Saturday, January 29, 2011

Gravitational Recoil

Supermassive black holes are the most massive type of black holes and they can contain between hundreds of thousands to billions of times the mass of our Sun. Almost all galaxies are known to harbour supermassive black holes within their cores. For example, the Milky Way Galaxy contains a supermassive black hole at its centre called Sagittarius A* and this behemoth packs around 4 million times the mass of our Sun. From time to time, galaxies are known to collide and eventually merge with one another. When two galaxies collide, the two supermassive black holes that are located in each of their galactic cores can eventually come close enough to each other to form a gravitationally bound binary pair.

As two supermassive black holes orbit around each other, the emission of gravitational waves will cause them to loose angular momentum and spiral towards each other, leading to an increasingly tighter orbit. The rate at which angular momentum is lost through the emission of gravitational waves increases dramatically as the two supermassive black holes get closer to each other, leading to a final inspiral that is followed by an eventual coalescence of the two supermassive black holes. During the final inspiral of the two supermassive black holes, the anisotropic emission of gravitational waves is able to impart a large gravitational wave recoil velocity to the final supermassive black hole. The final supermassive black hole can have a gravitational wave recoil velocity that is as large as 4000 kilometres per second.

A large gravitational wave recoil velocity can displace the supermassive black hole arbitrarily far from the core of its host galaxy, or even completely eject the supermassive black hole from its host galaxy if the gravitational wave recoil velocity is larger than the escape velocity of its host galaxy. In reality, the majority of coalescenced supermassive black holes will have gravitational wave recoil velocities that will be significantly less than the escape velocities of typical galaxies. Following a merger, the resultant supermassive black hole will be displaced at some maximum distance from the core of its host galaxy as a result of the gravitational wave recoil.

The supermassive black hole will oscillate a number of times through the core of its host galaxy with decaying amplitudes as it transfers its ‘excess’ kinetic energy gained from the gravitational wave recoil to the surrounding stars. This has the effect of reducing the density distribution of stars in the region of the host galaxy’s core that is near the supermassive black hole. The time required for the amplitude of the oscillatory motion of a ‘kicked’ supermassive black hole to eventually decay down to roughly the core radius of its host galaxy is on the order of a few hundred million years or less. For dwarf galaxies and globular clusters, the gravitational wave recoil velocities are more likely to completely eject most coalesced black holes from these systems due to their much lower escape velocities as compared to typical galaxies.

A supermassive black hole that is displaced from the core of its host galaxy is expected to carry with it an entourage of stars that remains gravitationally bound to the supermassive black hole as a densely packed cluster of stars. Such a cluster of stars is referred to as hypercompact stellar system and it differs from all the other types of star clusters by having an exceptionally high internal velocity distribution due to the deep gravitational potential well of the central supermassive black hole. The total mass of all the stars in a hypercompact stellar system is expected to be on the order of one percent of the mass of the supermassive black hole itself, making the cluster similar in size and luminosity to globular clusters. However, in extreme cases, the size and luminosity of a hypercompact stellar system can approach that of ultra-compact dwarf galaxies.

Although hypercompact stellar systems share similarities with globular clusters, they differ from globular clusters in two fundamental aspects which can help in the identification of these unique clusters. Firstly, hypercompact stellar systems have much higher velocity dispersions than globular clusters as the stars in hypercompact stellar systems have velocities that are on the order of a hundred to a thousand kilometres per second. This is comparable to the velocity imparted to a post-merger supermassive black hole from the gravitational wave recoil. Secondly, the stars in hypercompact stellar systems come from the cores of galaxies and they will exhibit higher metallicities than the stars in globular clusters. This is due to the multiple episodes of stellar evolution that occur in galaxies which enable subsequent generations of stars to be made up of a far higher proportion of elements that are heavier than hydrogen and helium as compared to the stars in globular clusters.