There are a lot more black holes in the Milky Way than
previously thought, according to a new UCI study by (from left)
James Bullock, chair and professor of physics & astronomy;
Manoj Kaplinghat, professor of physics & astronomy; and
Oliver Elbert, physics & and astronomy graduate student.
Credit: Steven Zylius / UCI
After conducting a cosmic inventory of sorts to calculate and
categorize stellar-remnant black holes, astronomers from the
University of California, Irvine have concluded that there
are probably tens of millions of the enigmatic, dark objects
in the Milky Way – far more than expected.
“We think we’ve shown that there are as many as 100 million
black holes in our galaxy,” said UCI chair and professor of
physics & astronomy James Bullock, co-author of a research
paper on the subject in the current issue of Monthly Notices
of the Royal Astronomical Society.
UCI’s celestial census began more than a year and a half ago,
shortly after the news that the Laser Interferometer
Gravitational-Wave Observatory, or LIGO, had detected ripples
in the space-time continuum created by the distant collision of
two black holes, each the size of 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,” Bullock said. “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?'”
He said that scientists assume most stellar-remnant black holes
– which result from the collapse of massive stars at the end of their lives – will be about
the same mass as our sun. To see evidence of two black holes of
such epic proportions finally coming together in a cataclysmic
collision had some astronomers scratching their heads.
UCI’s work was a theoretical investigation into the “weirdness
of the LIGO discovery,” Bullock said. The research, led by
doctoral candidate Oliver Elbert, 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,” Elbert said.
“Big galaxies are home to older stars, and they host older
black holes too.”
According to co-author Manoj Kaplinghat, UCI professor of
physics & astronomy, 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,” 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
In addition, to shed light on subsequent phenomena, the UCI
researchers sought to determine how often black holes occur in
pairs, how often they merge, and how long it takes. They
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 percent of the black holes formed
have to merge to explain what LIGO saw,” Kaplinghat said. “Of
course, the black holes have to get close enough to merge in a
reasonable time, which is an open problem.”
Elbert said he expects many more gravitation wave detections so
that he and other astronomers can determine if black holes
collide mostly in giant galaxies. That, he said, would tell
them something important about the physics that drive them to
According to Kaplinghat, 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,” he
Spiral arms allow school children to weigh black holes
More information: Oliver D. Elbert et al. Counting Black
Holes: The Cosmic Stellar Remnant Population and Implications
for LIGO, Monthly Notices of the Royal Astronomical
Society (2017). DOI:
Monthly Notices of the Royal Astronomical Society
University of California, Irvine