Large-scale BAO signatures of the smallest galaxies
  Recent work has shown that at high redshift, the relative velocity between
dark matter and baryonic gas is typically supersonic. This relative velocity
suppresses the formation of the earliest baryonic structures like minihalos,
and the suppression is modulated on large scales. This effect imprints a
characteristic shape in the clustering power spectrum of the earliest
structures, with significant power on 100 Mpc scales featuring highly
pronounced baryon acoustic oscillations. The amplitude of these oscillations is
orders of magnitude larger at z=20 than previously expected. This
characteristic signature can allow us to distinguish the effects of minihalos
on intergalactic gas at times preceding and during reionization. We illustrate
this effect with the example of 21 cm emission and absorption from redshifts
during and before reionization. This effect can potentially allow us to probe
physics on kpc scales using observations on 100 Mpc scales.
  We present sensitivity forecasts for FAST and Arecibo. Depending on
parameters, this enhanced structure may be detectable by Arecibo at redshifts
near z=15-20, and with appropriate instrumentation FAST could measure the BAO
power spectrum with high precision. In principle, this effect could also pose a
serious challenge for efforts to constrain dark energy using observations of
the BAO feature at low redshift.
