It was a dumbfounding discovery. Using Hubble telescope data, a scientist spotted a "lensing" galaxy that was enormous, extremely distant and acting as a magnifying glass to another ancient galaxy behind it.
She was seeing hot hydrogen gas -- a marker of star birth -- in an old, sedate galaxy. And fledgling stars were much farther away than expected. "I was like, oh my God, we screwed up," said Tran, an associated professor at the university. Her confusion centered around one blurred object.
Hubble imaging helped Tran and her team sort out that the one object was actually two: a monstrous elliptical lensing galaxy -- the most distant one the space agency has ever seen -- and, behind it, a tiny spiral galaxy rapidly churning out new stars. Light from the elliptical galaxy had taken 9.6 billion years to arrive here; for the spiral galaxy, it was 10.7 billion years.
So what does a gravitational lensing galaxy do?
It magnifies the background light behind it, bending and distorting light but also making it more visible. An observer is able to see rays of light he or she otherwise would have missed. Since the first cosmic gravitational lens was discovered in 1979, hundreds more have been found -- but never this far away.
Lensing, Tran explained, "increases the amount of light you get from an object; something a little bit faint, it makes brighter. It also makes the object appear bigger so that you can see more detail. It's like if you were looking at an ant and you blew up the ant to the size of a cat."
The lensing galaxy is uncommon in several ways. It's huge for its epoch, weighing 180 billion times more than our sun. But it's still smaller than the Milky Way.
"Even though this object is smaller than the Milky Way now," Tran said, "you're seeing it when the universe was much younger. If you could see the Milky Way at the same time, it would be much, much smaller than it is today."
Also rare is the amount of dark matter seen in the ancient elliptical galaxy.
"Galaxies nowadays have a lot of dark matter," the scientist said, "10 times more dark matter than there are stars. ... So the question is, was that always the case, did we always have this ratio of dark matter to light matter? With this measurement it seems that ratio is different -- less dark matter compared to number of stars we see looking back over 9 billion years."
But the most unusual thing about this lensing galaxy and the one behind it is that Tran spotted it at all.
"It's very unusual to get this time of alignment," Tran said. "We weren't expecting it at all. Space is basically empty so the fact that we found something like this in this very small region of space is very surprising."