Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have found the most distant oxygen yet seen in the Universe, in a galaxy 13.1 billion light-years from Earth.
Artist’s impression of SXDF-NB1006-2. Many young bright stars are located in this galaxy and ionize the gas inside and around it. Green color indicates the ionized oxygen detected by ALMA, whereas purple shows the distribution of ionized hydrogen detected by the Subaru Telescope. Image credit: NAOJ.
This galaxy, named SXDF-NB1006-2, is the most distant galaxy in which oxygen has ever been unambiguously detected, and it is most likely being ionized by powerful radiation from young giant stars.
SXDF-NB1006-2 was discovered in 2012 by NAOJ’s Subaru Telescope and the following observations with Keck Observatory showed that it was the most distant galaxy known at that time.
The galaxy lies at a redshift of 7.2, meaning that we see it only 700 million years after the Big Bang.
“Seeking heavy elements in the early Universe is an essential approach to explore the star formation activity in that period,” said lead author Dr. Akio Inoue of Osaka Sangyo University in Japan.
“Studying heavy elements also gives us a hint to understand how the galaxies were formed and what caused the cosmic reionization.”
Right panel: the red galaxy at the center of the image is the distant galaxy SXDF-NB1006-2. Left panels: close-ups of the galaxy. Image credit: NAOJ.
In the time before objects formed in the Universe, it was filled with electrically neutral gas.
But when the first objects began to shine, a few hundred million years after the Big Bang, they emitted powerful radiation that started to ionize the gas.
During this phase – known as the Epoch of Reionization – the whole Universe changed dramatically.
But there is much debate about exactly what kind of objects caused the reionization. Studying the conditions in very distant galaxies can help to answer this question.
Schematic diagram of the history of the Universe. This diagram depicts the major milestones in the evolution of the Universe since the Big Bang, about 13.8 billion years ago. The Universe was in a neutral state at 400 thousand years after the Big Bang and remained that way until light from the first generation of stars started to ionize the hydrogen. After several hundred million years, the gas in the Universe was completely ionized. Image credit: NAOJ.
The oxygen abundance of SXDF-NB1006-2 is estimated at about one-tenth that of the Sun.
“The small abundance is expected because the Universe was still young and had a short history of star formation at that time,” said co-author Dr. Naoki Yoshida from the University of Tokyo.
The astronomers unable to detect any emission from carbon in SXDF-NB1006-2, suggesting that this young galaxy contains very little un-ionized hydrogen gas, and also found that it contains only a small amount of dust, which is made up of heavy elements.
The detection of ionized oxygen indicates that many very luminous and massive stars have formed in the galaxy and are emitting the intense UV light needed to ionize the oxygen atoms.
The lack of dust in the galaxy allows the intense UV light to escape and ionize vast amounts of gas outside the galaxy.
“SXDF-NB1006-2 would be a prototype of the light sources responsible for the cosmic reionization,” Dr. Inoue said.
Details of the research were published online this week in the journal Science.
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Akio K. Inoue et al. Detection of an oxygen emission line from a high-redshift galaxy in the reionization epoch. Science, published online June 16, 2016; doi: 10.1126/science.aaf0714
