In search of the ghosts of dead galaxies

Associate Professor Ivo Labbé is hunting dead galaxies from the dawn of time. These cosmic relics passed their active star-making years and mysteriously ‘died’ more than 12 billion years ago, when they should have had at least another 10 billion years of life.

Labbé joined Swinburne’s Centre for Astrophysics and Supercomputing in 2018 to find the very faint markers of galaxies that met their demise in our universe’s infancy, when the cosmos itself was only a billion years old or so. His hope is that identifying these galaxies will help solve the mystery of how and why they lived fast and died young.

Born in the Netherlands, Labbé admits he “rolled into astronomy by accident”. As an undergraduate student at Leiden University, he was unclear on what kind of career path to pursue, but settled on studying physics because it underpins the workings of so much of our everyday technology, from our electric lights to our laptops and phones. “I figured these physicists are getting something right, so they must be clever cookies,” he says, laughing. While there, he made the change to astronomy and his passion was ignited. A masters and PhD at Leiden followed, and Labbé recalls plastering the walls of his dormitory with images of the cosmos he was studying.

Breaking the pattern of galactic life

Labbé has brought that enthusiasm to Swinburne with the aim of using just such images to unravel a conundrum about the life-cycle of odd dead galaxies, which he helped to discover just a couple of years ago.

Until recently, astronomers thought they had a pretty good handle on how galaxies form, grow, and then eventually, die. Our universe is known to be about 13.8 billion years old, with our galaxy, the Milky Way, thought to be only slightly younger, at 13.5 billion years of age.

The conventional wisdom was that all galaxies followed a similar life path to the Milky Way: the galaxy would start forming from a cloud of gas in the early universe, housing just a small collection of stars. It would then spend a good few billion years as a stellar factory, converting more and more gas from the neighbourhood into new stars, and getting bigger in the process. This growth rate was thought to speed up, until it hits a peak, roughly around 10 billion years ago, “when the whole universe was brimming with newly formed stars,” says Labbé. Eventually, after another five to ten billion years, or so, this activity grinds to a halt, and the galaxy is effectively dead.

Discovering the premature deaths of galaxies

That picture holds true for the mature, but likely not dead, Milky Way, and most of the galaxies that astronomers have spotted to date. But a couple of years ago, Labbé and his colleagues discovered that some galaxies — the peculiar ghosts that captivate him — defy this pattern. Instead of going through their life-cycle at the normal, sedate pace that takes 10–15 billion years, they raced through life at an astonishing speed and died when the cosmos had barely aged beyond its first billion years.

Labbé and his colleagues stumbled on this bizarre finding when, as a postdoc based at the Carnegie Institution of Science in Washington, DC, he led the ZFOURGE Project. Through this international collaboration he met many of his current Swinburne colleagues. The team were racing to find the most distant galaxy ever, using the Magellan optical telescope, hosted by Carnegie.

In astronomy, the farther away from Earth you peer with your telescope, the further back you can see in time. That’s because light from far-flung stars and galaxies takes so long to travel through space that, when it finally reaches our telescopes, it is a snapshot of how the universe appeared in the ancient past.

So, when Labbé trained the Magellan telescope on a particularly remote galaxy — snapped during the first billion years of our universe’s existence — he was pretty sure that he’d be viewing a young galaxy, newly born and just starting to cook its stars. But he was wrong.

Live fast and die young

What the team actually saw, stunned them: their ‘young’ galaxy had already grown to an enormous size, bigger than many much older galaxies, and had already stopped forming stars, and died. This was despite the fact the universe itself had not been around long enough for the galaxy to have matured so rapidly. “It’s something that was really quite unexpected,” says Labbé, “It’s like seeing a 200lb baby who is already old and grey, with a beard!”

Questions flew: How could this galaxy have grown at such a super-fast rate, effectively ageing instantly? And what caused it to die and become a ghost? But before tackling these puzzles, the team had to definitively prove that their eyes and the image from their optical telescope, weren’t deceiving them. To do this, Labbé needed access to state-of-the-art equipment that could confirm whether such ‘candidate ghosts’ really are as far away, and as dead, as they appeared to be in pictures.

Pinning down the age and distance to a galaxy requires a spectrograph, an instrument that splits the light coming from stars into its component wavelengths. Different kinds of stars have varying “spectral fingerprints”, explains Labbé, which give astronomers invaluable insight into the type of star they are looking at — young or old — and the galaxy’s location.

A spectrograph brought Labbé to Australia; astronomers at Swinburne have privileged access to the MOSFIRE spectrograph, which sits on one of the Keck telescopes in Hawaii. Over the past few years, as part of Swinburne’s ZFIRE project, led by Swinburne’s Distinguished Professor Karl Glazebrook, Labbé has been homing in on candidate ghosts he first picked out from optical images taken with the Magellan telescope. This involves carrying out a detailed spectral analysis of each one by focusing on them with the Keck telescope for a long period. “You basically point the telescope at them and burn a hole in the sky,” says Labbé.

Finding more and more ghosts

ZFIRE grabbed worldwide attention last year when Labbé and his colleagues confirmed that these prematurely-deceased galaxies are real — a result that was published in Nature. “People for the first time really got the proof that there were these ghost galaxies that were fully grown and are already dead, in the early universe,” says Labbé.

Discovering the first youthful ghost was a huge success for the ZFIRE team, but it was also an immense challenge to spot these faintest of objects in the sky, taking many nights of telescope-time in Hawaii, spread over many years. Now the team has added several more verified ghosts to their haul. “Our results show that there was already very significant activity in this super-young universe,” says Labbé.

But to really get to grips with what actually happened to prematurely age these ghost stars, they need many more examples to study in detail, taken from even earlier in cosmic history. This brings a new problem: older stars tend to emit light in the infrared, but detecting infrared light with even the best ground-based telescopes, such as Keck, is extremely tough because everything in the telescope’s surroundings,  people, and even the telescope’s own instruments,  also generate heat and infrared light, blinding the viewer with background light. “It’s like trying to look directly into car headlights,” says Labbé.

Astronomers get around this by cooling parts of their ground-based telescopes as best they can; but there is a fundamental limit to how well telescopes stuck on Earth can perform. It’s far better to put your telescope in space, away from infrared disturbances, for a cleaner signal that enables astronomers to probe deeper into the cosmos, and peer further back into the past.

Hubble and Spitzer recruited for the search

So, to hunt for thousands of new candidate ghost galaxies at even earlier times in the first billion years since our universe was born, Labbé and his colleagues are now pushing two ageing NASA space telescopes to their limits: the Hubble Telescope, which takes optical images, and tends to pick out young stars, and the Spitzer Space Telescope, which is sensitive to the infrared light given out by older stars. “This is basically to break ground in the earliest part of the universe,” says Labbé. “That’s where the current frontier is.”

And it is only just starting. NASA’s James Webb Space Telescope, scheduled to launch in March 2021, will carry a spectrograph on board. Labbé hopes that the Swinburne team will be able to use this to confirm their new ghost-candidates, scouted out using Hubble and Spitzer, are real, and to provide glorious new detail about the ghosts’ properties.

“This is where it becomes really interesting,” says Labbé. He notes that the quest for the most distant and earliest galaxies was revolutionised by advances in telescope technology over the past decade, and now, thanks to the new space telescope, it will gather speed.

“James Webb (telescope) will go a long way towards answering the question: how did we get here? All the way from the beginning, from the first stars to the first galaxies,” says Labbé. “We’re on the brink of filling in that final chapter of our cosmic history.”

This article was first published on Australia’s Science Channel, the original news platform of The Royal Institution of Australia.