The Milky Way might be hiding up to 100 million black holes from us, far more than previously thought. This number is the result of a statistical census trying to estimate how common black holes of certain sizes should be.
The research, published in the Monthly Notices of the Royal Astronomical Society, began by the detection of two large black holes by the gravitational wave observatory LIGO last year. LIGO saw the merger of two black holes, each with a mass equivalent to 30 Suns
"Fundamentally, the detection of gravitational waves was a huge deal, as it was a confirmation of a key prediction of Einstein's general theory of relativity," co-author Professor James Bullock said in a statement. "But then we looked closer at the astrophysics of the actual result, a merger of two 30-solar-mass black holes. That was simply astounding and had us asking, 'How common are black holes of this size, and how often do they merge?'"
Based on realistic estimations of the black hole merger rate, one can establish how many there should be. The researchers assumed that these black holes were formed from stars (so no primordial black holes shenanigans), which put constraints on their sizes. A black hole's mass depends on the stellar progenitor and stars are getting smaller.
Stars form by collapsing clouds of hydrogen. For the gas to turn into a star, it needs to be cold enough to first condense and then collapse under its own weight. The purer the hydrogen is, the longer it takes to cool down. Stars made almost exclusively of hydrogen are much, much bigger.
These stars exploded in supernovae that released a lot of elements heavier than hydrogen into the universe. These elements help cool down the hydrogen gas more quickly and make smaller stars. So, the size of a black hole in this scenario remains proportional to the stellar population.
"We have a pretty good understanding of the overall population of stars in the universe and their mass distribution as they're born, so we can tell how many black holes should have formed with 100 solar masses versus 10 solar masses," Bullock said. "We were able to work out how many big black holes should exist, and it ended up being in the millions – way more than I anticipated."
Black holes can only be seen directly in a handful of ways, and most of the time it requires these cosmic objects to be interacting. However, researchers have detected quiet ones in the past and this suggested a large population of undetected objects.
"We show that only 0.1 to 1 percent of the black holes formed have to merge to explain what LIGO saw," Manoj Kaplinghat, another co-author, said. "Of course, the black holes have to get close enough to merge in a reasonable time, which is an open problem."
Based on stellar population, the larger stellar black holes (up to 50 solar masses) are more likely to be found in smaller galaxies. Dwarf galaxies tend to have fewer heavy elements and bigger stars. And, thanks to current advancements, we might be able to detect mergers of 50-solar-mass black holes in the next few years.