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

Abstract

Wave dark matter ($\psi \mathrm{DM}$) composed of extremely light bosons ($m_{\psi }\sim 10^{-22}\mathrm{eV}$), 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 $\psi \mathrm{DM}$ ($\mathrm{FP}\psi \mathrm{DM}$), the extreme-axion $\psi \mathrm{DM}$ model ($\mathrm{EA}\psi \mathrm{DM}$) 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: $\mathrm{FP}\psi \mathrm{DM}$, $\mathrm{EA}\psi \mathrm{DM}$, and cold dark matter (CDM). We show that $\mathrm{EA}\psi \mathrm{DM}$ produces significantly more abundant low-mass haloes than $\mathrm{FP}\psi \mathrm{DM}$ 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 $\mathrm{EA}\psi \mathrm{DM}$ 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 $\mathrm{EA}\psi \mathrm{DM}$ haloes may exhibit more prominent starbursts at $z\lesssim 7$.

5 pages, 4 figures, only minor updates (accepted for publication in MNRAS letters)

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