Ultrarelativistic Black Hole Formation

We study the head-on collision of fluid particles well within the kinetic energy dominated regime ($\ensuremath{\gamma}=8$ 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 $0.014\text{ }{c}^{5}/G$, carrying $16\ifmmode\pm\else\textpm\fi{}2%$ of the total energy.

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