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.
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.
Finding an HCSS would confirm the theory of gravitational wave recoil, and would prove that supermassive black holes can exist outside galaxies.
Astronomers believe that supermassive black holes (SMBHs) can be ejected from the centers of galaxies by gravitational wave recoil.
[3] The biggest HCSSs would carry perhaps a few million stars, making them comparable in luminosity to a globular cluster or ultra-compact dwarf galaxy.
[4] In this case, the HCSS would only exist as a distinct object for a relatively short time, of order hundreds of millions of years, before disappearing back into the galaxy nucleus.
This is a challenging observation to make because an HCSS would be relatively faint, requiring many hours of exposure time even on a 10m class telescope.