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Gladders and M. Florian (University of Chicago), S. Baum, C. O'Dea, K.
Cooke (Rochester Institute of Technology), M. Bayliss
(Harvard-Smithsonian Center for Astrophysics), H. Dahle (University of
Oslo), T. Davis (European Southern Observatory), J. Rigby (Goddard Space
Flight Center), K. Sharon (University of Michigan), E. Soto (The
Catholic University of America), and E. Wuyts (Max Planck Institute for
Extraterrestrial Physics). Credit:NASA,ESA, and G. Tremblay (European Southern Observatory).Release Images
Astronomers routinely use the crisp view of NASA's Hubble Space
Telescope to study all kinds of intricate details in galaxy clusters.
By now you would think they've seen it all — but nature always has new
surprises in store.
The latest is an uncanny 100,000-light-year-long structure that looks
like a string of pearls twisted into a corkscrew shape that winds
around the cores of two colliding galaxies. Astronomers don't quite
know how to explain the origin and ultimate fate of the object, but the
answer must be extraordinary, say scientists.
The Slinky-structure's unique morphology may yield new insights into
the formation of stellar superclusters, the merger-driven growth of
galaxies, and gas dynamics in the rarely seen merger process of two
giant elliptical galaxies.
"We were surprised to find this stunning morphology, which must be
very short-lived" (perhaps about 10 million years, which is a fraction
of the time it takes for galaxies to merge), said Grant Tremblay of the
European Southern Observatory in Garching, Germany. "We've long known
that the 'beads on a string' phenomenon is seen in the arms of spiral
galaxies and in tidal bridges between interacting galaxies. However,
this particular supercluster arrangement has never been seen before in
giant merging elliptical galaxies," he said. "We have two monsters
playing tug-of-war with a necklace, and its ultimate fate is an
interesting question in the context of the formation of stellar
superclusters and the merger-driven growth of a galaxy's stellar
Like a string of pearls, these young, blue "super star clusters" are
evenly spaced along the chain at separations of 3,000 light-years from
one another. The pair of elliptical galaxies is embedded deep inside
the dense galaxy cluster SDSS J1531+3414. The cluster's powerful
gravity warps the image of background galaxies into blue streaks and
arcs that give the illusion of being inside the cluster. The
astronomers' first hypothesis was that the "string of pearls" was
actually a lensed image from one of these background galaxies, but
their recent follow-up observations with the Nordic Optical Telescope
definitively rules this out.
The galaxy cluster is part of a Hubble program to look at 23 clusters
that are so massive they create powerful gravitational lensing effects
on the sky. The clusters were first cataloged in the Sloan Digital Sky
Survey. Tremblay's team serendipitously discovered the highly bizarre
string of stellar superclusters while reviewing the images as they came
in from Hubble. "We were stunned by what we saw in SDSS J1531+3414,"
Tremblay said. "The uniqueness of this source spurred follow-up
observations with both ground- and space-based telescopes."
"This is a beautiful demonstration of the profound scale-invariance
of the fundamental laws of nature," Tremblay added. The underlying
physical processes that give rise to the "beads on a string" morphology
are related to the Jeans instability, describing the behavior of
self-gravitating clumps of gas. It's analogous to the process that
causes a falling column of water to disrupt, explaining why rain falls
in drops rather than in continuous filaments from clouds. Water coming
out of the kitchen tap eventually breaks into a series of droplets, and
a very similar process is happening in SDSS J1531+3414. "We see the
same physics on 100,000-light-year scales that we see in our kitchen
sinks and inkjet printers," said Tremblay.
Scientists are currently working on a better understanding of the
origin of the chain. One possibility is that the cold molecular gas
fueling the burst of star formation may have been native to the two
merging galaxies. Another possibility is a so-called "cooling flow"
scenario, where gas cools from the ultra-hot (10 million degree)
atmosphere of plasma that surrounds the galaxies, forming pools of cold
molecular gas that starts to form stars. The third possibility is that
the cold gas fueling the chain of star formation originates from a
high-temperature shock wave created when the two giant elliptical
galaxies crash together. This collision compresses the gas and creates a
sheet of dense, cooling plasma.
"Whatever the origin for this star-forming gas is, the result is
awesome. It's very exciting. You can't find a mundane explanation for
this," Tremblay said.
Space Telescope Science Institute, Baltimore, Md.