MAUNAKEA, Hawaii — UCLA astronomers have used the W. M. Keck Observatory on Maunakea, Hawaii to make 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 research is published online in the
Astrophysical Journal Letters.
“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, and co-author of the study. “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 said.
COSMOS-1908 contains approximately one billion stars. In contrast,
the Milky Way contains approximately 100 billion stars. Furthermore,
COSMOS-1908 contains approximately only 20 percent 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, said
Ryan Sanders, astronomy graduate student and the study’s lead author.
“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.
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 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, said UCLA physics and astronomy professor Ian McLean, co-project leader on MOSFIRE and director of UCLA’s Infrared Laboratory for Astrophysics.
McLean and co-principal investigator Chuck Steidel from the California
Institute of Technology built the instrument with colleagues colleagues
from UCLA, Caltech, UC Santa Cruz and industrial sub-contractors.
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, 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 said.
Keck Observatory’s 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 (colors), 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. “It’s an amazing instrument, which made Ryan’s
measurement possible,” Shapley said.
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.
The
research was funded by both the National Science Foundation and NASA. MOSFIRE
was also funded by the National Science Foundation (through the Telescope
System Instrumentation program), and by Gordon and Betty Moore.
The
W. M. Keck Observatory operates the largest, most scientifically productive
telescopes on Earth. The two, 10-meter optical/infrared telescopes near the
summit of Maunakea on the Island of Hawaii feature a suite of advanced
instruments including imagers, multi-object spectrographs, high-resolution
spectrographs, integral-field spectrographs and world-leading laser guide star
adaptive optics systems.
Keck
Observatory is a private 501(c) 3 non-profit organization and a scientific
partnership of the California Institute of Technology, the University of
California and NASA.
Media Contact
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