Hubble spies stars lighting up the cosmic dawn

By Alexandra Witze

Web edition: December 13, 2012
Print edition: January 12, 2013; Vol.183 #1 (p. 5)

Peering into the far reaches of the universe, astronomers have spotted seven galaxies so distant that they appear as they did less than 600 million years after the Big Bang.

Finding so many primordial galaxies allows scientists to pin down crucial questions about the newborn universe, such as when light from early stars and galaxies penetrated the early cosmic gloom.

“It’s the scientific study of Genesis,” says Avi Loeb, a Harvard astronomer who was not involved in the work.

The discovery comes from the hardworking Hubble Space Telescope, which in August and September spent more than 100 hours staring deeply into a small patch of sky. That region, in the southern constellation Fornax, is the same one that was targeted in 2009 for a long-duration exposure known as the Hubble Ultra Deep Field.

Astronomers led by Caltech’s Richard Ellis looked there again, but for longer exposure times and with an additional filter that’s sensitive to the faint, red light of faraway galaxies. The new census, to appear in an upcoming Astrophysical Journal Letters, includes seven galaxies at great distances — including one that might be the record-breaker of them all, seen as it was just 380 million years after the Big Bang.

Because the universe has been expanding since the Big Bang, 13.7 billion years ago, light from these very distant objects has only now finally arrived at Earth. That means these objects appear as they did during cosmic infancy. The distance to such faraway objects is usually stated in terms of redshift; the higher the redshift, the more distant the object.

The seven galaxies described by Ellis and his team all have redshifts higher than 8.5. One of them — a faint object spotted earlier by other astronomers (SN Online: 1/26/11) — may even be as high as redshift 11.9, Ellis says.

Another recent Hubble survey also found a handful of scattered, faraway galaxies with redshifts possibly in the range of 8.5 to 10. That survey relies on the gravitational influence of intervening galaxies to distort and magnify the light of distant galaxies behind them. In November, astronomers with that survey, known as CLASH, announced finding a galaxy with a redshift of 10.8 in the constellation Camelopardalis.

“Broadly speaking, their results are consistent with ours,” says James Dunlop, an astronomer at the University of Edinburgh who works with Ellis.

More important than any record-breaker, says Ellis, is what a census of distant galaxies can say about one of the early universe’s most important events, called cosmic reionization. Reionization happened when light from the first stars and galaxies permeated the opaque universe, breaking apart hydrogen atoms into electrons and protons and making the universe transparent to ordinary light.

That process started about 200 million years after the Big Bang, but astronomers aren’t sure exactly how it began. At some point the first stars ignited and gathered themselves into the first galaxies, but only when there were enough such stars and galaxies would they have illuminated the darkness. Knowing exactly how many galaxies there were, at what point in time, helps scientists understand whether reionization took place quickly or over time.

The new findings indicate that it might have been gradual. The number of early galaxies, spread out at different redshifts, suggests that they probably built up over time as stars amassed in the dense, hot environs of the early universe, says Ellis. “Cosmic dawn was probably not a single dramatic event,” he said in a December 12 news briefing.

Rogier Windhorst, an astronomer at Arizona State University in Tempe, says he’s surprised the new survey found only seven galaxies. More may be lurking in the data, hidden by the bright lights of other, closer stars or galaxies, he suggests. 

Any lingering questions may have to wait until the 2018 launch of the James Webb Space Telescope, the successor to Hubble that is designed to look in infrared wavelengths for the dim light of even more distant galaxies.

“We confidently predict there are many galaxies beyond this,” Ellis says.

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Back Story | Cosmic Dawn

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Credit: NASA, ESA, A. Feild

The universe might have begun with a bang, but it remained dark for quite some time. A bleak period known as the cosmic “dark ages” stretched from about 400,000 years to 200 million years after the Big Bang. During this period the universe was filled with neutral hydrogen gas, which rendered everything murky and opaque. When the first stars ignited, they released ultraviolet radiation that essentially burned off the hydrogen fog. At first, individual stars cleared pockets within the fog; later, more stars began to form and collect into galaxies, clearing larger pockets of space until the entire universe was transparent. Astronomers call this process “cosmic reionization.” Soon stars also began forging heavier chemical elements, like carbon and iron, in their hearts. When the stars blew up and died, they spewed those elements into interstellar space, there to be incorporated into planets and people. —Alexandra Witze