Cold collapse and the core catastrophe

Abstract

We show that a universe dominated by cold dark matter fails to reproduce the rotation curves of dark matter dominated galaxies, one of the key problems that it was designed to resolve. We perform numerical simulations of the formation of dark matter halos, each containing \gsim 10^6 particles and resolved to 0.003 times the virial radius, allowing an accurate comparison with rotation curve data. A good fit to both galactic and cluster sized halos can be achieved using the density profile rho(r) \propto [(r/r_s)^1.5(1+(r/r_s)^1.5)]^-1, where r_s is a scale radius. This profile has a steeper asymptotic slope, rho(r) \propto r^-1.5, and a sharper turnover than found by lower resolution studies. The central structure of relaxed halos that form within a hierarchical universe has a remarkably small scatter (unrelaxed halos would not host disks). We compare the results with a sample of dark matter dominated, low surface brightness (LSB) galaxies with circular velocities in the range 100-300 km/s. The rotation curves of disks within cold dark matter halos rise too steeply to match these data which require a constant mass density in the central regions. The same conclusion is reached if we compare the scale free shape of observed rotation curves with the simulation data. It is important to confirm these results using stellar rather than HI rotation curves for LSB galaxies. We test the effects of introducing a cut-off in the power spectrum that may occur in a universe dominated by warm dark matter. In this case halos form by a monolithic collapse but the final density profile hardly changes, demonstrating that the merger history does not play a role in determining the halo structure.Comment: Latex 13 pages, 4 figures. Submitted to MNRAS. High resolution colour version of figure 4 and other N-body images here: http://star-www.dur.ac.uk:80/~moore/images

Can Dark Matter See Itself?

“…Close to the center this profile approaches ρ ∝ r −1 , in strong disagreement with observations. More recent simulations with much higher resolution find an even steeper profile, approaching ρ ∝ r −3/2 [14][15][16][17]. Furthermore, it has been claimed that these CDM simulations are now of sufficiently high resolution that the results for the central halo profile have converged to the infinite resolution limit.…”

Halos of unified dark matter scalar field

“…Now, looking at the various CDM density profiles which have been proposed in the literature [31,32,33,34,35,36], we see that we can always take ρ CDM as…”

“…(6) and (8), the expression for the Lagrangian L. Such a procedure establishes a mapping between UDM and CDM solutions that predict the same halo rotation curve v c (r). As a starting point we could, of course, use very different CDM density profiles to this aim, such as the modified isothermal-law profile [31], the Burkert profile [32], the Moore profile [33], the Navarro-Frenk-White profile [34,35] or the profile proposed by Salucci et al (see for example [36]). …”

“…where κ is a proper positive constant which depends on the particular profile that is chosen [31,32,33,34,35,36]. For example for many of the density profiles the slope is n = 3 for large radii [32,33,34,35,36].…”

“…For example for many of the density profiles the slope is n = 3 for large radii [32,33,34,35,36]. In this case a positive energy density ρ > 0 requires n ≥ 2.…”