Navarro , Frenk And White Research Articles

Isolated and non-isolated dark matter haloes and the Navarro, Frenk and White profile

We compare the conclusions reached using the coarse-graining technique employed by Henriksen for a one degree of freedom (per particle) collisionless system to those presented in a paper by Binney based on an exact one degree of freedom model. We find agreement in detail, but in addition we show that the isolated 1D system is self-similar and therefore unrelaxed. Fine graining of this system recovers much less prominent wave-like structure than in a spherically symmetric isotropic 3D system. The rate of central flattening is also reduced in the 1D system. We take this to be evidence that relaxation of collisionless systems proceeds ultimately by way of short wavelength Landau damping. N-body systems, both real and simulated, can be trapped in an incompletely relaxed state because of a break in the cascade of energy to small scales. This may be due to the rapid dissipation of the small-scale oscillations in an isolated system to the existence of conserved quantities such as angular momentum, or to the failure in simulations to resolve sub-Jeans length scales. Such a partially relaxed state appears to be the Navarro, Frenk and White (NFW) state and is to be expected especially in young systems. The NFW core is shown to be isolated. In non-isolated systems, continuing coarse-grained relaxation should be towards a density core in solid body rotation.

On the dynamics of the satellite galaxies in NGC 5044

The NGC 5044 galaxy group is dominated by a luminous elliptical galaxy that is surrounded by ∼160 dwarf satellites. The projected number density profile of this dwarf population deviates within ∼1/3 of the virial radius from a projected Navarro, Frenk and White (NFW) profile, which is assumed to approximate the underlying total matter distribution. By means of a semi-analytic model, we demonstrate that the interplay between gravitation, dynamical friction and tidal mass loss and destruction can explain the observed number density profile. We use only two parameters in our models: the total to stellar mass fraction of the satellite haloes and the disruption efficiency. The disruption efficiency is expressed by a minimum radius. If the tidal radius of a galaxy (halo) falls below this radius, it is assumed to become unobservable. The preferred parameters are an initial total to stellar mass fraction of ∼20 and a disruption radius of 4 kpc. In that model, about 20 per cent of all the satellites are totally disrupted on their orbits within the group environment. Dynamical friction is less important in shaping the inner slope of the number density profile because the reduction in mass by tidal forces lowers the impact of the friction term. The main destruction mechanism is tide. In the preferred model, the total B-band luminosity of all disrupted galaxies is about twice the observed luminosity of the central elliptical galaxy, indicating that a significant fraction of stars are scattered into the intragroup medium. Dwarf galaxy satellites closer to the centre of the NGC 5044 group may exhibit optical evidence of partial tidal disruption. If dynamical friction forces the satellite to merge with the central elliptical, the angular momentum of the satellite tends to be removed at the apocentre passage. Afterwards, the satellite drops radially towards the centre.

Modelling the Milky Way through adiabatic compression of cold dark matter haloes

We use the adiabatic compression theory to build a physically well-motivated Milky Way mass model in agree- ment with observational data. The visible mass of the Galaxy is distributed in a spheroidal bulge and a multi-component disc parametrized by three galactic parameters, the Sun distance to the galactic centre, R0, the total bulge mass, Mbulge ,a nd the local disc surface density, Σ� . To model the dark matter component, we adiabatically compress a Navarro, Frenk and White (NFW) halo (with concentration c and total mass Mvir) for fixed values of the spin parameter λ, the fraction of the mass in baryons mb, and the thin disc contribution to total angular momentum jd. An iterative selection procedure is used to explore in detail the wide space of parameters only selecting those combinations of R0, Mbulge, Σ� ,λ ,mb, jd, c, Mvir that give rise to a Milky Way model in agreement with observational constraints. This analysis leads us to conclude that only models with R0 = 8.5 kpc, 0.8 × 10 10 M� < Mbulge < 1.6 × 10 10 Mand 49 Mpc −2 ≤ Σ� ≤ 56 Mpc −2 can be reconciled with the set of observational constraints. As regards the parameters entering the adiabatic compression, we find 0.03 ≤ λ ≤ 0.10 and 0.04 ≤ mb ≤ 0.10, while final estimates of the parameters describing the initial halo profile turn out to be 5 < c < 12 and 7 × 10 11 M� < Mvir < 17 × 10 11 M� (all at 95.7% CL).

The spacetime associated with galactic dark matter halos

We show how an adequate post--Newtonian generalization can be obtained for Newtonian dark matter halos associated with an empiric density profile. Applying this approach to halos that follow from the well known numerical simulations of Navarro, Frenk and White (NFW), we derive all dynamical variables and show that NFW halos approximatelly follow an ideal gas type of equation of state which fits very well to a polytropic relation in the region outside the core. This fact suggests that ``outer'' regions of NFW halos might be related to equilibrium states in the non--extensive Statistical Mechanics formalism proposed by Tsallis.

Dark matter profiles in clusters of galaxies: a phenomenological approach

There are some basic differences between the observed properties of galaxies and clusters and the predictions from current hydrodynamical simulations. These are particularly pronounced in the central regions of galaxies and clusters. The popular NFW (Navarro, Frenk and White) profile, for example, predicts a density cusp at the center, a behavior that (unsurprisingly) has not been observed. While it is not fully clear what are the reasons for this discrepancy, it perhaps reflects (at least partly) insufficient spatial resolution of the simulations. In this paper we explore a purely phenomenological approach to determine dark matter density profiles that are more consistent with observational results. Specifically, we deduce the gas density distribution from measured X-ray brightness profiles, and substitute it in the hydrostatic equilibrium equation in order to derive the form of dark matter profiles. Given some basic theoretical requirements from a dark matter profile, we then consider a number of simple profiles that have the desired asymptotic form. We conclude that a dark matter profile of the form ρ=ρ 0 1+r/r a −3 is most consistent with current observational results.

The Orbits of Cluster Galaxy Populations as Evolutionary Constraints

We determine the mass profile of a synthetic cluster built from the combination of 59 nearby clusters observed in the ESO Nearby Abell Cluster Survey (ENACS). We use ellipticals and S0s as tracers of the cluster potential, and solve the Jeans equation assuming isotropic orbits. Such an assumption is justified by the analysis of the shape of the velocity distribution of ellipticals and S0s. We find that the cluster mass profile is consistent with the Navarro, Frenk and White (NFW) model. We use this cluster mass profile to search for equilibrium solutions for the other cluster galaxy populations: very bright ellipticals (M R ≤ −22 + 5 log h), early-type spirals (Sa-Sb), and late-type spirals and irregulars (Sbc-Ir), together with emission-line galaxies. We find equilibrium solutions for both the early- and the late-spirals, but not for the very bright ellipticals. The dynamics of very bright ellipticals is probably affected by dissipative processes which invalidate the use of the collisionless Jeans equation. The equilibrium solution found for the early-spirals implies them to move on nearly-isotropic orbits. Late-spirals are instead found to be on mildly radial orbits, with the radial anisotropy increasing outwards. We discuss the implications of these results for the evolutionary histories of the different populations of cluster galaxies.