)0 = 0.72 ± 0.04, (b/a)0 = 0.78 ± 0.08], but it becomes less axisymmetric and more spherical in the outer regions. The principal axes of the isopotential surfaces remain well aligned, and in most haloes the angular momentum tends to be parallel to the minor axis and perpendicular to the major axis. This suggests that galactic discs may form in a plane where the potential is elliptical and where its ellipticity varies rapidly with radius. This can result in significant deviations from circular motion in systems such as low surface brightness (LSB) galaxies, even for relatively minor deviations from circular symmetry. Simulated long-slit rotation curves often appear similar to those of LSBs cited as evidence for constant density ‘cores’. This suggests that taking into account the 3D shape of the dark mass distribution might help to reconcile such evidence with the cuspy mass profile of CDM haloes.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/225342c3828e5484ecdd0fcc9c27528ba7722540

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Citations in the corpus

Galaxy Masses: A Review (2013, background): “Numerical simulations of CDM have shown that the resulting dark matter halos are triaxial with a typical ellipticity of ∼ 0.3 (e.g. Dubinski & Carlberg 1991; Jing & Suto 2002b; Hayashi et al. 2007).”
The Core-Cusp Problem (2009, background): “Kuzio de Naray et al. (2009) also note that the kind of rapidly varying ellipticity of the potential as proposed by Hayashi et al. (2007) might help in better describing the data, but that the problem of a preferred viewing angle will remain.”
The shape of dark matter haloes in the Aquarius simulations: evolution and memory (2011, methodology): “Taking advantage of this feature, Hayashi et al. (2007) have implemented a method to characterize the structural shape of dark matter haloes by fitting ellipses to the iso-potential contours computed on three orthogonal 2D planes.”
THE SAGITTARIUS DWARF GALAXY: A MODEL FOR EVOLUTION IN A TRIAXIAL MILKY WAY HALO (2010, background): “While there is a tendency for haloes to be more spherical at larger radii (Allgood et al. 2006; Hayashi et al. 2007), the halo of Milky Way-like systems at a distance of ∼ 50 kpc is expected to be prolate with TΦ ≈ 0.88 (based on the Via Lactea simulation of Diemand et al. 2007; Kuhlen et al. 2007).”
The diversity and similarity of simulated cold dark matter haloes (2008, background): “…(2) strongly triaxial shapes, with a slight preference for nearly prolate systems (e.g., Frenk et al., 1988; Jing & Suto, 2002; Allgood et al., 2006; Hayashi et al., 2007), (3) abundant, but non-dominant, substructure (Klypin et al., 1999; Moore et al., 1999a; Ghigna et al., 2000; Gao et al.,…”
GHASP: an Hα kinematic survey of spiral and irregular galaxies – V. Dark matter distribution in 36 nearby spiral galaxies (2007, background): “Hayashi et al. 2007 suggest that galactic disks may be forming in elliptical gravitational potential.”
THE SPACE MOTION OF LEO I: THE MASS OF THE MILKY WAY'S DARK MATTER HALO (2012, background): “%) of symmetric scatter at a given radius due to nonsphericity of the gravitational potential (see, e.g., Hayashi et al. 2007).”