Probing Oscillons of Ultra-Light Axion-like Particle by 21cm Forest

TL;DR

This paper explores how ultra-light axion-like particles forming oscillons could be detected through 21cm line observations, especially for particles with very low mass and significant dark matter fractions.

Contribution

It demonstrates that oscillons from ULAP can enhance 21cm signals, providing a new observational probe for ultra-light axion-like particles.

Findings

Oscillons can significantly boost 21cm signals for ULAP mass below 10^{-19} eV.

Detection likelihood increases with ULAP fraction exceeding 1% of dark matter.

Potential to constrain ULAP properties through 21cm line observations.

Abstract

Ultra-Light Axion-like Particle (ULAP) is motivated as one of the solutions to the small scale problems in astrophysics. When such a scalar particle oscillates with an $\mathcal{O}(1)$ amplitude in a potential shallower than quadratic, it can form a localized dense object, oscillon. Because of its longevity due to the approximate conservation of the adiabatic invariant, it can survive up to the recent universe as redshift $z \sim \mathcal{O}(10)$. The scale affected by these oscillons is determined by the ULAP mass $m$ and detectable by observations of 21cm line. In this paper, we examine the possibility to detect ULAP by 21cm line and find that the oscillon can enhance the signals of 21cm line observations when $m \lesssim 10^{-19}\ {\rm eV}$ and the fraction of ULAP to dark matter is much larger than $10^{-2}$ depending on the form of the potential.