Abstract
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 $10^4 - 10^8$ M$_ødot$.
This effect produced a distinct baryon acoustic oscillation (BAO) feature in
the EoR 21 cm power spectrum at wave numbers $k0.1$ 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 $5$ 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 $k 0.1 (0.06)$ 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$_ødot$ was efficient enough to drive the first 10\%
of reionization.
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