Constraining the nature of ultra light dark matter particles with the 21 cm forest

The ultralight scalar fields can arise ubiquitously, for instance, as a result of the spontaneous breaking of an approximate symmetry such as the axion and, more generally, the axionlike particles. In addition to the particle physics motivations, these particles can also play a major role in cosmology by contributing to dark matter abundance and affecting the structure formation at sub-Mpc scales. In this paper, we propose to use the 21 cm forest observations to probe the nature of ultralight dark matter. The 21 cm forest is the system of narrow absorption lines appearing in the spectra of high redshift background sources due to the intervening neutral hydrogen atoms, similar to the Lyman-α forest. Such features are expected to be caused by the dense neutral hydrogen atoms in a small starless collapsed object called a minihalo. The 21 cm forest can probe much smaller scales than the Lyman-α forest, that is, k≳10 Mpc−1. We explore the range of the ultralight dark matter mass mu and fu, the fraction of ultralight dark matter with respect to the total matter, which can be probed by the 21 cm forest. We find that 21 cm forest can potentially put the dark matter mass lower bound mu≳10−18 eV for fu=1, which is a 3 orders of magnitude bigger mass scale than those probed by the current Lyman-α forest observations. While the effects of the ultralight particles on the structure formation become smaller when the dominant component of dark matter is composed of the conventional cold dark matter, we find that the 21 cm forest is still powerful enough to probe the subcomponent ultralight dark matter mass up to the order of 10−19 eV. The Fisher matrix analysis shows that (mu,fu)∼(10−20 eV,0.3) is the most optimal parameter set which the 21 cm forest can probe with the minimal errors for a subcomponent ultralight dark matter scenario.