Gas whirls in between monster black holes in galaxy crash

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When two cars crash into each other, you end up with a lot of wreckage. The same is true for two
colliding galaxies. Over time, they may merge into one larger galaxy, but right after the crash,
everything looks very chaotic.
That’s the case with the galaxy NGC 6240, which has now been studied in detail by ALMA. Earlier
telescopes could easily see the weird overall shape of the galaxy merger. But thanks to the sharp
vision of ALMA, astronomers have now also observed what’s going on in the galaxy’s core.
Apparently, each of the colliding galaxies had a central supermassive black hole. The two black holes
are now orbiting each other in the very center of NGC 6240. Swirling around these gluttonous
monsters is a huge amount of cold molecular gas.
Using ALMA, a team of astronomers have now studied the distribution of this gas in much detail. It
turns out that most of the gas is actually in between the two supermassive black holes. Moreover,
the gas is very turbulent and chaotic. Part of it will likely fall into one of the black holes in the not-
too-distant future; another part of the gas will be ejected out of the galaxy’s center altogether.
The ALMA observations will help astronomers understand the interaction between massive black
holes and star formation in a galaxy’s core. After all, the cold molecular gas studied by ALMA not only
feeds the black holes; the same gas is also used in the formation of new stars.
ALMA also turned up a new mystery about NGC 6240. Earlier observations with the European Very
Large Telescope (VLT) appeared to show evidence for a third supermassive black hole in the galaxy’s
core. However, ALMA couldn’t find any evidence for this. The object that looked like a third black
hole in the VLT observations may actually be a dense star cluster instead, the team suggests.

What?

NGC 6240 is a galaxy at a distance of some 400 million light-years in the constellation Ophiuchus, the
Serpent Bearer. For a long time, it has been known that the galaxy has a weird, chaotic shape and a
double nucleus. This suggests that NGC 6240 is the result of a collision between two smaller galaxies.
NASA’s Chandra X-ray Observatory has revealed that each nucleus harbors a supermassive black
hole. ALMA has now mapped the distribution and velocities of cold molecular gas in the galaxy’s
core, revealing ten times more detail than earlier observations.

Who?

The ALMA observations of NGC 6240 were carried out by an international team of astronomers, led
by Ezequiel Treister of the Pontificia Universidad Católica de Chile in Santiago and Anne Medling of
the University of Toledo in Ohio. Ezequiel and Anne worked together with 20 other astronomers
from Chile, the United States, Italy, Norway, and Switzerland. The results of their research have been
published in two separate articles in The Astrophysical Journal and The Astrophysical Journal Letters.
They also presented their work at the 235 th meeting of the American Astronomical Society in
Honolulu, Hawaii.

ALMA URL