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Hypercompact stellar system


A hypercompact stellar system (HCSS) is a dense cluster of stars around a supermassive black hole that has been ejected from the center of its host galaxy. Stars that are close to the black hole at the time of the ejection will remain bound to the black hole after it leaves the galaxy, forming the HCSS.

The term "hypercompact" refers to the fact HCSSs are small in size compared with ordinary star clusters of similar luminosity. This is because the gravitational force from the supermassive black hole keeps the stars moving in very tight orbits about the center of the cluster.

The luminous X-ray source SDSS 1113 near the galaxy Markarian 177 would be the first candidate for an HCSS. Finding an HCSS would confirm the theory of gravitational wave recoil, and would prove that supermassive black holes can exist outside of galaxies.

Astronomers believe that supermassive black holes (SMBHs) can be ejected from the centers of galaxies by gravitational wave recoil. This happens when two SMBHs in a binary system coalesce, after losing energy in the form of gravitational waves. Because the gravitational waves are not emitted isotropically, some momentum is imparted to the coalescing black holes, and they feel a recoil, or "kick," at the moment of coalescence. Computer simulations suggest that the kick can be as large as , which exceeds escape velocity from the centres of even the most massive galaxies.


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