Sun. Nov 29th, 2020

Astronomers find galaxy with intact disc in early universe

6 min read

In a spin: Astronomers discover an ancient galaxy that was already the shape of a giant disc 1.5 billion years after the Big Bang – when the Milky Way was still a ‘train wreck’ with no real form

  • The Wolfe Disk galaxy is the most distant discovered by astronomers to date
  • It was found using the Atacama LArge Millimeter/submillimeter Array (ALMA)
  • When galaxies like the Milky Way were going through violent mergers and were ‘a mess’ the Wolfe Disk galaxy already had a well-ordered, cold and intact disc 

A huge rotating disc galaxy has been discovered by astronomers and 1.5 billion years after the Big Bang it was already ‘well-ordered, cold’ and intact.

The Wolfe Disk galaxy was found by astronomers from the National Radio Astronomy Observatory using the Atacama Large Millimeter/submillimeter Array (ALMA).

The universe is 13.8 billion years old and most galaxies, like the Milky Way, were formed gradually over billions of years, reaching their current size relatively late.

In the early years of the universe the Milky Way and other galaxies were like a ‘train wreck’ going through violent mergers for the first 6 billion years.

Within 1.5 billion years of the Big Bang the Wolfe Disk was already well formed, according to astronomers, who say this presents a new view of the early universe.

Artist impression of the Wolfe Disk, a massive rotating disk galaxy in the early, dusty universe. The galaxy was initially discovered when ALMA examined the light from a more distant quasar (top left)

ALMA radio image of the Wolfe Disk, seen when the universe was only ten percent of its current age. The universe is 13.8 billion years old and most galaxies, like the Milky Way, were formed gradually over billions of years, reaching their current size relatively late

The discovery of the Wolfe Disk provides a challenge for many galaxy formation simulations, which predict that massive galaxies at this point in the evolution of the cosmos grew through many mergers of smaller galaxies and hot clumps of gas. 

‘Most galaxies that we find early in the universe look like train wrecks because they underwent consistent and often ‘violent’ merging,’ said author Marcel Neeleman. 

‘These hot mergers make it difficult to form well-ordered, cold rotating disks like we observe in our present universe.’ 

Galaxy DLA0817g was nicknamed the Wolfe Disk after the late astronomer Arthur M. Wolfe and is the most distant rotating disc galaxy ever observed. 

It is about 100,00 light-years across and is as large as 70 to 80 billion Suns – putting it in the same category as the Milky Way galaxy. 

‘While previous studies hinted at the existence of these early rotating gas-rich disk galaxies, thanks to ALMA we now have unambiguous evidence that they occur as early as 1.5 billion years after the Big Bang,’ said Neeleman.

In most cases of galaxy formation they only start to show a well-formed disc about six billion years after the Big Bang.

That is what makes the discovery of the Wolfe Disk so fascinating for astronomers – it shows that the growth process can start earlier and so other processes must have dominated in terms of galactic evolution in the earliest years of the universe.

‘We think the Wolfe Disk has grown primarily through the steady accretion of cold gas,’ said J. Xavier Prochaska, of the University of California, Santa Cruz and coauthor of the paper.

‘Still, one of the questions that remains is how to assemble such a large gas mass while maintaining a relatively stable, rotating disk.’ 

The team also used the National Science Foundation’s Karl G. Jansky Very Large Array (VLA) and the NASA/ESA Hubble Space Telescope to learn more about star formation in the Wolfe Disk. 

In radio wavelengths, ALMA looked at the galaxy’s movements and mass of atomic gas and dust while the VLA measured the amount of molecular mass – the fuel for star formation. 

In UV-light, Hubble observed massive stars within the ancient galactic disc.

‘The star formation rate in the Wolfe Disk is at least ten times higher than in our own galaxy,’ explained Prochaska. 

The Wolfe Disk as seen with ALMA (right – in red), VLA (left – in green) and the Hubble Space Telescope (both images – blue). In the early years of the universe the Milky Way and other galaxies were like a ‘train wreck’ going through violent mergers. Wolfe Disk was already well formed

‘It must be one of the most productive disk galaxies in the early universe.’  

Neeleman and his team found the Wolfe Disk galaxy when they examined the light from a more distant quasar. 

The light from the quasar was absorbed as it passed through a massive reservoir of hydrogen gas surrounding the galaxy – which is how it revealed itself. 

Rather than looking for direct light from extremely bright, but more rare galaxies, astronomers used this ‘absorption’ method to find fainter, and more ‘normal’ galaxies in the early universe.

‘The fact that we found the Wolfe Disk using this method, tells us that it belongs to the normal population of galaxies present at early times,’ said Neeleman. 

‘When our newest observations with ALMA surprisingly showed that it is rotating, we realized that early rotating disk galaxies are not as rare as we thought and that there should be a lot more of them out there.’

‘This observation epitomizes how our understanding of the universe is enhanced with the advanced sensitivity that ALMA brings to radio astronomy,’ said Joe Pesce, astronomy program director at the National Science Foundation. 

The ‘train wrecks’ of large galaxies in the early universe still make up 90 per cent of all action observed by astronomers but this shows that in that primordial mix – large rotating discs were already part of the picture.

According to the current understanding of cosmology, galaxies are expected to be built up in a hierarchical order. 

Dark matter ‘halos’ are thought to develop, drawing in surrounding gas and merging into larger structures from which stars form, leading to the growth of a galaxy. 

The traditional view of galaxy formation suggests that the infalling gas is heated, resulting in a spherical structure that can only support the formation of a disk once the central region cools.

The Wolfe Disk led to the authors presenting evidence for an alternative theory – so-called cold-mode accretion. 

The finding suggests that the infalling gas may have been cold, allowing the rapid condensation of a disk. 

The galaxy is estimated to have a mass 72 billion times that of our Sun, and the disk is spinning at around 169 miles per second.

This work indicates that massive gas disks could form 2.5 billion years earlier than other recent observational studies had suggested, notes Alfred Tiley in an accompanying News & Views article. 

He said that this finding is based on a single galaxy, and states that similar observations of larger numbers of galaxies are needed to determine whether cold-mode accretion was a common mode of galaxy formation.

While the Wolfe Disk is the first galaxy of its type to be found in the early universe, researchers say there are 20 candidate objects that could be similar galaxies.

Arthur M Wolfe, who gave his name to the galaxy, proposed the idea of a large disk galaxy in the early universe. He died in 2014. 

The findings have been published in the journal Nature. 

THE BIG BANG THEORY DESCRIBES THE BEGINNING AND EVOLUTION OF THE UNIVERSE

The Big Bang Theory is a cosmological model, a theory used to describe the beginning and the evolution of our universe.

It says that the universe was in a very hot and dense state before it started to expand 13,7 billion years ago.

This theory is based on fundamental observations.

In 1920, Hubble observed that the distance between galaxies was increasing everywhere in the universe. 

The Big Bang Theory is a cosmological model, a theory used to describe the beginning and the evolution of our universe, based on observations – including the cosmic background radiation (pictured), which is a like a fossil of radiation emitted during the beginning of the universe, when it was hot and dense

This means that galaxies had to be closer to each other in the past.

In 1964, Wilson and Penzias discovered the cosmic background radiation, which is a like a fossil of radiation emitted during the beginning of the universe, when it was hot and dense. 

The cosmic background radiation is observable everywhere in the universe.

The composition of the universe – that is, the the number of atoms of different elements –  is consistent with the Big Bang Theory. 

So far, this theory is the only one that can explain why we observe an abundance of primordial elements in the universe.

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