Concentration, spin and shape of dark matter haloes: scatter and the dependence on mass and environment

Abstract

We use a series of cosmological N-body simulations for a flat cold dark matter ( CDM) cosmology to investigate the structural properties of dark matter haloes, at redshift zero, in the mass range 3 × 10 9 h −1 M vir 3 × 10 13 h −1 M . These properties include the concentration parameter, c, the spin parameter, λ, and the mean axis ratio,q. For the concentration-mass relation we find c ∝ M −0.11 vir in agreement with the model proposed by Bullock et al., but inconsistent with the alternative model of Eke et al. The normalization of the concentrationmass relation, however, is 15 per cent lower than suggested by Bullock et al. The results for λ andq are in good agreement with previous studies, when extrapolated to the lower halo masses probed here, while c and λ are anticorrelated, in that high-spin haloes have, on average, lower concentrations. In an attempt to remove unrelaxed haloes from the sample, we compute for each halo the offset parameter, x off , defined as the distance between the most bound particle and the centre of mass, in units of the virial radius. Removing haloes with large x off increases the mean concentration by ∼10 per cent, lowers the mean spin parameter by ∼15 per cent, and removes the most prolate haloes. In addition, it largely removes the anticorrelation between c and λ, though not entirely. We also investigate the relation between halo properties and their largescale environment density. For low-mass haloes we find that more concentrated haloes live in denser environments than their less concentrated counterparts of the same mass, consistent with recent correlation function analyses. Note, however, that the trend is weak compared to the scatter. For the halo spin parameters we find no environment dependence, while there is a weak indication that the most spherical haloes reside in slightly denser environments. Finally, using a simple model for disc galaxy formation we show that haloes that host low surface brightness galaxies are expected to be hosted by a biased subset of haloes. Not only do these haloes have spin parameters that are larger than average, they also have concentration parameters that are ∼15 per cent lower than the average at a given halo mass. We discuss the implications of all these findings for the claimed disagreement between halo concentrations inferred from low surface brightness rotation curves, and those expected for a CDM cosmology.

Orbit-based dynamical models of the Sculptor dSph galaxy

“…Assuming an NFW profile for the dark matter profile, we derive a mass within 1 kpc of M 1kpc = (1.03 ± 0.07) × 10 8 M⊙, and find the concentration (c ∼ 15) to be compatible with current ΛCDM predictions, given the expected scatter in the mass-concentration relation (Macciò et al 2007). When we try to constrain the inner slope of the dark matter density profile of Sculptor, we can exclude very cuspy profiles (α < −1.5).…”

“…In the right panel of the Fig. 9, we show as the dashed black line the mass-concentration relation of Macciò et al (2007): log c200 = −0.109 log(M200/M⊙) + 2.34. Sculptor is not compatible with the current ΛCDM cosmology.…”

Statistical Decoupling of a Lagrangian Fluid Parcel in Newtonian Cosmology

“…Therefore, two proto-halos with exactly the same initial density and shape might evolve into very different states as a result of their distinct environments. Furthermore, for various purposes, we are also interested in other cosmic-web morphologies Lemson & Kauffmann 1999) including filaments, walls, and voids (Sheth & Tormen 2004;Avila-Reese et al 2005;Wechsler et al 2005;Bett et al 2007;Hahn et al 2007aHahn et al , 2007bMacciò et al 2007;Wetzel et al 2007). The same problem arises when we concentrate on discrete matter patches and attempt to understand their Lagrangian evolutions 3 as well.…”

Principal properties of the velocity distribution of dark matter particles near the Solar System

“…Meanwhile, according to modern cosmological conceptions, not only the total angular momentum of the halo but also the momentum of each particle should have been negligibly small on the linear stage of the structure formation (Gorbunov & Rubakov 2011). The halo could gain some angular momentum later, as a result of tidal perturbations or merging of smaller halos; numerical simulations show, however, that it cannot be large (Macciò et al 2007). …”