An old-looking galaxy in a young universe
Astronomers have made the first detection of dust in one of the most distant galaxies ever observed. The discovery, published in Nature by an international team including Kirsten Knudsen at Chalmers University of Technology, is tantalising evidence that galaxies evolved rapidly after the Big Bang.
The scientists used the Atacama Large Millimeter/submillimeter Array (Alma) and the X-shooter instrument on ESO’s Very Large Telescope (VLT) to pick up the faint glow from cold dust in one of the youngest and most remote galaxies ever found. The astronomers were surprised to discover a far more evolved system than expected. It had a fraction of dust similar to a very mature galaxy, such as the Milky Way. Such dust is vital to life, because it helps form planets, complex molecules and normal stars.
Their observations targeted the galaxy A1689-zD1. It is observable only by virtue of its brightness being amplified more than nine times by a gravitational lens in the form of the spectacular galaxy cluster, Abell 1689, which lies between the young galaxy and the Earth. Without the gravitational boost, the glow from this very faint galaxy would have been too weak to detect. This galaxy was noticed earlier in the Hubble images, and suspected to be very distant, but the distance could not be confirmed at that time.
We are seeing A1689-zD1 when the Universe was only about 700 million years old – five percent of its present age (a redshift of 7.5). It is a relatively modest system – much less massive and luminous than many other objects that have been studied before at this stage in the early Universe and hence a more typical example of a galaxy at that time.
A1689-zD1 is being observed as it was during the period of reionisation, when the earliest stars brought with them a cosmic dawn, illuminating for the first time an immense and transparent Universe and ending the extended stagnation of the Dark Ages. Expected to look like a newly formed system, the galaxy surprised the observers with its rich chemical complexity and abundance of interstellar dust.
The scientists were able to confirm the galaxy’s distance using the VLT, but they also discovered that the galaxy had been clearly detected by Alma. One of the main goals of the Alma Observatory was to find galaxies in the early Universe from their cold gas and dust emissions.
"No one had ever seen such a distant galaxy shine so bright because of its dust, so it was a big surprise for us to find it with Alma as well. This galaxy is a really impressive star factory", says Kirsten Knudsen, astronomer at Chalmers University of Technology.
This galaxy was a cosmic infant – but it proved to be precocious. At this age it would be expected to display a lack of heavier chemical elements – anything heavier than hydrogen and helium, defined in astronomy as metals. These are produced in the bellies of stars and scattered far and wide once the stars explode or otherwise perish. This process needs to be repeated for many stellar generations to produce a significant abundance of the heavier elements such as carbon, oxygen and nitrogen.
Surprisingly, the galaxy A1689-zD1 seemed to be emitting a lot of radiation in the far infrared. This radiation is stretched by the expansion of the Universe into the millimetre wavelength range by the time it gets to Earth and hence can be detected with Alma. Its brightness indicates that it had already produced many of its stars and significant quantities of metals, and revealed that it not only contained dust, but had a dust-to-gas ratio that was similar to that of much more mature galaxies.
“Although the exact origin of galactic dust remains obscure,” explains Darach Watson, astronomer at the University of Copenhagen. “Our findings indicate that its production occurs very rapidly, within only 500 million years of the beginning of star formation in the Universe – a very short cosmological time frame, given that most stars live for billions of years”, he says.
The findings suggest A1689-zD1 to have been consistently forming stars at a moderate rate since 560 million years after the Big Bang, or else to have passed through its period of extreme starburst very rapidly before entering a declining state of star formation.
“This amazingly dusty galaxy seems to have been in a rush to make its first generations of stars. In the future, Alma will be able to help us to find more galaxies like this, and learn just what makes them so keen to grow up”, concludes Kirsten Knudsen.
Caption: This spectacular view from the NASA/ESA Hubble Space Telescope shows the rich galaxy cluster Abell 1689. The huge concentration of mass bends light coming from more distant objects and can increase their total apparent brightness and make them visible. One such object, A1689-zD1, is located in the box – although it is still so faint that it is barely seen in this picture. Credit: NASA; ESA; L. Bradley; R. Bouwens; H. Ford; G. Illingworth
More about the research
This research was presented in a paper entitled “A dusty, normal galaxy in the epoch of reionization” by D. Watson et al., to appear online in the journal Nature on 2 March 2015.
The team is composed of D. Watson (Niels Bohr Institute, University of Copenhagen, Denmark), L. Christensen (University of Copenhagen), Kirsten Kraiberg Knudsen (Chalmers University of Technology, Sweden), J. Richard (CRAL, Observatoire de Lyon, Saint Genis Laval, France), A. Gallazzi (INAF-Osservatorio Astrofisico di Arcetri, Firenze, Italy) and M. J. Michalowski (SUPA, Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh, UK).
More about Alma
Alma (Atacama Large Millimeter/submillimeter Array) – with its 66 gigantic 12-metre and 7-metre antennas – is an international astronomy facility located at 5000 metres altitude at Chajnantor in northern Chile. Alma is a partnership of Europe, North America and East Asia in cooperation with the Republic of Chile and is the world’s largest astronomy project. Chalmers University of Technology and Onsala Space Observatory have been part of Alma since its inception; receivers for the telescope are one of many contributions. Onsala Space Observatory is host to the Nordic Alma Regional Centre, which provides technical expertise to the Alma project and supports astronomers in the Nordic countries in using Alma.
Contacts:
Robert Cumming, astronomer and communcations officer, Onsala Space Observatory, Chalmers University of Technology, Sweden, +46 31-772 5500, +46 70-493 31 14, robert.cumming@chalmers.se
Kirsten Knudsen, associate professor in astronomy, Chalmers University of Technology, Sweden, +46 31-772 5526 or 5500, +46 7- 975 0956, kirsten.knudsen@chalmers.se
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