July 11, 2014
Hubble Finds Star Bridge Linking Ancient Galaxies
John P. Millis, Ph.D. for redOrbit.com - Your Universe Online
As astronomers have sought to understand the formation and evolution of galaxies, a handful of simple truths have emerged. Despite many questions remaining, we know that galaxies form in clusters, and create interlinked chains and structures that create a spider web across the cosmos.
[ Watch the Video: Hubblecast 76: Merging Galaxies And Droplets Of Starbirth ]
Because they often move through the Universe in such close proximity, collisions and mergers play an important role in their evolution. And sometimes astronomers catch two galaxies in their gravitational tango, occasionally with spectacular and unusual results.
Now astronomers have observed a phenomenon called 'beads on a string' in an unexpected place. Sometimes when spiral galaxies merge, the gases they host collide, creating a burst of star formation.
[ Watch the Video: Water Droplet Animation And The Link To Stellar Superclusters ]
However, other galaxy types, such as elliptical galaxies, can be gas poor, so 'beads on a string' are rare in collisions of these galaxies. So, a recent Hubble image of a pair of merging elliptical galaxies, embedded deep inside the dense galaxy cluster known as SDSS J1531+3414, has astronomers excited.
"We were surprised to find this stunning morphology. 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," said Grant Tremblay of the European Southern Observatory in Garching, Germany in a recent NASA statement.
The image clearly shows a string of young, hot, blue star clusters spaced at about 3,000 light-years along the chain. Researchers are now hoping that this study will provide new insights into the formation of star clusters within merging galaxies.
Also of interest, the image is clearly warped by the intense gravity provided by the merging galaxies and their dark matter halos. Initially, scientists believed that the string of blue star clusters might have been an artifact of this warping – a process called gravitational lensing. However, that hypothesis has since been ruled out by follow-up observations.
One thing is now clear: the elliptical galaxies contained sufficient gas reserves to initiate star formation along the observed chain. They clump together due to a phenomenon known as the Jeans instability. Essentially, the gas pressure is not sufficient to keep the gas fluid. Instead, it begins to clump together, leading to gravitational collapse and eventually star formation. A similar effect occurs when rain clouds condense and rain begins to fall in individual drops instead of a fluid stream.
[ Watch the Video: Zooming In On Merging Galaxies And A String Of Star Formation In SDSS J1531+3414 ]
However, questions still remain. Specifically, astronomers want to understand how this specific chain formed, and how this event could be extrapolated to explain cluster formations in other galaxy collisions. While it is possible that the fueling gas was native to the galaxies before the collision, it is also possible that the gas was generated during a cooling process from the hot plasma surrounding the galaxies. Finally, the seed gas may have come from the intergalactic medium itself – the space between the galaxies – and was compressed by the shockwave created when the massive galaxies collided.
"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.
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