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Scientists using the Atacama Large Millimeter/submillimeter
Array (ALMA) to look deep into the heart of a pair of merging galaxies known as
UGC 4211 discovered two black holes growing side by side, just 750 light-years
apart. This artist’s conception shows the late-stage galaxy merger and its two
central black holes. The binary black holes are the closest together ever
observed in multiple wavelengths. ALMA ESO/NAOJ/NRAO; M. Weiss NRAO/AUI/NSF (CC
BY 3.0) |
Our galaxy, the Milky Way, is on a collision course. Some four billion years from now, the Milky Way and its large neighboring galaxy, Andromeda, will begin a spiraling gravitational dance, merging over hundreds of millions of years to form one larger object.
Such galaxy mergers are happening all the time, all over the
universe, and they are important building blocks for larger-scale cosmic
structure. Astrophysicists have many questions about this extraordinary
process, but one mystery has proved especially captivating: When large galaxies
merge, what happens to the supermassive black holes that decades of
observations have revealed to be lurking at their centers?
Logically, these giant black holes—each millions to billions
of times heavier than our sun—must collide and merge, too. Such mergers can
channel huge volumes of material into the black holes, sparking violent
astrophysical outbursts that shape star formation and other processes in their
host galaxies. But astronomers have so far only seen snapshots of this long
process, from when the black holes are still tens to hundreds of light-years
apart. The closer the merging black holes get, the harder they become to
distinguish from each other, blurring the picture for theorists seeking to
understand how this process works.
Now an international team of scientists has announced the
discovery of two active supermassive black holes close to Earth in a new study
in the Astrophysical Journal Letters. At an estimated 125 million and 200
million times the mass of the sun, respectively, these black holes sit about
500 million light-years away from us, gobbling up gas and dust at the center of
UGC 4211, a galaxy that is still reeling from a merger.
“This pair was really exciting because they’re so close to
each other and they’re so nearby,” says Chiara Mingarelli, one of the study’s
authors and an astrophysicist at the Flatiron Institute in New York City and
the University of Connecticut. Separated by only some 750 light-years, or 230
parsecs, they are the closest pair of black holes that scientists have been
able to confirm by measuring multiple wavelengths of light. These black holes’
proximity to Earth and each other may provide a unique opportunity for
fundamental studies of giant black hole mergers, as well as one of their most
elusive by-products: ripples in spacetime called gravitational waves.
The project began nearly 10 years ago, when astrophysicist
Michael Koss started using some of the world’s largest telescopes to search the
sky for “active” pairs of supermassive black holes—that is, those that are
feeding and burping out blasts of intense radiation. After examining nearly 100
objects, he found six that were actually hidden merging pairs. Of these, UGC
4211’s stood out as being much closer together than the rest.
“You can see it. I mean, literally, you can see on the
[images] there are two sources,” says Koss, who works at Eureka Scientific, an
astrophysics research institute in California. The high resolution was
possible, in part, because of UGC 4211’s proximity to Earth.
This paper “has really pushed the limit” of what is possible
to observe, says Zoltan Haiman, an astronomer at Columbia University, who was
not involved in the study. Astronomers have observed one pair of binary black
holes in even closer proximity to each other before but only with radio
telescopes. In the new study, the team confirmed its findings using
multiwavelength data from several different telescopes—an important step
because the field has had false positives in the past, Mingarelli says. “You can
only trick the eye in so many wavelengths,” she says. The team studied the
system using the W. M. Keck Observatory in Hawaii, the Very Large Telescope and
the Atacama Large Millimeter/submillimeter Array in Chile, and the Hubble Space
Telescope in orbit around Earth.
UGC 4211’s two black holes are thought to be midway through
their merger. As their extended cosmic duet progresses, the pair will draw even
closer together as surrounding swarms of stars and gas siphon away their
orbital momentum. The authors predict that the dance will end in approximately
200 million years, when the two supermassive black holes at last fully merge to
become one.
“This seems to be right smack in line with a lot of our
paradigms, so that’s a good thing. It’s not breaking astrophysics,” says Jeremy
Schnittman, an astrophysicist at NASA’s Goddard Space Flight Center, who was
not involved in the recent study.
At least, that’s the case for now. Much remains unknown
about the mechanics of merging monstrous black holes, especially during later
stages, when the black holes approach each other so closely that they cannot be
clearly distinguished. “What happens once they’re even closer than this, down
at parsecs or less, is still a really big question,” says Sarah Burke-Spolaor,
an astronomer at West Virginia University, who was not involved in the new
results.
Like many of her peers, Burke-Spolaor is especially
fascinated by each merger’s final phase, when the black holes spiral together
so violently that they shake the fabric of spacetime itself, producing copious
gravitational waves. After first detecting such emissions in 2015—a discovery
that netted a Nobel Prize in Physics—astronomers now routinely study these last
gasps from merging black holes using specialized observatories that are as
different from light-gathering telescopes as ears are from eyes. Most of those
studies, however, concern pairs of black holes that are far smaller than their
supermassive kin. Tuning in to the immense gravitational waves from the
universe’s largest merging black holes requires a new generation of even more
advanced observatories that scientists and engineers have scarcely begun to
build.
The payoff should be worthwhile: Such huge mergers are
thought to be the most common contributor to the “gravitational wave
background,” the as-yet-undetected totality of spacetime ripples from sources
scattered across the entire observable universe, imprinted across the entire
sky. Detecting and mapping this background, Mingarelli says, would yield “the
cosmic merger history of supermassive black holes” and a wealth of other
cosmologically vital information. And studying the fine details of merging systems
like UGC 4211 can help researchers better understand what they’re seeing if or
when they finally manage to glimpse the universe’s gravitational wave
background.
Koss hopes that future studies of UGC 4211 will be
especially fruitful, given its proximity to Earth. This system is easier to
observe from our place in the universe, “just like you can see the leaves on
the nearby trees in the forest, not distant ones,” he says.
The discovery also suggests that there may be more merging
black holes to observe than we previously thought because Koss found it from a
sample of fewer than 100 active black holes. “There’s always a chance that the
study just got lucky,” Burke-Spolaor says. But even if it was a fluke,
scientists will continue searching for more examples to fill in the final steps
of this cosmic choreography.