Astronomers from Japan, Sweden, the United Kingdom and the European Southern Observatory (ESO) have used the Atacama Large Millimeter/submillimeter Array (ALMA) to observe one of the most distant galaxies known. SXDF-NB1006-2 lies at a redshift of 7.2, meaning that we see it only 700 million years after the Big Bang.
The team was hoping to find out about the chemical elements present in the galaxy, as they can tell us about the level of star formation, and provide clues about the period in the history of the Universe known as cosmic reionisation. Their results are reported today in the journal Science.
“Seeking heavy elements in the early Universe is an essential approach to explore the star formation activity in that period,” said Akio Inoue of Osaka Sangyo University, Japan, the paper’s lead author. “Studying heavy elements also gives us a hint to understand how the galaxies were formed and what caused the cosmic reionisation.”
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In the time before objects formed, the Universe 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 break up those neutral atoms, ionising the gas. During this phase — known as cosmic reionisation — the whole Universe changed dramatically. But there is much debate about exactly what kind of objects caused the reionisation. Studying the conditions in very distant galaxies can help to answer this question.
Before observing the distant galaxy, the researchers performed computer simulations to predict how easily they could expect to see evidence of ionised oxygen with ALMA. They also considered observations of similar galaxies that are much closer to Earth, and concluded that the oxygen emission should be detectable, even at vast distances.
They then carried out high-sensitivity observations with ALMA and found light from ionised oxygen in SXDF-NB1006-2, making this the most distant unambiguous detection of oxygen ever obtained. It is firm evidence for the presence of oxygen in the early Universe, only 700 million years after the Big Bang.
“Many astronomers have believed that ionised carbon emits the strongest light from very distant galaxies in the far infrared range and tried to detect them in the carbon emission using ALMA,” said co-author Dr Kazuaki Ota from the Kavli Institute for Cosmology at the University of Cambridge. “Most of such attempts have failed. However, this study has solely predicted that ionized oxygen emits the strongest light and even detected it from one of the most distant galaxies known. We believe that oxygen could be used as a powerful probe for very distant galaxies.”
Oxygen in SXDF-NB1006-2 was found to be ten times less abundant than it is in 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 Naoki Yoshida at the University of Tokyo. “Our simulation actually predicted an abundance ten times smaller than the Sun. But we have another, unexpected, result: a very small amount of dust.”
The team was unable to detect any emission from carbon in the galaxy, suggesting that this young galaxy contains very little un-ionised hydrogen gas, and also found that it contains only a small amount of dust, which is made up of heavy elements. “Something unusual may be happening in this galaxy,” said Inoue. “I suspect that almost all the gas is highly ionised.”
The detection of ionised oxygen indicates that many very brilliant stars, several dozen times more massive than the Sun, have formed in the galaxy and are emitting the intense ultraviolet light needed to ionise the oxygen atoms.
The lack of dust in the galaxy allows the intense ultraviolet light to escape and ionise vast amounts of gas outside the galaxy. “SXDF-NB1006-2 would be a prototype of the light sources responsible for the cosmic reionisation,” said Inoue.
“This is an important step towards understanding what kind of objects caused cosmic reionisation,” explained Yoichi Tamura of the University of Tokyo. “Our next observations with ALMA have already started. Higher resolution observations will allow us to see the distribution and motion of ionised oxygen in the galaxy and provide vital information to help us understand the properties of the galaxy.”
Reference:
Inoue et al. “Detection of an oxygen emission line from a high redshift galaxy in the reionization epoch.” Science (2016). DOI: 10.1126/science.aaf0714
Adapted from a press release by NAOJ/ESO
An international team of astronomers have detected glowing oxygen in a distant galaxy seen just 700 million years after the Big Bang. This is the most distant galaxy in which oxygen has ever been unambiguously detected, and it is most likely being ionised by powerful radiation from young giant stars. This galaxy could be an example of one type of source responsible for cosmic reionisation in the early history of the Universe.
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