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

Abstract

Wave dark matter ($ψ\rm{DM}$) composed of extremely light bosons ($m_ψ \sim 10^{-22}\,\rm eV$), 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 $ψ{\rm DM}$ (${\rm FP} ψ{\rm DM}$), the extreme-axion $ψ\rm{DM}$ model (${\rm EA} ψ{\rm DM}$) 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: ${\rm FP} ψ{\rm DM}$, ${\rm EA} ψ{\rm DM}$, and cold dark matter (CDM). We show that ${\rm EA} ψ{\rm DM}$ produces significantly more abundant low-mass haloes than ${\rm FP} ψ{\rm DM}$ with the same $m_ψ$, and therefore could alleviate the tension in $m_ψ$ required by the Ly$α$ forest data and by the kpc-scale dwarf galaxy cores. We also find that, compared to the CDM counterparts, massive ${\rm EA} ψ{\rm DM}$ 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 ${\rm EA} ψ{\rm DM}$ haloes may exhibit more prominent starbursts at $z \lesssim 7$.