Predicting the density profiles of the first halos

Abstract

The first dark matter halos form by direct collapse from peaks in the matter density field, and evidence from numerical simulations and other analyses suggests that the dense inner regions of these objects largely persist today. These halos would be the densest dark matter structures in the Universe, and their abundance can probe processes that leave imprints on the primordial density field, such as inflation or an early matter-dominated era. They can also probe dark matter through its free-streaming scale. The first halos are qualitatively different from halos that form by hierarchical clustering, as evidenced by their ρ ∝ r −3/2 inner density profiles. In this work, we present and tune models that predict the density profiles of these halos from properties of the density peaks from which they collapsed. These models predict the coefficient A of the ρ = Ar −3/2 small-radius asymptote of the density profile along with the maximum circular velocity vmax and associated radius rmax. These models are universal; they can be applied to any cosmology, and we confirm this by validating them using six N -body simulations carried out in wildly disparate cosmological scenarios. We find that these models can even predict the full population of halos with reasonable accuracy in scenarios with narrowly supported power spectra, although for broader power spectra, an understanding of the impact of halo mergers is needed. With their connection to the primordial density field established, the first dark matter halos will serve as probes of the early Universe and the nature of dark matter.

Large-scale dark matter simulations

“…Finally, another interesting question concerns the internal structure of WDM halos near the free streaming scale. As in the case of neutralino simulations, simulations have found indications that WDM halos have inner density profiles that are steeper than their CDM counterparts (Colı ´n et al 2008;Polisensky and Ricotti 2015;Ogiya and Hahn 2018;Delos et al 2019).…”

“…In contrast, most neutralino simulations find that the initial internal structure of microhalos is better described by a single power law profile $ r À1:5 , as first pointed out by Diemand et al (2005) (see also Ishiyama et al 2009;Anderhalden and Diemand 2013;Ishiyama 2014;Ogiya et al 2016;Angulo et al 2017;Delos et al 2019). This very steep profile would make microhalos very resilient to tidal disruption by the Milky Way or by binary stars and also would enhance their corresponding emission from dark matter self annihilation, making them potentially detectable by future experiments (Diemand et al 2006;Ishiyama et al 2009;Ishiyama 2014;Delos 2019), although their abundance is affected by free streaming (Ishiyama and Ando 2020).…”

The First Three Seconds: a Review of Possible Expansion Histories of the Early Universe

“…The peak scale is determined by the cut-off scale in the matter power spectrum, so the formation time of the first halos is most sensitive to the ratio k cut /k reh , with only a weak dependence on the reheat temperature [43,359]. The formation time of a halo determines its density, and Delos et al [634] demonstrated that the density profiles of these first halos can be predicted directly from ∆ 2 (k).…”

“…Ref. [634] showed that microhalo profiles and their abundance can be predicted in a simple way from properties of the precursor linear density field, enabling rapid computation of Γ/M in a given cosmological scenario [824]. Other works [359,358,360] have employed Press-Schechter theory [635] to characterize microhalo populations and assumed that these microhalos have NFW profiles with low concentrations at formation.…”

A dynamics-based density profile for dark haloes. I. Algorithm and basic results

“…The secondary infall models also predicts the full density profile inside and outside the splashback radius, but it is well known that these predictions are not realistic, at least not unless the model is coupled with collapse from cored peaks (e.g., Dalal et al 2010;Ogiya & Hahn 2018;Delos et al 2019), realistic accretion histories (Lu et al 2006), or mergers (Ogiya et al 2016;Angulo et al 2017). One reason is that, by itself, the model can predict only power-law density profiles because it evolves a self-similar initial density perturbation.…”