Probing oscillons of ultra-light axion-like particle by 21 cm forest

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 \U0001d4aa(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 ∼\U0001d4aa(10). The scale affected by these oscillons is determined by the ULAP mass m and detectable by observations of 21 cm line. In this paper, we examine the possibility to detect ULAP by 21 cm line and find that the oscillon can enhance the signals of 21 cm line observations when m ≲ 10-19 eV and the fraction of ULAP to dark matter is much larger than 10-2 depending on the form of the potential.