A trio of astronomers at the University of California, Irvine, claims that stellar-remnant black holes, which result from the collapse of massive stars at the end of their lives, are so common that there could be as many as 100 million in the Milky Way alone.
According to Elbert et al, there are probably millions of stellar-remnant black holes in our Milky Way Galaxy — far more than expected. Image credit: NASA’s Goddard Space Flight Center.
The team’s work began in 2016, shortly after the announcement that the LIGO collaboration had detected gravitational waves produced by the distant merger of two black holes, one 29 solar masses and the other 36 solar masses.
“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,” said Professor James Bullock, co-author of the study.
“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?”
“Scientists assume most stellar-remnant black holes will be about the same mass as our Sun,” he added.
“To see evidence of two black holes of such epic proportions finally coming together in a cataclysmic collision had some scientists scratching their heads.”
“Our work was a theoretical investigation into the ‘weirdness’ of the LIGO discovery.”
The study was an attempt to interpret the gravitational wave detections through the lens of what is known about galaxy formation and to form a framework for understanding future occurrences.
“Based on what we know about star formation in galaxies of different types, we can infer when and how many black holes formed in each galaxy. Big galaxies are home to older stars, and they host older black holes too,” said Oliver Elbert, lead author on the study.
The number of black holes of a given mass per galaxy will depend on the size of the galaxy. The reason is that larger galaxies have many metal-rich stars, and smaller dwarf galaxies are dominated by big stars of low metallicity.
Stars that contain a lot of heavier elements, like our Sun, shed a lot of that mass over their lives.
When it comes time for one to end it all in a supernova, there isn’t as much matter left to collapse in on itself, resulting in a lower-mass black hole.
Big stars with low metal content don’t shed as much of their mass over time, so when one of them dies, almost all of its mass will wind up in the black hole.
“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,” Professor 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.”
The team also sought to determine how often black holes occur in pairs, how often they merge, and how long it takes.
The researchers wondered whether the 30-solar-mass black holes detected by LIGO were born billions of years ago and took a long time to merge or came into being more recently — within the past 100 million years — and merged soon after.
“We show that only 0.1% to 1% of the black holes formed have to merge to explain what LIGO saw,” said co-author Professor Manoj Kaplinghat.
“Of course, the black holes have to get close enough to merge in a reasonable time, which is an open problem.”
“We expect many more gravitation wave detections so that we and other astronomers can determine if black holes collide mostly in giant galaxies. That would tell them something important about the physics that drive them to coalesce,” Elbert said.
“They may not have to wait too long, relatively speaking. If the current ideas about stellar evolution are right, then our calculations indicate that mergers of even 50-solar-mass black holes will be detected in a few years,” Professor Kaplinghat added.
The research is published in the Monthly Notices of the Royal Astronomical Society.
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Oliver D. Elbert et al. Counting Black Holes: The Cosmic Stellar Remnant Population and Implications for LIGO. MNRAS, published online August 2, 2017; doi: 10.1093/mnras/stx1959
