University of California (UCLA) astronomers have made the first accurate measurement of the abundance of oxygen in a distant galaxy called COSMOS-1908.
“This is by far the most distant galaxy for which the oxygen abundance has actually been measured,” said Alice Shapley, a UCLA professor and co-author of the study. “We’re looking back in time at this galaxy as it appeared 12 billion years ago.”
COSMOS-1908, contains approximately 1 billion stars, while the Milky Way contains approximately 100 billion stars. Moreover, COSMOS-1908 contains approximately only 20 percent the abundance of oxygen that is observed in the sun.
Ryan Sanders, astronomy graduate student and the study’s lead author, said: “Close galaxies are much brighter, and we have a very good method of determining the amount of oxygen in nearby galaxies,” Sanders said. In faint, distant galaxies, the task is dramatically more difficult, but COSMOS-1908 was one case for which Sanders was able to apply the “robust” method commonly applied to nearby galaxies. “We hope this will be the first of many.”
Shapley said that prior to Sanders’ discovery, researchers didn’t know if they could measure how much oxygen there was in these distant galaxies. “Ryan’s discovery shows we can measure the oxygen and compare these observations with models of how galaxies form and what their history of star formation is,” Shapley said.
The amount of oxygen in a galaxy is determined primarily by three factors: how much oxygen comes from large stars that end their lives violently in supernova explosions — a ubiquitous phenomenon in the early universe, when the rate of stellar births was dramatically higher than the rate in the universe today; how much of that oxygen gets ejected from the galaxy by so-called “super winds,” which propel oxygen and other interstellar gases out of galaxies at hundreds of thousands of miles per hour; and how much pristine gas enters the galaxy from the intergalactic medium, which doesn’t contain much oxygen.
“If we can measure how much oxygen is in a galaxy, it will tell us about all these processes,” said Shapley.
Oxygen, the third-most abundant chemical element in the universe, is created inside stars and released into interstellar gas when stars die. Quantifying the amount of oxygen is key to understanding how matter cycles in and out of galaxies.
The research findings were published in the Astrophysical Journal of Letters.