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We investigate the feasibility of using the velocity acoustic oscillations(VAO) features on the Cosmic Dawn 21 cm power spectrum to probe small-scaledensity fluctuations. In the standard cold dark matter (CDM) model, Pop IIIstars form in minihalos and affect the 21 cm signal through Ly$\alpha$ andX-ray radiation. Such a process is modulated by the relative motion betweendark matter and baryons, generating the VAO wiggles on the 21 cm powerspectrum. In the fuzzy or warm dark matter models for which the number ofminihalos is reduced, the VAO wiggles are weaker or even fully invisible. Weinvestigate the wiggle features in the CDM with different astrophysical modelsand in different dark matter models. We find: 1) In the CDM model the relativestreaming velocities can generate the VAO wiggles for broad ranges ofparameters $f_*$, $\zeta_X$ and $f_{\rm esc,LW}\zeta_{\rm LW}$, though fordifferent parameters the wiggles would appear at different redshifts and havedifferent amplitudes. 2) For the axion model with $m_{\rm a} \lesssim10^{-19}$eV, the VAO wiggles are negligible. In the mixed model, the VAO signal issensitive to the axion fraction. For example, the wiggles almost disappear when$f_{\rm a} \gtrsim 10\%$ for $m_{\rm a}=10^{-21}$ eV. Therefore, the VAO signalcan be an effective indicator for small-scale density fluctuations and a usefulprobe of the nature of dark matter. The SKA-low with $\sim$2000 hourobservation time has the ability to detect the VAO signal and constraint darkmatter models.

Velocity acoustic oscillations on Cosmic Dawn 21 cm power spectrum as a probe of small-scale density fluctuations