The 21-cm signals from ultracompact minihaloes as a probe of primordial small-scale fluctuations

Ultracompact minihaloes (UCMHs) can form after the epoch of matter-radiation equality, if the density fluctuations of dark matter have significantly large amplitude on small scales. The constraint on the UCMH abundance allows us to access such small-scale fluctuations. In this paper, we present that, through the measurement of 21-cm fluctuations before the epoch of reionization, we can obtain a constraint on the UCMH abundance. We calculate the 21-cm signal from UCMHs and show that UCMHs provide the enhancement of the 21-cm fluctuations. We also investigate the constraint on the UCMH abundance and small-scale curvature perturbations. Our results indicate that the upcoming 21-cm observation, the Square Kilometre Array (SKA), provides the constraint on amplitude of primordial curvature power spectrum, A(zeta) less than or similar to 10(-6) on 100 less than or similar to k less than or similar to 1000 Mpc(-1). Although it is not stronger than the one from the non-detection of gamma-rays induced by dark matter annihilation in UCMHs, the constraint by the SKA will be important because this constraint is independent of the dark matter particle model.