“Rogue” black holes hundreds or thousands of times the mass of our sun could be hurtling through space at enormous speed, hurled by enormous collisions in the distant past.
Most massive galaxies harbour a central “supermassive” black hole, and the researchers say that it's possible that when galaxies collide, the collisions "spit out" rogue black holes.
When two galaxies merge, it produces gravitational waves carrying off extreme amounts of energy, comparable to an atomic bomb with the mass of several suns.
The researchers said that a black hole, plus a group of stars, could be hurled out in these collisions, similar to the recoil from a gun.
They added that these hyper-compact stellar clusters (HCSCs) could be spotted in telescope images, and offered a prediction of what they will look like.
If the researchers spot one, they will be able to work out where it came from, said lead author Davide Lena.
He said: “That has already been calculated from simulations of gravitational waves, but those are based on theories that need to be confirmed by observations.”
The researchers' predictions of how these objects would appear in a two-dimensional telescope image could also offer confirmation of a new class of black hole, Lena said.
The “rogue” black holes are predicted to be just hundreds or thousands of times the mass of our sun - far smaller than the black holes we currently know of.
“The existence of intermediate mass black holes is debated,” Lena said.
"If we indeed find HCSCs, we will at the same time show the existence of intermediate mass black holes. We can then confirm this by measuring the mass of the black holes through spectroscopic observations of the HCSC."
Any ejected black holes in the Milky Way’s outskirts would have been the result of mergers between a dwarf galaxy and a young Milky Way.
Co-author Peter Jonker said: “We think that mergers play an important part in forming massive black holes. ESA’s LISA satellite, to be launched in 2034, will be able to detect their gravitational waves.”