Astronomers detect faintest ever early-universe galaxy


The discovery sheds light on the epoch of re-ionisation and used a giant cluster of galaxies as a celestial magnifying glass, writes Cathal O’Connell.


A Hubble Space Telescope image showing gravitational lensing of a blue background galaxy.
NASA/HST

Sometimes an ordinary galaxy can be an extraordinary discovery. For the first time ever, astronomers have spied what they believe is a ‘typical’ galaxy from the universe’s early childhood, just 700 million years after the Big Bang.

Discovering an average galaxy doesn’t sound like much of a big deal – but in this case it’s a whopper.

That’s because this tiny galaxy could provide vital clues about what happened during one of the landmark events in the history of the universe: the epoch of reionisation, when light first shone and the universe emerged from the cosmic dark ages.

Astronomers had never been able to detect the ancient, faint and relatively tiny galaxies they think were behind the reionisation. But, last year, astronomers noticed a tiny smudge in the background of a Hubble Space Telescope image, and realized they could be onto something big.

Now, the astronomers have followed up that discovery with telescopes on the ground, and confirmed that smudge is the faintest early-universe galaxy ever detected. The work is published today in Nature Astronomy.

For the first few hundred million years after the Big Bang, a cloud of cold hydrogen gas lay over the universe, blocking all light at certain wavelengths. Then the first stars and galaxies formed and radiation blazed through the gas, breaking hydrogen atoms into ions. This let the light through, like the sun clearing an early morning fog.

This event, called re-ionisation, was the last event to globally affect all the visible matter in the universe.

Though astronomers can piece together, more or less, what happened, they don’t know what exactly was responsible for the radiation.

The chief suspects are the small, ancient galaxies, which astronomers expect were abundant at the time. The only problem with this idea was that we’ve never seen one.

Astronomers have only detected about four galaxies from that epoch, and these were exceptionally bright. Their brightness made these monster galaxies easier for us to see, but would have been too rare to have been big players in re-ionisation.

Average Joe galaxies, on the other hand, have been too faint for even our best telescopes to detect.

That’s why astronomers at the University of California, at Davis, have been increasing the power of their telescopes using giant clusters of galaxies, billions of light years away.

A century ago, in his general theory of relativity, Albert Einstein described how mass distorts space and time, and can cause rays of light to bend as if they were passing through a glass lens.

The more massive the object, the greater the lensing. So a team of astronomers, led by physicist Maruša Bradač, at the University of California, Davis, have been using some of the largest structures in the universe, galaxy clusters, to magnify the light of faint, ancient objects.

Last year they got lucky when they happened upon the perfect alignment: a huge cluster of 155 galaxies, sitting in front of another, ancient and extremely faint galaxy, 13.1 billion years old. The lensing effect made the ancient galaxy about ten times brighter than it would otherwise be – just bright enough to be picked up by the Hubble Space Telescope, in the form of a tiny smudge.

The UC Davis team announced the discovery in May last year though the instruments on Hubble weren’t sensitive enough to confirm its key features.

Now Bradač’s team have used a ground-based telescope, the Keck I Telescope on Mauna Kea, Hawaii, to analyse the light in detail.

The new analysis confirms the object, MACS1423-z7p64 (presumably named after the astronomers’ router password), is a young, star-forming galaxy with a mass of about 300 million suns, or about 1/2000th the mass of our galaxy, the Milky Way.

That’s also about ten times smaller than any other galaxy ever detected from that epoch.

“We think this is much more representative of galaxies of the time,” says Austin Hoag, a graduate student at UC Davis and co-author of the work.

The team have shown that telescopes augmented by natural lenses in the sky can bring ultra-faint galaxies into view. Things can only improve when Hubble’s successor, the James Webb Space Telescope, is launched next year. “We will truly witness the birth of the first galaxies,” says Bradač, “which will allow us to answer the longstanding question of where did we come from.”

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Cathal O'Connell is a science writer based in Melbourne.
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