# Halo Abundance and Assembly History with Extreme-Axion Wave Dark Matter at $z\ge 4$

## TL;DR

This study uses cosmological simulations to compare halo formation and growth in extreme-axion wave dark matter, free-particle wave dark matter, and cold dark matter, revealing distinctive assembly histories and potential observational signatures.

## Contribution

It introduces and tests the extreme-axion wave dark matter model, showing it produces more low-mass haloes and different assembly patterns than free-particle wave dark matter and CDM.

## Key findings

- EA ψDM yields more low-mass haloes than FP ψDM.
- EA ψDM haloes are 3-4 times more massive at high redshift.
- EA ψDM haloes have higher merger rates at z=4-7.

## Abstract

Wave dark matter ($\psi \rm{DM}$) composed of extremely light bosons ($m_{\psi} \sim 10^{-22}\,\rm eV$), with quantum pressure suppressing structures below a kpc-scale de Broglie wavelength, has become a viable dark matter candidate. Compared to the conventional free-particle $\psi {\rm DM}$ (${\rm FP} \psi {\rm DM}$), the extreme-axion $\psi \rm{DM}$ model (${\rm EA} \psi {\rm DM}$) proposed by Zhang & Chiueh (2017) features a larger cut-off wavenumber and a broad spectral bump in the matter transfer function. Here we conduct cosmological simulations to compare the halo abundances and assembly histories at $z=4-11$ between three different scenarios: ${\rm FP} \psi {\rm DM}$, ${\rm EA} \psi {\rm DM}$, and cold dark matter (CDM). We show that ${\rm EA} \psi {\rm DM}$ produces significantly more abundant low-mass haloes than ${\rm FP} \psi {\rm DM}$ with the same $m_{\psi}$, and therefore could alleviate the tension in $m_{\psi}$ required by the Ly$\alpha$ forest data and by the kpc-scale dwarf galaxy cores. We also find that, compared to the CDM counterparts, massive ${\rm EA} \psi {\rm DM}$ haloes are on average $3-4$ times more massive at $z=10-11$ due to their earlier formation, undergo a slower mass accretion at $7 \lesssim z \lesssim 11$, and then show a rapidly rising major merger rate exceeding CDM by $\sim 50\%$ at $4 \lesssim z \lesssim 7$. This fact suggests that ${\rm EA} \psi {\rm DM}$ haloes may exhibit more prominent starbursts at $z \lesssim 7$.