Formation of First Stars Triggered by Collisions and Shockwaves: Prospect for High Star Formation Efficiency and High Ionizing Photon Escape Fraction

Abstract

We show that large, high-redshift (z>10) galaxies with virial temperature in excess of 10^4K may be mostly comprised of cold atomic clouds which were formerly minihalos. These clouds move at a speed of 15-30km/s and collide with one another on a time scale of 10^7yr. The supersonic collisions result in spatially distributed star formation with high efficiency. We then show that most, subsequent star formation in cold atomic clouds may be triggered by shockwaves launched from the first stars formed in collisions. Those shockwave-compressed clouds are even more widespread spatially because of large imparted velocities and some can possibly escape into the intergalactic medium. More importantly, with respect to cosmological reionization, widespread star formation would allow a much higher ionizing photon escape fraction. These favorable conditions may form the physical basis to enable the standard cosmological model to produce a reasonably high Thomson optical depth \tau_e=0.10-0.14. A chain reaction of star formation in minihalos in the intergalactic space may be triggered by explosions in the intergalactic medium, if minihalos are strongly clustered. In this case, a still higher \tau_e would be achievable.