Early structure formation constraints on the ultralight axion in the postinflation scenario

Many works have concentrated on the observable signatures of dark matter being an ultralight axionlike particle (ALP). We concentrate on a particularly dramatic signature in the late-time cosmological matter power spectrum that occurs if the symmetry breaking that establishes the ALP happens after inflation - white-noise density fluctuations that dominate at small scales over the adiabatic fluctuations from inflation. These fluctuations alter the early history of nonlinear structure formation. We find that for symmetry-breaking scales of fA∼1013-1015 GeV, which require a high effective maximum temperature after inflation, ALP dark matter with a particle mass of mA∼10-13-10-20 eV could significantly change the number of high-redshift dwarf galaxies, the reionization history, and the Lyα forest. We consider all three observables. We find that the Lyα forest is the most constraining of current observables, excluding fA 1015 GeV (mA 10-17 eV) in the simplest model for the ALP and considerably lower values in models coupled to a hidden asymptotically free strongly interacting sector (fA 1013 GeV and m 10-13 eV). Observations that constrain the extremely high-redshift tail of reionization may disfavor similar levels of isocurvature fluctuations as the forest. Future z∼20-30 21 cm observations have the potential to improve these constraints further using that the supersonic motions of the isocurvature-enhanced abundance of ∼104 M halos would shock heat the baryons, sourcing large baryon acoustic oscillation features.