Across the Universe, there are numerous examples of galaxies coming together in colossal mergers, a billion-year-old process that stains stars and gas in the surrounding space. For the most part, we know how this unfolds, but we don’t know what happens to supermassive black holes within their galactic centers.
Astronomers think that these two galactic nuclei could merge into a larger black hole, sending huge gravitational waves that would undulate in space-time. But if there is any asymmetry in galactic fusion, the newly forged black hole could be ejected from the galaxy and sent through the Universe by a recoil of gravitational waves, taking away a swarm of stars.
Such fugitive objects, theorized in the late 1990s, are called hypercompact star systems (HCSS): a supermassive black hole with a tight group of gravitationally bonded stars, drifting away into the cosmos.
The existence of HCSS would be quite convincing evidence of the pullback of the gravitational wave. There’s only one big problem: We haven’t found any conclusively, not yet anyway.
But what happens on the largest scale should also occur on smaller scales. We know that stellar-mass black holes can merge; Thanks to gravitational wave interferometers, we have detected quite a few black hole fusions across the Universe.
When the Milky Way was much younger, and its possibly much smaller central black hole, a merger with a dwarf galaxy that also contained a smaller central black hole could have resulted in one of these smaller mergers.
This would produce a black hole between the stellar mass and the supermassive classes: a black hole of intermediate mass, between 100 and 10,000 times the mass of the Sun, with a retinue of stars: basically a small HCSS.
Simulations predict that there should be a few hundred rogue black holes in the halo of a galaxy like the Milky Way.
Which led astronomers from the Netherlands Institute for Space Research SRON and Radboud University in the Netherlands to the idea that HCSS might already have been picked up by previous surveys like Gaia and Sloan Digital Sky Survey, and are hiding in somewhere in the data.
But when the team went to find out more, they realized that no one had made detailed predictions about what these hypercompact star clusters should look like. So even if they were in the data, there was no toolkit to identify it.
In a new article, the team has simulated these groups, producing detailed predictions of their colors, appearance, and spectra. And they have gone one step further, adapting these results to show how groups would appear in data from nine separate ongoing surveys, as well as how they would appear in a two-dimensional telescope image.
They have not yet identified one of these groups; That is the next step in your research. But they have published the results of their simulations, which means that other astronomers can also search.
If they find them, it will be a big problem, and not just because of the recoil of the gravitational wave. The evidence for intermediate-mass black holes is pretty thin on the ground – they’re considered a ‘missing link’ to the black hole because, well, we’ve had trouble detecting them. There have been some pretty interesting detections that strongly suggest its existence, but nothing conclusive.
“The existence of intermediate-mass black holes is debated,” said astronomer Davide Lena of the Netherlands Institute for Space Research SRON.
“If we do indeed find hypercompact star groups, at the same time we will show the existence of intermediate mass black holes. Then we can confirm this by measuring the mass of the black holes through spectroscopic observations of the hypercompact star group.”
We also don’t know whether supermassive black holes, those that are millions to billions of times the mass of the Sun, and form the nuclei of mature galaxies, can merge at all. Calculations suggest that by the time two orbiting supermassive black holes are separated by a parsec (3.2 light years), their orbital energy keeps them rocking each other in a stable orbit for billions of years.
But we know that stellar-mass black holes (between 5 and 100 times the mass of the Sun) can merge. If HCSS can be identified in the galactic halo, this will show that intermediate mass black holes can also merge. In turn, that will strongly suggest that even more massive black holes can merge as well.
So if we can find them, these elusive star balls could be the key to solving quite a few space mysteries. We can hardly wait.
The research has been published in the Monthly notices from the Royal Astronomical Society.