Shedding light on cosmic history NASA/JPL-Caltech/UCLA

The early universe was filthy. That much can be garnered from a new detection of cosmic dust in a galaxy whose light reaches us from when the universe was only 600 million years old.

In the past 10 years, astronomers have learned that dust is forged during the aftermath of the supernova deaths of massive, short-lived stars. But many mysteries surround dust’s origin. Astronomers, for example, don’t know how dust can withstand the violent shock waves from supernovae and precisely how long it takes to form.

With that in mind, Nicolas Laporte at University College London and his colleagues turned ALMA, the Atacama Large Millimeter/submillimeter Array, towards the early universe. They studied a star-forming galaxy called A2744_YD4, whose light dates back to just 200 million years after the birth of the earliest stars.


With a little help from a foreground galaxy cluster called Abell 2744, which acted as a gravitational lens and thus magnified the distant galaxy by a factor of two, Laporte’s team discovered the dust. To boot, there’s so much of it that it could fill the sun 6 million times over.

So much dust so early on provides a strict limit on the time it takes to form, which should help astronomers better understand some of the mysteries surrounding the origins of dust. It also hints that the early universe might have looked familiar, with protoplanetary discs or even Earth-like planets circling those early stars, says Darach Watson at the University of Copenhagen in Denmark.

That’s because dust is a crucial building block in all molecules – from the molecular hydrogen within stars to the complex molecules inside planets and even you. “You need dust to do anything actually interesting in the universe at all,” says Watson.

Game changer

The findings also suggest that tracing cosmic dust could be a useful probe for studying these early galaxies.

Astronomers usually study the universe’s first galaxies by counting their numbers, measuring their luminosities and studying their colours, says co-author Richard Ellis at University College London. That’s much less information than we can get from observations of nearby galaxies, of which we can take crystal-clear pictures and detect spectral lines – spikes or drops in light that appear at specific wavelengths based on the chemical elements they contain.

But the detection of early dust is a game changer. It stands as a proxy for the presence of heavier elements, which similarly form from supernova explosions. Ultimately, it may show how quickly those first galaxies evolved.

Next, astronomers want to peer back to a time in cosmic history when the emission from dust disappears. That will point towards the first galaxies, which were so pristine that they contained only the hydrogen and helium left over from the big bang.

“That’s what we’re looking for,” says Watson. “We’re trying to push back far enough where we see the formation of the first galaxies.”

Reference: https://arxiv.org/pdf/1703.02039.pdf