Astronomers have made the first accurate measurement of the abundance of oxygen in a distant galaxy. 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.
“This is by far the most distant galaxy for which the oxygen abundance has actually been measured,” says Alice Shapley, a University of California, Los Angeles (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, Shapley says.
COSMOS-1908, contains approximately 1 billion stars. In contrast, the Milky Way contains approximately 100 billion stars – some galaxies in the universe contain many more, while others contain many fewer. Furthermore, COSMOS-1908 contains approximately only 20 per cent 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, says Ryan Sanders, astronomy graduate student and the study’s lead scientist.
“Close galaxies are much brighter, and we have a very good method of determining the amount of oxygen in nearby galaxies,” says Sanders. 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 says.
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,” says Shapley.
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,” says 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.
Shapley expects the measurements of oxygen will 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 says.
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. This five-ton instrument was designed to study the most distant, faintest galaxies, says UCLA physics and astronomy professor Ian McLean, project leader on MOSFIRE and director of UCLA’s Infrared Laboratory for Astrophysics. McLean built the instrument with colleagues from UCLA, the California Institute of Technology and UC Santa Cruz and industrial sub-contractors.
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 type of instrument known as a “spectrograph,” which spreads the light from astronomical objects out into a spectrum of separate wavelengths (colours), 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 characterise the chemical contents of COSMOS-1908, Sanders analysed a particular wavelength in the MOSFIRE spectrum of this galaxy that is sensitive to the amount of oxygen. “MOSFIRE made Ryan’s measurement possible,” says Shapley, who described it as an “amazing instrument.”
Data for COSMOS-1908 were collected as part of the MOSFIRE Deep Evolution Field (MOSDEF) survey, a large Keck Observatory project that Shapley and Sanders have carried out in collaboration with astronomers at UC Berkeley, UC Riverside and UCSD. Between 2012 and 2016, the MOSDEF survey was allocated roughly 50 nights of MOSFIRE time on the Keck I telescope to study distant galaxies forming in the early universe.