Galaxy COSMOS-1908 is in the center of this Hubble Space Telescope image, indicated by the arrow. Nearly everything in the image is a galaxy; many of these galaxies are much closer to Earth than COSMOS-1908. Credit: Ryan Sanders and the CANDELS team
Quantifying the amount of oxygen is key to understanding how #matter #cycles in and out of galaxies. Oxygen, the third-most abundant chemical element in the universe, is created inside stars and released into interstellar gas when stars die. “This is by far the most distant galaxy for which the oxygen abundance has actually been measured,” said Alice Shapley, a UCLA professor of astronomy. “We’re looking back in time at this galaxy as it appeared 12 billion years ago.”
Knowing the abundance of oxygen in the galaxy called #COSMOS-1908 is an important stepping stone toward allowing astronomers to better understand the population of faint, distant galaxies observed when the universe was only a few billion years old and galaxy evolution. COSMOS-1908, contains ~1 billion stars. In contrast, the Milky Way contains ~100 billion stars; some galaxies in the universe contain many more, while others contain many fewer. Furthermore, COSMOS-1908 contains approximately only 20% the abundance of oxygen that is observed in the sun.
Typically, astronomers rely on extremely indirect and imprecise techniques for estimating oxygen abundance for the vast majority of distant galaxies. But in this case, UCLA researchers used a #direct #measurement. “#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,” he said.
“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 3 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, who, along with Sanders, is interested in learning how galaxies form and evolve, why galaxies have different structures, and how galaxies exchange material with their intergalactic environments.
The measurements of oxygen should reveal that super winds are very important in how galaxies evolved. “Measuring the oxygen content of galaxies over cosmic time is one of the key methods we have for understanding how galaxies #grow, as well as how they #spew out #gas into the intergalactic medium,” she said.
The researchers used an extremely advanced and sophisticated instrument called #MOSFIRE (Multi-Object Spectrometer for Infra-Red Exploration) installed on the #Keck I telescope at the Keck Observatory. MOSFIRE collects #visible–#light photons from objects billions of light years away whose wavelengths have been stretched or “#redshifted” to the #infrared by the expansion of the universe. Due to the finite speed of light, MOSFIRE is providing a view of these galaxies as they existed billions of years ago, when the light first started traveling to Earth.
MOSFIRE is a “#spectrograph,”, indicating the specific amount of energy emitted at each wavelength. Spectrographs enable astronomers to determine the #chemical contents of galaxies, because different chemical elements – such as oxygen, carbon, iron or hydrogen – each provide a unique #spectral #fingerprint, emitting light at specific wavelengths.
To characterize the chemical contents of COSMOS-1908, Sanders analyzed a particular wavelength in the MOSFIRE spectrum of this galaxy that is sensitive to the amount of oxygen. “MOSFIRE made Ryan’s measurement possible,” said Shapley. http://newsroom.ucla.edu/releases/ucla-astronomers-make-first-accurate-measurement-of-oxygen-in-distant-galaxy
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