Implications of Baryonic-Dark Matter Streaming velocity on Reionization

Relative velocity between dark matter and baryon can prevent gas from falling in potential wells of dark matter structures below ~10^5 solarmasses. I discuss two majorimplications of this in reionization. In small scales, gas clumps in intergalactic space act as sink of UV photons. These clumps would consume more ionizing photons per mass due to its high recombination rate, thereby increasing the UV photon budget for reionization locally. The streaming velocity suppresses formation of those sinks and galaxies will be able to form larger HII bubbles for a given amount of radiation. Since the level of the streaming velocity modulates at around a hundred of megaparsec scale, the geometry of HII regions would has acquire large-scale modulations detectable through upcoming observations like 21cm emission line survey. Another implication is a formation mechanism for direct collapse blackholes at high redshifts. The streaming can prevent collapse of gas in dark matter halos until they grow above certain threshold and suddenly accrete a large amount of gas. Recent simulations find that direct collapse blackholes of 10^5 solarmasses can form at around redshift of 30, which might explain the 10^9 solarmass supermassive blackholes observed in some quasars at z=6.