# Axion Structure Formation II: The Wrath of Collapse

## TL;DR

This paper investigates the non-linear collapse and virialization of QCD axion dark matter, revealing unique Bose structures and dynamics that survive violent gravitational collapses, expanding understanding of axion structure formation.

## Contribution

It introduces N-body simulations tailored for axion Bose fluids to study full gravitational collapse and identifies novel post-collapse structures and behaviors.

## Key findings

- Bose structures survive violent collapses.
- Discovery of new substructures and violations of classical energy conditions.
- Bose fluid exhibits unique dynamical features post-collapse.

## Abstract

The first paper in this series showed that QCD axion dark matter, as a highly-correlated Bose fluid, contains extra-classical physics on cosmological scales. The nature of the derived extra-classical physics is exchange-correlation interactions induced by the constraints of symmetric particle exchange and inter-axion correlations from self-gravitation. The paper also showed that the impact of extra-classical physics on early structure formation is marginal, as the exchange-correlation interaction is inherently non-linear. This paper continues the study of axion structure formation into the non-linear regime by now considering the case of full collapse and virialization. The N-body method is chosen to solve the collapse problem, and its algorithms are derived for a condensed Bose fluid. Simulations of isolated gravitational collapse are performed for both Bose and cold dark matter fluids using a prototype N-body code. Unique Bose structures are found to survive even the most violent collapses. Bose post-collapse features include dynamical changes to global structures, creation of new broad substructures, violations of classical binding energy conditions, and new fine structures. Effective models of the novel structures are constructed and possibilities for their discovery are discussed.