A. BOLTON (UH/IFA) FOR SLACS AND NASA/ESA
An international team of astronomers, including Adam S. Bolton of the University of Hawaii's Institute for Astronomy, has recently announced a finding that helps to settle a long-standing debate over the relationship between mass (the amount of matter) and luminosity (brightness) in galaxies. A Hubble Space Telescope image shows the Einstein ring of one of the SLACS gravitational lenses, with the lensed background galaxy enhanced.
‘Einstein rings’ show gravity
A UH scientist uses the images to learn that galaxy size affects the amount of dark matter
A University of Hawaii astronomer and an international team have discovered a phenomenon they say could help settle a controversy over the relationship between the amount of matter and brightness in galaxies.
"It's something that might be made up in a science fiction movie, but it's in the sky every night," said Adam S. Bolton, lead author of two papers on the finding that will be published in the Astrophysical Journal in August and September.
The astronomers compiled images of 70 "gravitational lens" galaxies, the largest single collection, identified by the Sloan Digital Sky Survey with images from the Hubble Space Telescope.
A gravitational lens is similar to a terrestrial mirage, but it creates multiple images, Bolton said in an interview. When two galaxies happen to be aligned one behind another, the image of the more distant galaxy is distorted by the gravity of the nearer galaxy into multiple arc-shaped images, or what is known as an "Einstein ring."
"It's probably the most dramatic visual demonstration of the force of gravity in the universe," Bolton said. "It can actually take one image and make two or four images out of it."
Albert Einstein predicted the existence of gravitational lenses in the early 1900s with his theory of general relativity, but the first one was not discovered until 1979.
Einstein ring images can be up to 30 times brighter than the distant galaxy would be without the lensing effect, Bolton said. "They have an almost artistic beauty to them."
What is significant is the number of gravitational lenses found by his six-member team and the scientific use they have made of them, he said.
They used the Hubble images to measure the apparent sizes of the Einstein rings, and they measured the distances to the two galaxies of the aligned pair. Combining measurements, they were able to estimate the mass of the nearer galaxy, he said.
Considering the galaxy masses and the measurements of their sizes and luminosity and other data, the astronomers were able to infer the presence of dark matter in addition to visible stars in the galaxies.
The astronomers found that the fraction of dark matter relative to stars increases as the mass of galaxies increases, Bolton said.
Scientists think unseen dark matter comprises most of the total mass of the universe. They have been trying to find it to explain the gravitational motions of galaxies and test theories about Earth's formation and ultimate fate.
It has been difficult to measure the masses through traditional observations, and there have been different explanations for the effects seen, Bolton said. "We can't put a galaxy on a scale, and there are competing explanations for those relationships."
The Sloan Digital Sky Survey used a 2.5-meter telescope in New Mexico to measure distances to nearly 1 million distant galaxies and quasars in one quarter of the sky. The astronomers used the Advanced Camera for Surveys aboard the Hubble Space Telescope to measure details of the Einstein ring images.
Bolton said the collection of lenses "is especially powerful for science."
With his team's findings, he said, they are able to make direct measurements of the amount of mass within the ring-shaped images and help to answer questions about the evolution of galaxies and dark matter.