Formation of First Stars Triggered by Collisions and Shock Waves: 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 104 K may be composed mostly of cold atomic clouds that were formerly minihalos. These clouds move at a speed of ~15-30 km s-1 and may collide with one another on an average time interval of ~107 yr. The supersonic collisions may result in spatially distributed and efficient star formation. Most of the subsequent star formation in cold atomic clouds may be triggered by shock waves launched from the first stars formed in colliding clouds. Those shock-wave-compressed clouds may be more widespread spatially, because of the large imparted velocities, and some may escape into the intergalactic medium. The resultant widespread star formation would allow the possibility of a much higher ionizing photon escape fraction. These favorable conditions may form a physical basis to enable the standard cosmological model to produce a reasonably high Thomson optical depth τe = 0.11-0.14. Furthermore, a chain reaction of star formation in minihalos in the intergalactic space may be triggered by explosions in intergalactic medium, if minihalos are strongly clustered. In this case, a still higher τe would be achievable.