Impact of dark matter-baryon relative velocity on the 21-cm forest

We study the effect of the relative velocity between the dark matter (DM) and the baryon on the 21-cm forest signals. The DM-baryon relative velocity arises due to their different evolutions before the baryon-photon decoupling epoch, and it gives an additional anisotropic pressure that can suppress the perturbation growth. It is intriguing that the scale k (1010^3) h/Mpc at which the matter power spectrum is affected by such a streaming velocity turns out to be the scale which the 21-cm forest signal is sensitive to. We demonstrate that the 21-cm absorption line abundance can decrease by more than a factor of a few due to the small-scale matter power spectrum suppression caused by the DM-baryon relative velocity.