Using Albert Einstein’s cosmic magnifying glass, UCSB researchers have spied one of the smallest known galaxies in the universe.
Assistant physics professor Tommaso Treu and postdoctoral physics candidate Phil Marshall made the find with their team of 13 international scientists last March. Researchers discovered the object using a phenomenon Einstein first predicted in 1912. His theory states that massive objects bend space-time, thereby brightening light emitting from objects behind them like a magnifying glass. According to Marshall, the galaxy remains nameless because of the vast amount of such celestial formations, which number in the billions.
The results of the study will appear in the December issue of the Astrophysical Journal, though the article is currently available online.
Halfway Across the Universe, Half the Galaxy Size
According to Marshall, galaxies are measured in kiloparsecs, a unit equivalent to about 3,260 light years. A typical galaxy is between 10 and 50 kiloparsecs. However, the newly identified tiny galaxy is about half that size, he said.
“It’s about a factor of 10 smaller than the typical galaxies we can measure at that distance,” Marshall said. “The galaxy is halfway across the universe.”
This distance has important implications for researchers interested in the age of the galaxy. Due to the vast distance the light has to travel before reaching earth, Marshall’s team now has an idea of how old the galaxy is at a minimum.
“[Halfway across the universe] means it’s taken half the age of the universe to reach us,” Marshall said. “It takes several billion years for light to get to us.”
Therefore, on Earth, the galaxy appears the way it was six billion years ago, Marshall said. In other words, assuming nothing unusual has happened to this galaxy, it is now six billion years old and looks like a dwarf elliptical galaxy. It would look yellow and contain only older stars. Younger galaxies would appear bluer in color, he said.
“If you see a bright blue galaxy and it’s far away, you know it must be quite young,” Marshall said.
Celestial Magnifying Glasses
Marshall said scientists can see these tiny galaxies in very specific places. They must first look for bright yellow galaxies that can create the gravitational lens needed for the observation.
According to Marshall, such a lens works because the massive gravitational displacement of a galaxy causes light that travels through the field to experience a slight pull and deflect. It then bends twice around the larger galaxy and continues through space into view of the telescope, he said. The two light rays come from different angles and scientists see two blue images.
“It’s the same effect as if you were to hold a lit candle behind the bottom of a wine glass and observe the ring of light it creates,” Marshall said. “The bottom of the wine glass is similar to the gravitational lens because it is dense in the center but not in the outskirts.”
Treu said the result works like a magnifying glass by focusing on the bent light rays, thus amplifying the size and brightness of the galaxy in the background.
A Complicated Image
Treu and Marshall began collecting data in December 2006 alongside Jason Melbourne, a member from the Center for Adaptive Optics at UC Santa Cruz, at the W.M. Keck Observatory in Hawaii.
The scientists used advanced tricks to enhance the images they collected. The Earth’s atmosphere is filled with patches of dense air blowing through it. This phenomenon, which causes stars to twinkle at night, also means that images taken with long exposure times are of low quality.
“Normally, ground-based images are not nearly as high-resolution because the atmosphere blurs them,” Marshall said.
The laser guide star adaptive optics system conquers this interference through two techniques. The Keck image uses adaptive optics to beat atmospheric blurring. A mirror in the telescope quickly changes shape to compensate for the air pockets by capturing images at the precise moment when the atmosphere is not in the way, Marshall said. This creates a sharp image that has a resolution limited only by the size of the telescope, he said.
In order to make the adaptive optics work, however, a bright star must appear in the field to use as a focus point for the corrections to the mirror, Marshall said. The rarity of such bright objects would normally curtail use of the system, but he said a laser beam was substituted.
“It turns out that bright stars are relatively rare,” he said. “With the laser guide, you make your own star. You take the yellow laser and fire it up in the atmosphere where it makes an illumination spot.”
The clear image from the Keck was then combined with two others from the Hubble orbital telescope, which bypasses the atmosphere altogether. By combining the power of the gravitational lens with these techniques, scientists gained an unprecedented view into the depths of the universe that was not otherwise possible.