The 21-cm forest as a simultaneous probe of dark matter and cosmic heating history

The hyperfine structure absorption lines of hydrogen atoms against high-redshift radio point sources, known as the 21 cm forest. They provide a unique probe of small-scale structures during the epoch of reionization, and can be used to constrain the properties of the dark matter thought to govern small-scale structure formation. However, the signals are easily suppressed by heating processes that are degenerate with a warm dark matter model. We propose a probe of both the dark matter particle mass and the heating history of the Universe, using the one-dimensional power spectrum of the 21-cm forest. This approach not only breaks the dark matter model degeneracy but also increases the sensitivity. Using the 21-cm forest observation with the upcoming Square Kilometre Array, it is promising to simultaneously determine both the dark matter particle mass and the heating level of the early Universe, shedding light on the nature of dark matter and the first galaxies.