Clustering of cosmic string loops within a Milky-way like halo

TL;DR

This study uses N-body simulations to analyze how cosmic string loops with the rocket effect are captured by a Milky-Way-like halo, revealing a peak in captured loops at a specific size parameter and their distribution within the halo.

Contribution

It introduces a novel simulation approach modeling cosmic string loops with recoil forces, providing new insights into their capture efficiency and spatial distribution in galaxy halos.

Findings

Peak capture of loops at a specific size parameter $\xi_{peak}\, extasciitilde 12.5$

Captured loops with weaker rocket forces follow dark matter distribution

Stronger recoil loops are concentrated toward halo centers

Abstract

Loops of cosmic string experience a recoil from anisotropic gravitational radiation, known as the rocket effect, which influences the extent to which they are captured by galaxies during structure formation. Analytical studies have reached different conclusions regarding loop capture in galaxies: early treatments argued for efficient capture, while later analyses incorporating the loop rocket force throughout halo formation found that capture efficiency is reduced and strongly dependent on loop size. In this work, we employ the N-body simulation code GADGET-4, introducing non-backreacting tracer particles subject to a constant recoil force to model cosmic string loops with the rocket effect. We simulate the formation of a Milky-Way-like halo from redshift $z=127$ to $z=0$, considering loop populations characterized by a range of length parameters $\xi$, inversely proportional to the rocket acceleration. We find that the number of captured loops exhibits a pronounced peak at $\xi_{\textrm{peak}}\simeq 12.5$, arising from the competition between rocket-driven ejection at small $\xi$ and the declining intrinsic loop abundance at large $\xi$. For fiducial string tensions, this corresponds to $\mathcal{O}(10^6)$ loops within the halo. We further find that loops with weak rocket forces closely trace the dark-matter distribution, while those subject to stronger recoil but still captured -- particularly the most abundant loops near $\xi_{\textrm{peak}}$ -- are preferentially concentrated toward the central regions of the halo.