What a Shock! Galaxies Caught Colliding
Ker Than
Staff Writer
SPACE.com
Tue Mar 7, 7:00 AM ET
A galaxy in Stephan's Quintet is rushing headlong into a cluster of
neighboring galaxies at more than one million mph and generating
one of the largest shock waves scientists have ever seen.
The finding, made using NASA's Spitzer Space Telescope, gives
astronomers a chance to see a galactic collision in action and
could help explain why some galaxies are more luminous in the
infrared than others.
A cosmic pileup
Stephen's Quintet is a cluster of five galaxies located about 300
million light-years away in the constellation Pegasus. For decades,
astronomers using optical telescopes have known that galaxies in
the cluster had experienced encounters in the past and that some
were colliding even now.
But it wasn't until they used Spitzer's Infrared Spectrograph on
the galaxy cluster that they could make out the details of what was
happening.
They discovered that one of the galaxies, called NGC7318b, is
currently falling toward three of the others at a very high speed
and generating a giant shock wave, or "bow shock," in front of it.
A bow shock is akin to the ripple raised by a boat's bow as it
moves through the water.
NGC7318b is estimated to be moving at more than 540 miles per
second (873 kms) and generating a colossal shockwave larger than
our own Milky Way Galaxy.
Faster than sound
Shock waves are generated when an object moves faster than the
speed of sound through some kind of medium. On Earth, the medium is
usually air or water; in space, it's interstellar gas.
One familiar type of shock wave is the "sonic boom" created by
supersonic jets as they fly by overhead. As the jet moves through
the air, it creates pressure waves in front and behind it. These
waves travel at the speed of sound, called Mach 1. As the jet
approaches this speed, it compresses the waves together into a
cone-shaped "shock" which travels outward toward the ground,
producing an audible boom.
In Stephan's Quintet, the shock wave generated by the infall of
NGC7318b is estimated to be moving at speeds greater than Mach
100.
Time machine
The researchers discovered the shockwave after detecting infrared
radiation emitted from molecular hydrogen in the region that had
been disturbed. Unlike atomic hydrogen, molecular hydrogen emits
infrared energy when jostled.
The finding could lead to a better understanding of so-called
Ultra-luminous Infrared Galaxies, which typically have infrared
luminosities 100 to 1,000 times greater than the Milky Way.
"We know that these galaxies are involved in vast mergers and
collisions," said study leader Philip Appleton of the California
Institute of Technology. "It's possible that some of the emission
we see from them is created not by stars, but by vast shocks in the
gas between colliding galaxies."
The finding also gives scientists a glimpse into what the universe
was like in the first few billion years after the Big Bang. It's
thought that many galaxies, including our own, grew as a result of
galactic mergers and that these events were more common during the
early years of the universe.
"In this respect these observations are a bit like stepping into a
time machine," said study team member Cristina Popescu from the Max
Planck Institute for Nuclear Physics in Germany.
In addition to acting like a window into the far distant past, the
collision in Stephen's Quintet could give scientists a preview of
our own galaxy's fate in the far distant future.
Scientists predict that in about 2 or 3 billion years, the Milky
Way will collide with the slightly larger Andromeda Galaxy. It's
expected that both galaxies will lose their spiral shape and merge
to become a single elliptical galaxy.
The finding is detailed in the March 10 issue of Astrophysical
Journal Letters.
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