Peering back in time to find the very earliest objects in the universe, an international team of astronomers has discovered a galaxy that shouldn't be there at all.
The problem, the scientists report Monday in Nature, is that while the tiny galaxy dates from just 700 million years or so after the big bang, it's far more dusty than something this young and small has any right to be.
It's surprising, says Daniel Marrone, a University of Arizona expert on galaxy formation who wasn't involved in the research, because although dust is essential for the formation of planets and other solid material, the cosmos started out with no dust at all. In the aftermath of the big bang, the universe consisted only of hydrogen and helium gas (along with dark matter, but that's invisible).
The dusty galaxy is just one of the recent surprises astronomers have found. "Last week," says Marrone, "we learned of an incredibly massive black hole in the early universe. Now we have this average galaxy with significant amounts of dust. We've had this cartoon picture of the early universe, but it's clear that we really don't know what's going on."
What Astronomers Think They Know
The gas in the post-big bang universe condensed to form the first stars, which forged heavier elements, including carbon, silicon, and oxygen, then died and released those elements into space. It was from these that the first particles of dust formed.
Those first stars had already lived and died by the time this newly discovered galaxy, known as A1689-zD1, was up and running, so the universe was hardly dust free at that point. But most of the dust should have been in large, bright galaxies that formed lots of stars. A1689-zD1 is relatively small and dim—no bigger than the Large Magellanic Cloud, a dwarf galaxy that orbits the Milky Way.
Few early galaxies have been spotted, because even the brightest among them is far away and thus relatively dim. To get around that problem, astronomers use the magnifying effect of gravity. Einstein's general theory of relativity says the gravity from an object closer to Earth can warp light rays from a more distant one. (Einstein thought we'd never actually observe this so-called gravitational lensing, but he was dead wrong about that.)
In this case, the Nature paper's lead author, Darach Watson, of the deliciously named Dark Cosmology Centre at the University of Copenhagen, used the Very Large Telescope (VLT) in Chile's Atacama Desert to look at a massive cluster of galaxies called Abell 1689. The cluster's gravity magnified A1689-zD1 by a factor of nine, says Watson, which allowed him and several colleagues to gauge its distance from Earth and thus how long ago its light began traveling in our direction.
The VLT can see starlight but not dust, so Watson called on a colleague who was using the dust-sensitive ALMA radio telescope (see "Cosmic Dawn" in National Geographic magazine). "She had a look," says Watson, "and bingo!"
Like Marrone, Watson was taken aback by how dusty A1689-zD1 turned out to be. In bigger galaxies with more massive, short-lived stars, dust can pile up quickly since giant stars explode violently as supernovae after just a few million years. In smaller galaxies, most of the dust should emerge from the more gentle deaths of smaller stars that live for billions of years. So maybe A1689-zD1's dust comes from supernovae after all, says Watson. "But they'd have to produce the maximum possible dust," he says, to account for what ALMA sees, "and the dust can't be destroyed."
The only way to find out for sure is to uncover more galaxies like A1689-zD1. Unfortunately, they're extremely difficult to find—and the one other example astronomers have is almost entirely dust free, so it's impossible to say which type is more common. "We don't have any other candidates at this point," says Watson.
With ALMA, completed two years ago, and a new generation of giant optical telescopes now under construction, however, they're likely to find more of these galaxies before long. (See "Cosmic Vision: Telescopes.") The very early universe doesn't make a lot of sense at the moment—but that's almost certainly going to change.
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