How supermassive black holes are formed

Washington: Over billions of years, small black holes can slowly grow supermassive by taking on mass from their surroundings and also by merging with other black holes but this slow process can`t explain the problem of supermassive black holes existing in the early universe, as these would have formed less than one billion years after the Big Bang.

To investigate the origins of young supermassive black holes, lead author Christian Reisswig, NASA Einstein Postdoctoral Fellow in Astrophysics at Caltech, in collaboration with Christian Ott, assistant professor of theoretical astrophysics, and their colleagues turned to a model involving supermassive stars. In a very massive star, photon radiation-the outward flux of photons that is generated due to the star`s very high interior temperatures-pushes gas from the star outward in opposition to the gravitational force that pulls the gas back in. When the two forces are equal, this balance is called hydrostatic equilibrium. Previous studies predicted that when supermassive stars collapse, they maintain a spherical shape that possibly becomes flattened due to rapid rotation. This shape is called an axisymmetric configuration. Incorporating the fact that very rapidly spinning stars are prone to tiny perturbations, Reisswig and his colleagues predicted that these perturbations could cause the stars to deviate into non-axisymmetric shapes during the collapse. Such initially tiny perturbations would grow rapidly, ultimately causing the gas inside the collapsing star to clump and to form high-density fragments. These fragments would orbit the center of the star and become increasingly dense as they picked up matter during the collapse; they would also increase in temperature. And then, Reisswig says, "an interesting effect kicks in." At sufficiently high temperatures, there would be enough energy available to match up electrons and their antiparticles, or positrons, into what are known as electron-positron pairs. The creation of electron-positron pairs would cause a loss of pressure, further accelerating the collapse; as a result, the two orbiting fragments would ultimately become so dense that a black hole could form at each clump. The pair of black holes might then spiral around one another before merging to become one large black hole. These findings have been published in Physical Review Letters.