VITAL clues to the evolution of galaxies and stars billions of years ago have been discovered by an international group of astronomers led by a Scottish team from Edinburgh University.
Searches in the furthest reaches of the universe by the team led by Professor James Dunlop of Edinburgh’s School of Physics and Astronomy have found clues as to why there was a “golden age” of the birth of galaxies around 10 billion years ago.
Using the Atacama Large Millimeter/submillimeter Array (ALMA) located high in the Atacama Desert in Chile to explore a distant corner of the universe, the team probed the Hubble Ultra Deep Field – a “dark” part of the universe – and found a rising gas content in galaxies the further back in time that they looked.
The team traced the previously unknown abundance of star-forming gas and dust over cosmic time. This is likely to be the root of a remarkable increase in star formation rates during the peak of galaxy “births” some 10 billion years ago.
The ALMA observations confirmed the existence of galaxies previously indicated by the Hubble Space Telescope and went much further, with the findings showing for the first time how the rate of star formation in young galaxies is closely related to their total mass in stars, which is assembled from previous star-formation events.
In 2004 the Hubble Ultra Deep Field images — pioneering deep-field observations from the Hubble Space Telescope — were first published.
To the amazement of scientists and astronomers alike, these spectacular pictures probed more deeply than ever before and revealed a menagerie of galaxies stretching back to less than a billion years after the Big Bang. The area was observed several times by Hubble and many other telescopes, resulting in the deepest view of the universe to date.
However, Hubble cannot reveal the emission from young stars that is absorbed and then re-emitted by clouds of interstellar gas and dust, within which the youngest stars form.
Astronomers had to wait for ALMA to arrive to view those galaxies both deeply and sharply in the millimetre range of wavelengths which allows them to see the faint glow from gas clouds and also the emission from warm dust in galaxies in the early universe.
Consisting of 66 six- and seven-metre diameter radio telescopes, ALMA’s ability to see a completely different portion of the electromagnetic spectrum from Hubble allows astronomers to study a different class of astronomical objects, such as massive star-forming clouds, as well as objects that are otherwise too faint to observe in visible light, but visible at millimetre wavelengths.
According to the European Southern Observatory, which is a partner in ALMA, this latest study showed clearly for the first time that the stellar mass of a galaxy is the best predictor of star formation rate.
They detected essentially all of the high-mass galaxies – galaxies with stellar masses greater than 20 billion times that of the sun – and virtually nothing else.
Prof Dunlop said: “This is a breakthrough result. For the first time we are properly connecting the visible and ultraviolet light view of the distant universe from Hubble and far-infrared/millimetre views of the universe from ALMA.”
Chris Carilli, an astronomer with the National Radio Astronomy Observatory in Socorro, New Mexico, USA and member of the research team, said: “We discovered a population of galaxies that is not clearly evident in any other deep surveys of the sky.”
Roberto Decarli, another member of the research team, said: “These newly detected carbon monoxide-rich galaxies represent a substantial contribution to the star-formation history of the universe. We have opened a pathway for studying the early formation and assembly of galaxies.”
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