Mysterious, dust-filled galaxy seems very mature for its age

Why so much dust? This galaxy was sighted just 700 million years after the Big Bang -- but it looks much older

Looking deep into the universe's past, astronomers have discovered a galaxy that’s surprisingly mature for its age. The star-forming galaxy A1689-zD1, described in the journal Nature, reveals that dust – the stuff we’re made of – might have been able to accumulate much earlier than once thought.

Scientists study extremely distant celestial objects because it’s a way to look back in time. For example, if you were to take a picture of the sun right now, that snapshot would actually show the sun as it was eight minutes ago, because it took that sunlight eight minutes to reach your camera on Earth. The same principle applies for a galaxy that is billions upon billions of light-years away – the image we see today shows the galaxy as it was many billions of years ago.

The problem is, it’s very difficult to see galaxies from the early eons of the universe’s 13.8-billion-year history. Because they’re so distant, they’re also exceedingly faint. But researchers managed to pick up A1689-zD1 because its light was being bent by a massive galaxy cluster called Abell 1689, which sits between A1689-zD1 and Earth.

Abell 1689 acts rather like a magnifying glass, bending the galaxy’s faint light and amplifying it, making A1689-zD1 look 9.3 times as bright as it would otherwise. This phenomenon is known as gravitational lensing, and it’s supremely helpful for scientists wanting to study distant objects that are far out of a telescope’s range.

The scientists used the European Southern Observatory's Very Large Telescope to determine the galaxy’s distance and the Atacama Large Millimeter/submillimeter Array, or ALMA, to examine its dust emissions. This star-forming galaxy was putting out about 12 suns' worth of new stars per year.

But the scientists quickly noticed something strange: A1689-zD1 was rich in dust.

“The galaxy is highly evolved: it has a large stellar mass and is heavily enriched in dust, with a dust-to-gas ratio close to that of the Milky Way,” the study authors wrote.

This is really weird, and here’s why: Dust is a more "modern" phenomenon. In the early universe, stars were almost entirely made of lightweight hydrogen and helium – and when they died, often in supernovae, the "stardust" from the explosion seeded the surrounding cosmos with heavier elements that were incorporated into future stars, which then died and made more heavy elements, and so on. It took a long time for a decent amount of dust to accumulate, the thinking goes - and so the early galaxies, within several hundred million years of the Big Bang, should not have much dust.

But A1689-zD1 defies that expectation: It’s only about 700 million years old but holds a dust fraction that rivals that of the Milky Way (which, for comparison’s sake, is about 13 billion years old).

Such "evolved" galaxies, then, may have appeared much earlier than scientists may have expected.

“Although the exact origin of galactic dust remains obscure, 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,” lead author Darach Watson of the University of Copenhagen said in a statement.

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