Ancient oxygen: ALMA detects distant gas

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  • Published: Jul 1, 2016
  • Author: David Bradley
  • Channels: UV/Vis Spectroscopy
thumbnail image: Ancient oxygen: ALMA detects distant gas

Reionization

Many young bright stars are located in the galaxy and ionise the gas inside and around the galaxy. Green colour indicates the ionised oxygen detected by ALMA, whereas purple shows the distribution of ionised hydrogen detected by the Subaru Telescope.  Credit: NAOJ

Oxygen, glowing because it is bathed in ultraviolet radiation has been detected in a galaxy that existed 700 million years after the Big Bang and whose light has been detected by ALMA, the Atacama Large Millimeter/submillimeter Array.

Astronomers from Japan, Sweden, the United Kingdom and European Southern Observatory (ESO) have detected oxygen in the galaxy known as SXDF-NB1006-2, the most distant galaxy, and as such, the oldest, in which the gas has been detected unambiguously. The search for heavy chemical elements in the early universe is part of ongoing research that will astronomers piece together the nature of early star formation, according to lead author Akio Inoue of Osaka Sangyo University, Japan. The new findings hint at how galaxies themselves formed during this early period of the evolution of the universe and might hint at new understanding of the process known as cosmic reionization.

Red shift

SXDF-NB1006-2 lies at a red shift of 7.2 a cosmic time and distance associate with a period just before which the universe was pervaded by an electrically neutral gas. When the first stars ignited, several hundred million years after the Big Bang, the radiation they emitted began to ionize this ubiquitous gas forming a plasma in a process known as the cosmic reionization. The change was dramatic and the subsequent evolution of the universe hinges on this epoch. However, there is still a lot of debate about exactly what kind of objects triggered this cosmic reionization and exactly how it occurred. Studying the conditions present in the most distant galaxies could help to settle the arguments.

In preliminary work, ahead of the ALMA observations, the researchers carried out a range of computer simulations to predict how easily they could expect to see evidence of ionized oxygen. "ALMA is the only telescope which can detect the redshifted oxygen emission line within a reasonable exposure time," Inoue told SpectroscopyNOW. They also took into account observations of similar galaxies that are much closer to Earth. Their calculations suggest that they should be able to detect oxygen emissions even at the vast distance of red shift 7.2. Doubly ionized oxygen emits at a wavelength of 0.088 millimetres but given the red shift, should be stretched to 0.725 millimetres making it visible to ALMA.

The team then undertook high-sensitivity observations with ALMA and saw light from ionized oxygen in SXDF-NB1006-2; this is solid evidence oxygen's existence in the early Universe. Nevertheless, oxygen in this galaxy is 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," explains Naoki Yoshida of 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."

Cosmic team work

Intriguingly, the data showed a lack of emission due to carbon in this galaxy, which also suggests that the early galaxy contains very little un-ionized hydrogen gas. Moreover, there was only a small amount of cosmic dust, which is comprised of heavy elements, atomic mass greater than lithium by astronomical definition. "Something unusual may be happening in this galaxy," adds Inoue. "I suspect that almost all the gas is highly ionized." The lack of dust in the galaxy, however, is what allows intense ultraviolet 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," explains Inoue.

The detection of ionized oxygen indicates that many very brilliant stars, several dozen times more massive than the Sun, are present in the galaxy and are emitting the intense ultraviolet light needed to ionize the oxygen atoms. "This is an important step towards understanding what kind of objects caused cosmic reionization," says Yoichi Tamura also 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 ionized oxygen in the galaxy and provide vital information to help us understand the properties of the galaxy."

"Now, we are observing the oxygen emission line from a bit more distant galaxy candidate with ALMA. And we will observe more galaxies in the early Universe in order to know how common the galaxy we observed is in the early Universe, in respect of oxygen abundance, little dust, and ionizing photons' emissivity to the intergalactic medium. Eventually, we hope to know how galaxies made cosmic reionization," Inoue told us.

The team comprised: Akio Inoue (Osaka Sangyo University, Japan), Yoichi Tamura (The University of Tokyo, Japan), Hiroshi Matsuo (NAOJ/Graduate University for Advanced Studies, Japan), Ken Mawatari (Osaka Sangyo University, Japan), Ikkoh Shimizu (Osaka University, Japan), Takatoshi Shibuya (University of Tokyo, Japan), Kazuaki Ota (University of Cambridge, United Kingdom), Naoki Yoshida (University of Tokyo, Japan), Erik Zackrisson (Uppsala University, Sweden), Nobunari Kashikawa (NAOJ/Graduate University for Advanced Studies, Japan), Kotaro Kohno (University of Tokyo, Japan), Hideki Umehata (ESO, Garching, Germany; University of Tokyo, Japan), Bunyo Hatsukade (NAOJ, Japan), Masanori Iye (NAOJ, Japan), Yuichi Matsuda (NAOJ/Graduate University for Advanced Studies, Japan), Takashi Okamoto (Hokkaido University, Japan) and Yuki Yamaguchi (University of Tokyo, Japan).

Related Links

Science 2016, 352, 1559-1562: "Detection of an oxygen emission line from a high redshift galaxy in the reionization epoch"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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