NS 1S 212 ASTRONOMY • APRIL 2020
When galaxies merge, so do their super-
massive black holes. In the interim,
catching two supermassive black holes
sharing close quarters isn’t unusual.
But now, according to a study pub-
lished online January 14 in Astronomy
& Astrophysics, an international team
of astronomers has discovered a single
galaxy that’s home to not two, but three
supermassive black holes.
NGC 6240 is about 300 million light-
years away. Its shape has been visibly
disturbed by the gravitational effects of
a merger between galaxies slamming
together at hundreds of miles per second.
Therefore, the researchers expected to
find two supermassive black holes hiding
out near the center of the cosmic collision.
Instead, when they peered into NGC
6240’s core with the 3D-mapping capa-
bility of the Multi Unit Spectroscopic
Explorer on the Very Large Telescope in
Chile, the team found three supermas-
sive black holes. Each weighs more than
90 million Suns. (For comparison, the
Milky Way’s supermassive black hole,
Sagittarius A*, weighs about 4 million solar
masses.) What’s more, NGC 6240’s three
behemoth black holes are all crammed
into a region less than 3,000 light-yearsacross, which is less than 1 percent of the
size of the galaxy in which they reside.
“Up until now, such a concentration
of three supermassive black holes had
never been discovered in the universe,”
study co-author Peter Weilbacher of the
Leibniz Institute for Astrophysics Potsdam
said in a press release. Although astrono-
mers have previously found instances of
three separate galaxies and their associ-
ated black holes on a collision course,
this is the first time they’ve witnessed
a trifecta of supermassive black holes
crammed into such a small space.
The finding is not only an exciting
discovery, it also shows how multiple
galaxies can come together simultane-
ously to build the universe’s biggest gal-
axies. That’s a process that has mystified
astronomers, who see galaxies today too
large to have been built up by slower,
two-galaxy mergers, despite the uni-
verse’s almost 14-billion-year age.
“If ... simultaneous merging processes
of several galaxies took place, then the
largest galaxies with their central super-
massive black holes were able to evolve
much faster,” Weilbacher said. “Our
observations provide the first indication
of this scenario.” — A.K.One galaxy, three
supermassive black holes
GIANT PLANET
FOUND AROUND
WHITE DWARF
For the first time, astronomers
have discovered evidence for
a giant planet orbiting a tiny,
dead white dwarf star. And
surprisingly, according to a paper
published December 4 in Nature,
the Neptune-sized planet is more
than four times the diameter of
the Earth-sized star it orbits.
“This star has a planet that
we can’t see directly,” study
author Boris Gänsicke from the
University of Warwick said in a
press release. “But because the
star is so hot, it is evaporating the
planet, and we detect the atmo-
sphere it is losing.” In fact, the
searing star is sending a stream
of vaporized material away from
the planet at a rate of more than
314 million tons (285 million
metric tons) per day.
The system, found around
a star dubbed WDJ0914+1914,
serves as the first evidence of a
gargantuan planet surviving a
star’s transition to a white dwarf.
It suggests that evaporating
planets around dead stars may be
somewhat common throughout
the universe. And because our
Sun, like most stars, will also
eventually evolve into a white
dwarf, the find could even shed
light on the fate of our own
solar system. — J.P.QUANTUM GRAVITY
TRIPLE PLAY.
Astronomers
already knew
about NGC 6240’s
active, northern
black hole (N).
Thanks to cutting-
edge 3D-mapping
techniques, they’ve
now identified two
more — S 1 and S 2.
The white bar at
the bottom right of
the inset image
represents a
distance of
1,000 light-years.
WEILBACHER (AIP), NASA, ESA,
THE HUBBLE HERITAGE (STSCI/
AURA)-ESA/HUBBLE
COLLABORATION, AND A. EVANS
(UNIVERSITY OF VIRGINIA,
CHARLOTTESVILLE/NRAO/
STONY BROOK UNIVERSITY)MARK^GA
RLI
CK