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We study the head-on collision of fluid particles well within the kinetic energy dominated regime (=8 to 12) by numerically solving the Einstein-hydrodynamic equations. We find that the threshold for black hole formation is lower (by a factor of a few) than simple hoop conjecture estimates, and, moreover, near this threshold two distinct apparent horizons first form postcollision and then merge. We argue that this can be understood in terms of a gravitational focusing effect. The gravitational radiation reaches luminosities of 0. 014 c^5/G, carrying 162% of the total energy.
East et al. (Thu,) studied this question.