Yan Dong, Bin Yue, Chen Zhang, Yuewei Wen, Yidong Xu, Ayodeji Ibitoye, Xuelei Chen

In the Dark Ages, minihalos above the Jeans mass accrete gas from the intergalactic medium (IGM). The accreted gas would be heated to virial temperature and produces 21 cm signal in emission, while the majority of gas retained in the IGM is cold and has 21 cm signal in absorption. The 21 cm signal of the IGM generally dominates over that of minihalos, making it difficult to extract the contribution of minihalos from the detected 21 cm signal. However, in the late Dark Ages, in regions where the gas has not yet been influenced by the radiation from Pop III stars, the 21 cm signal from the IGM is weak because collisional coupling has become less efficient. More importantly, the relic supersonic dark matter-baryon relative streaming velocities suppress the gas accretion of smaller minihalos and encode their large-scale coherent features into the 21 cm signal, generating the velocity acoustic oscillation (VAO) features on the 21 cm power spectrum. This offers a promising avenue for detecting minihalos. We found that, at $z\sim 15$ ($\sim 20$), the first peak of the net VAO wiggle at $k=0.05$ Mpc$^{-1}$ is $\sim 1.4\times 10^{-3}$ ($\sim 3.1\times 10^{-4}$) mK$^2$. With a total of 24 lunar nights/8064 hr (60 lunar nights/20160 hr) of integration, the signal from $z\sim 15$ ($\sim 20$) is detectable for an array composed of 82944 (278784) dipoles, spread over $\sim 3.5$ ($\sim 6.3$) km. If the small-scale matter power spectrum is enhanced and minihalos are much more numerous than in the standard $Λ$CDM model, the signal is detectable for an even smaller scale array.