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Early galaxies full of cosmic dust

Space dust Astronomers have found dusty giant galaxies were already in existence 13 billion years ago, far earlier than previously thought.

The discovery reported in the Astrophysical Journal, means planets, which are made from coalescing dust particles, may also have already formed that far back in time.

The study's lead author Assistant Professor Steven Finkelstein from the University of Texas, says the discovery came as a complete surprise.

"I don't think we really expected that," says Finkelstein.

"We thought that 13 billion years ago would have been so early in the universe, that dust really didn't have a chance to form."

"But we now know that's simply not the case, at least in the most massive galaxies."

Using NASA's Hubble Space Telescope, Finkelstein and colleagues found that on average, galaxies appear less dusty the further back in time they look.

"If you go far enough back, dust doesn't exist in galaxies," says Finkelstein.

"That's what you would expect, because only hydrogen and helium were made in the big bang, and dust is made up of heavier elements like carbon, silicon and magnesium produced by the first generations of stars."

Size matters

Finkelstein hypothesises why dust is only found in these early massive galaxies.

"Galaxies have large outflows of gas and dust from their interstellar medium, and it's a lot easier for that outflow to occur in low mass galaxies, where there's less gravity," says Finkelstein.

"Dust may be forming in all early epoch galaxies, but it's only sticking around in big galaxies because they have enough mass to hang on to their dust."

The findings are based on data from CANDELS, the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey, a huge two-month study carried out by the Hubble Space Telescope.

Finkelstein and colleagues examined the colour of galaxies in the Hubble images to see how red they look.

"Dust makes things appear red, so the redder a galaxy looks, the more dust it has," says Finkelstein.

"As well as being important for planet formation, the dust also blocks out some of the light, making it more difficult to determine how luminous a galaxy is, and consequently how much star formation is taking place in it."

More surveys

Finkelstein and colleagues have more work to do, including taking spectroscopy of these galaxies to work out what they're made of.

"We can do some of that now with today's ten-metre telescopes," says Finkelstein.

"But we're really waiting for the next generation of big telescopes, such as the 25-metre Giant Magellan Telescope, and the space-based James Webb Telescope which will let us look even farther back in space-time to see what's there."