Probing axion dark matter with 21cm fluctuations from minihalos

TL;DR

This paper investigates how axion dark matter with isocurvature perturbations influences small-scale structure formation, leading to potential 21cm fluctuation signals detectable by future radio telescopes like SKA.

Contribution

It introduces a novel analysis of axion-induced small-scale density fluctuations and estimates their impact on 21cm signals, providing new bounds on axion mass.

Findings

Future telescopes can constrain axion mass to $m_a ceil 10^{-13}$ eV for temperature-independent models.

Bounds extend to $m_a ceil 10^{-8}$ eV for temperature-dependent axion models.

Enhanced small-scale structure formation due to axion perturbations affects 21cm fluctuation signals.

Abstract

If the symmetry breaking inducing the axion occurs after the inflation, the large axion isocurvature perturbations can arise due to a different axion amplitude in each causally disconnected patch. This causes the enhancement of the small-scale density fluctuations which can significantly affect the evolution of structure formation. The epoch of the small halo formation becomes earlier and we estimate the abundance of those minihalos which can host the neutral hydrogen atoms to result in the 21cm fluctuation signals. We find that the future radio telescopes, such as the SKA, can put the axion mass bound of order $m_a \gtrsim 10^{-13}$ eV for the simple temperature-independent axion mass model, and the bound can be extended to of order $m_a \gtrsim 10^{-8}$eV for a temperature-dependent axion mass.