Tidal stripping of dark matter subhalos by baryons from analytical perspectives: disk shocking and encounters with stars

Abstract

The cold dark matter (CDM) scenario predicts that galactic halos should host a huge amount of subhalos possibly as light as or lighter than planets, depending on the nature of dark matter.Predicting their abundance and distribution on such small scales has important implications for dark matter searches and searches for subhalos themselves, which could provide a decisive test of the CDM paradigm. A major difficulty in subhalo population model building is to account for the gravitational stripping induced by baryons, which strongly impact on the overall dynamics within the scale radii of galaxies. In this paper, we focus on these "baryonic" tides from analytical perspectives, summarizing previous work on galactic disk shocking, and thoroughly revisiting the impact of individual encounters with stars. For the latter, we go beyond the reference calculation of Gerhard and Fall (1983) to deal with penetrative encounters, and provide new analytical results. Based upon a full statistical analysis of subhalo energy change during multiple stellar encounters possibly occurring during disk crossing, we show how subhalos lighter than ∼ 1 M are very efficiently pruned by stellar encounters, and how that modifies their mass function in a stellar environment. If reasonably resilient, surviving subhalos have lost all their mass but the inner cusp, with a tidal mass function strongly departing from the cosmological one; otherwise, their number density can drop by an order of magnitude at the solar position in the Milky Way with respect to disk-shocking effects only. For illustration, we integrate these results into our analytical subhalo population model. They can easily be incorporated to any other analytical or numerical approach. This study complements those using cosmological simulations, which cannot resolve dark matter subhalos on such small scales.

Classification of gamma-ray targets for velocity-dependent and subhalo-boosted dark-matter annihilation

“…[35] (SL17 henceforth) -see also Refs. [68][69][70][71]. Considering subhalos is particularly important when annihilation is velocity-dependent because the internal velocity dispersion in these objects is much smaller than that of the host halo, typically scaling like ∝ m 1/3 , where m is the virial self-gravitating (sub)halo mass.…”

“…This section gives a short introduction on the SL17 subhalo population model [35], which was built to address DM searches in general (including the search for subhalos themselves), as flexible as possible to be applied to a diversity of DM candidates and to easily account for changes in relevant cosmological parameters-see also Refs. [68][69][70][71]. This model is analytical in its formulation, but semi-analytical in practice (numerical integrations, iterations, or interpolations are necessary).…”

“…For definiteness, we use two types of tidal disruption criteria: We can now give a few details about the PDFs introduced above (see Refs. [35,71] for an exhaustive presentation). For the initial spatial PDF, we simply assume dP V /dV = ρ host (R)/M host , where M host is the total DM mass of the host.…”

“…[200] and we recover that the mass function can be well fitted by [182,183] We find γ 1 = 0.014, γ 2 = 0.41, α 1 = 1.965, α 2 = 0.57, β = 20, ζ = 3.4, roughly independent of the cosmology and of the host mass-still, we used the cosmological parameters from the latest Planck analysis [204]. Contrarily to previous works (only interested in large masses) in this fit we also constrain the low mass part of the spectrum [71,205]. Note that the above mass function is close to a power law in mass ∝ m −α with a spectral index α α such that the total number of subhalos, after tidal disruption effects are plugged, is given by…”

Disruption of Dark Matter Minihaloes in the Milky Way environment: Implications for Axion Miniclusters and Early Matter Domination

“…In those models, adiabatic fluctuations produced at the end of inflation can also seed small CDM subhaloes down to the kinetic decoupling and free-streaming limit (𝑘 ∼ O (pc −1 )), roughly corresponding to the Earth mass (∼ 10 −6 M ; e.g., Hofmann et al 2001;Berezinsky et al 2003;Green et al 2005;Loeb & Zaldarriaga 2005). The evolution of these subhaloes in the Milky Way environment has been studied in the past (e.g., Goerdt et al 2007;Green & Goodwin 2007;Zhao et al 2007;Schneider et al 2010;Berezinsky et al 2014;Delos 2019;Facchinetti et al 2022). However, WIMP-like CDM formed its first non-linear structures at relatively late times with correspondingly low density (e.g.…”