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Scientists Turn Into Space Archaeologists During Galaxy Studies

How a team of scientists, led by UCSB researchers, found and measured an infant galaxy 6 billion light years away from Earth, and why.

An international team of scientists, led by two UCSB researchers, have identified a tiny galaxy, estimated to be 100 times lighter in mass than our own Milky Way. The scientists believe that if this galaxy is just one of a larger population of similar galaxies in the area, then what they found could be a young galaxy cluster in the making.

But if it weren’t for a much larger galaxy positioned in front of this much more distant one, the team may not have been able to see it. Using data collected by NASA’s Hubble Space Telescope and the W.M. Keck Observatory in Hawaii, the scientists, led by postdoctoral fellow Phil Marshall and Tommaso Treu, assistant professor of physics, were able to see the galaxy in the form of an Einstein ring, or a ring of light around a galaxy.{mosimage}

The galaxy in the foreground, said Marshall, acts as a gravitational lens, magnifying, focusing and bending the light of the smaller galaxy in the background, which is more than 6 billion light years away, or roughly about halfway across the observable universe.

“We already knew it was there,” said Marshall, referring to research done by Treu and his colleagues in the Sloan Lens ACS Survey, which studies Einstein ring gravitational lenses. What they had to do, said Marshall, was trace it back through the lens, account for the optical lensing, and try to reconstruct the galaxy without the gravitational lens in front of it. 

What they came up with, by measuring its brightest stars, was a galaxy approximately one-tenth the mass of the smallest distant galaxies typically observed.

Using the optic and near infrared images from the Hubble and the Keck telescopes, the researchers also inferred that many of the stars in this distant galaxy have only recently formed.

“If the galaxy is representative of a larger population, it could be one of the building blocks of today’s spiral galaxies, or perhaps a progenitor of modern dwarf galaxies,” said Treu, who is the second author of the paper, which will be published in the Dec. 20 issue of the Astrophysical Journal.

What this infant galaxy will become, however, will be a mystery. The images the scientists are seeing now reflects how the galaxy looked 6 billion light years ago. It has taken the light from that galaxy that long to reach us, and it would take billions more years to see its evolution.

“If we could freeze the cosmic expansion, stop the clock and walk 6 billion light years to this galaxy, it wouldn’t look small and blue. It would probably look small and red. And it would have the right size to be a dwarf elliptical galaxy, or possibly the bulge of a small spiral galaxy,” Marshall said. 

The researchers are interested in seeing galaxies at different times in the universe’s history, by looking at galaxies of various sizes and at various distances from the Earth.

 “What we’re really doing is a bit like archaeology . . . we can try to put together stories of how an object in the distant past might turn into an object that we see today.” 

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