Merging black holes are cosmic train wreck
Pastor Dale Morgan
Merging black holes are cosmic train wreck
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By Ker Than, SPACE.com
Scientists have pinpointed the precise locations of a pair of
supermassive black holes at the centers of two colliding galaxies 300
million light-years away.
Infrared images made by the Keck II telescope in Hawaii reveal the two
black holes at the center of the galaxy merger known as NGC 6240 are
each surrounded by a rotating disk of stars and cloudy stellar nurseries.
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The new images are detailed in Science Express, an online publication of
the journal Science.
Cosmic train wreck
Scientists had previously observed NGC 6240 in a piecemeal fashion,
using different wavelengths of light. Images captured by the Hubble
Space Telescope showed the outer parts of the colliding galaxies in
visible light, and revealed long tidal tails made of orphaned stars,
gas, and dust.
Subsequent X-ray observations by NASA's Chandra X-ray Observatory
revealed the presence of two supermassive black holes at the center of
each galaxy, and the Very Long Baseline Array spotted two radio sources
in the galaxies' central regions. The problem was combining all of the
images to form a single coherent picture.
"You have all these different telescopes and they look at the same piece
of sky but see wildly different things," said study team member Willem
de Vries of the University of California, Davis.
Keck II uses a relatively new type of imaging called adaptive optics,
which uses an artificial laser-generated "guide star" as a reference and
a rapidly deforming mirror to correct image distortions from the Earth's
atmosphere in real time.
The Keck II images were of such high spatial resolution that scientists
could identify features in NGC 6240 captured by the other telescopes in
the optical, X-ray and radio wavelengths.
Like a Rosetta Stone, the Keck II images allowed scientists for the
first time to calibrate the different images of NGC 6420.
"Now we can really see it all — the hot dust in the infrared, the stars
in the visible and infrared, and the X-rays and radio emissions from
right around the black holes," said study leader Claire Max at the
University of California, Santa Cruz.
Co-evolution
Scientists think galactic mergers are one of the primary ways galaxies
form. Like heated wax in a lava lamp, two small galaxies can come
together to form one larger one, or a blob of gas and stars might pinch
off during a particularly messy galactic smash-up and, over cosmic time,
the result evolves into a diminutive dwarf galaxy.
Our own Milky Way galaxy is expected to collide and merge with its
neighbor Andromeda in a few billion years to form a large elliptical
galaxy some scientists jokingly call "Milkomeda" or "Andromeda Way."
When that occurs, a black hole merger similar to that of NGC 6420 could
occur.
"We have a wimpier black hole than the two galaxies [in NGC 6420], but
we have a black hole and we believe Andromeda does too," Max told SPACE.com.
When galaxies merge, the behemoth black holes at their centers merge and
grow as well. Recent studies have shown that supermassive black holes
help determine many properties of their host galaxies. Scientists say
this idea, sometimes called "co-evolution," is among the most surprising
realizations to emerge from astronomy in recent years.
"The gravitational influence of the black hole is actually limited to a
relatively small region right around it, so how can it affect the rest
of the galaxy?" Max said. "But if the black hole and the galaxy around
it evolved together through the same sequence of merger events, that
would explain the correlations."
Scientists estimate the two black holes in NGC 6240 will spiral into
each other and merge in 10 million to 100 million years.