Connecting the structure of dark matter haloes to the primordial power spectrum

Abstract

A large body of work based on collisionless cosmological N-body simulations going back over two decades has advanced the idea that collapsed dark matter (DM) haloes have simple and approximately universal forms for their mass density and pseudo-phase-space density (PPSD) distributions. However, a general consensus on the physical origin of these results has not yet been reached. In the present study, we explore to what extent the apparent universality of these forms holds when we vary the initial conditions (i.e. the primordial power spectrum of density fluctuations) away from the standard CMB-normalized case, but still within the context of lambda cold dark matter with a fixed expansion history. Using simulations that vary the initial amplitude and shape, we show that the structure of DM haloes retains a clear memory of the initial conditions. Specifically, increasing (lowering) the amplitude of fluctuations increases (decreases) the concentration of haloes and, if pushed far enough, the density profiles deviate strongly from the NFW form that is a good approximation for the CMB-normalized case. Although, an Einasto form works well. Rather than being universal, the slope of the PPSD (or pseudo-entropy) profile steepens (flattens) with increasing (decreasing) power spectrum amplitude and can exhibit a strong halo mass dependence. Our results therefore indicate that the previously identified universality of the structure of DM haloes is mostly a consequence of adopting a narrow range of (CMB-normalized) initial conditions for the simulations. Our new suite provides a useful test-bench against which physical models for the origin of halo structure can be validated.

The BAHAMAS project: evaluating the accuracy of the halo model in predicting the non-linear matter power spectrum

“…Our approach is to allow for additional freedom relative to the original NFW form, but with fewer free parameters than in the generalised NFW case. In particular, we adopt the so-called Einasto profile (Einasto 1965), which recent work has shown better reproduces the matter distribution in haloes in collisionless simulations (Springel et al 2008;Navarro et al 2010;Dutton & Macciò 2014;Brown et al 2020). This is due to its additional flexibility relative to NFW (it has an additional free parameter) which ought to allow it to better describe hydrodynamical simulations as well (indeed we show this below, in Section 4).…”

Towards a universal model for the density profiles of dark matter haloes

“…In this work, we primarily study a subset of the cosmologies first presented in Brown et al ( 2020 ), particularly examining the two suites closest to our own Universe. We discuss here briefly these different cosmologies.…”

“…We discuss here briefly these different cosmologies. For a more in-depth description of how these cosmologies were chosen, we refer the reader to section 2.3 of Brown et al ( 2020 ). The cosmologies presented in Brown et al ( 2020 ) were chosen to systematically study the effects of changes to both the amplitude and shape (i.e.…”

“…For a more in-depth description of how these cosmologies were chosen, we refer the reader to section 2.3 of Brown et al ( 2020 ). The cosmologies presented in Brown et al ( 2020 ) were chosen to systematically study the effects of changes to both the amplitude and shape (i.e. slope) of the linear power spectrum at different k -scales on the internal properties of DM haloes, such as the density and velocity dispersion profiles.…”

The principle of maximum entropy explains the cores observed in the mass distribution of dwarf galaxies

“…Their settling into this particular configuration could be due either to some fundamental physical process (as it happens with the velocities of the molecules in a gas) or to the initial conditions that had given rise to the system (Binney & Tremaine 2008). The mass distribution in galaxies is currently explained as the outcome of the initial conditions (Cen 2014;Nipoti 2015;Ludlow & Angulo 2017;Brown et al 2020). The option of a fundamental process determining the configuration is traditionally discredited because, in following the principles of statistical physics, it should correspond to the most probable configuration of a self-gravitating system and, thus, it should result from maximizing the entropy.…”