Astronomers using the Hubble Space Telescope have spotted an elliptical galaxy 9.6-billion light years from Earth that is so colossal, its gravity bends and magnifies the light from a galaxy behind it. The rare alignment is the most distant example of a lensing galaxy ever discovered, and could yield insights into the formation of the early universe.
Image Credits: NASA, ESA, K.-V. Tran (Texas A&M University), and K. Wong (Academia Sinica Institute of Astronomy & Astrophysics).
Researchers led by Texas A&M astronomer Kim-Vy Tran were studying star formation in a distant galaxy cluster named IRC 0218 when they happened upon an unusual sight. An old galaxy, whose star-making days were long since over, appeared to be teeming with young stars. "I thought we had made a major mistake with our observations," said Tran, but further analysis revealed that they had in fact stumbled upon a lensing galaxy – a star cluster so massive, it bends, amplifies and magnifies the light from objects behind it.
"When you look more than nine billion years ago in the early universe, you don't expect to find this type of galaxy-galaxy lensing at all," said Tran, in a statement. She continues:
It's very difficult to see an alignment between two galaxies in the early universe. Imagine holding a magnifying glass close to you and then moving it much farther away. When you look through a magnifying glass held at arm's length, the chances that you will see an enlarged object are high. But if you move the magnifying glass across the room, your chances of seeing the magnifying glass nearly perfectly aligned with another object beyond it diminishes.
According to lead author Kenneth Wong of Academia Sinica Institute of Astronomy & Astrophysics (ASIAA), the lens provides astronomers a unique opportunity to study the dark-matter content of galaxies from the Universe's infancy. "By comparing our analysis of this lens galaxy to the more nearby lenses, we can start to understand how that dark-matter content has evolved over time."
The image at the top of this post are two insets, which depict the galactic pair as seen by Hubble, set against the larger cosmic context in which the lens was discovered. In the foreground of the inset on the left is the giant, lensing elliptical galaxy, seen as it appeared 9.6 billion years ago. Its red light indicates the predominance of ancient stars. The spots of blue in the inset image on the left is the light of the background galaxy, a small, spiral system located about 10.7-billion light years from Earth:
In the absence of the gravitational lensing provided by the foreground galaxy, the background galaxy might be too faint to observe. In the inset on the right, the foreground cluster has been removed, to show only the distant spiral galaxy. Here it is up close:
Pixel-tastic! According to NASA, the blue hues of the more distant galaxy is due to the glow of young stars, and the white glob of pixels at the upper right is likely a region of rapid star formation.
By measuring the intensity of its lensing effect on the light from the galaxy behind it, Wong and his team were able to determine that the foreground galaxy weighs roughly 180-billion times as much as our Sun. That's not as big as today's galaxies (our own Milky Way is 3-4 times as massive), but it's huge for so far-flung a galaxy. What's more, most of the Milky Way's mass is made up of dark matter. The lensing galaxy? Not so much.
"The unusually small amount of dark matter in this massive, elliptical, lensing galaxy is very surprising," said study coauthor Sherry Suyu. "Other elliptical galaxies that are closer to us have much more dark matter and have inventories of stars that appear to be different from this super-distant lensing galaxy."
The researchers' findings are published in the July 10th issue of The Astrophysical Journal Letters. Read more about this surprising discovery over at HubbleSite.