A Model-Insensitive Baryon Acoustic Oscillation Feature in the 21 cm Signal from Reionization

We examine the impact of the baryon-dark matter relative velocity on intergalactic small-scale structure and the 21 cm signal during the epoch of reionization (EoR). Streaming velocities reduced the clumping of the intergalactic medium (IGM) on mass scales of $\sim 10^4 - 10^8$ M$_{\odot}$. This effect produced a distinct baryon acoustic oscillation (BAO) feature in the EoR 21 cm power spectrum at wave numbers $k\sim 0.1$ h/Mpc, near which forthcoming surveys will be most sensitive. In contrast to the highly uncertain impact of streaming velocities on star formation, the effect on clumping can be calculated more precisely because it is set mainly by cosmology and straightforward gas dynamics. We quantify the latter using fully coupled radiation-hydrodynamics simulations that capture the Jeans scale of the pre-EoR gas. The clumping factor of ionized gas is reduced by 5-10\% in regions that had typical streaming velocities of 30 km/s at recombination. The suppression peaks $\approx 5$ Myr after a region is reionized, but washes out within 200 Myr due to pressure smoothing of the gas. Using these results, we model the corresponding impact on the EoR 21 cm power spectrum and find that the BAO feature is most likely to appear early in reionization ($\approx$ 10 \% ionization).During this phase, the signal may appear at the 1 \% (5 \%) level at $k \sim 0.1 (0.06)$ h/\Mpc with an amplitude that varies by less than a factor of 10 across a range of reionization histories. We also provide a simple model for the signal originating from streaming velocity's impact on ionizing sources, which can vary by 4 orders of magnitude depending on highly uncertain source properties. We find that the clumping-driven signal is likely to dominate the source-driven one unless population III star formation in halos of masses $10^6 - 10^8$ M$_{\odot}$ was efficient enough to drive the first 10\% of reionization.