Early disc galaxy puts theories in a spin

Astronomers have found a massive rotating disc galaxy that formed just 1.5 billion years after the Big Bang, when the Universe was only 10% of its current age.

Galaxy DLA0817g is the most distant such galaxy ever observed and challenges traditional models of galaxy formation, Marcel Neeleman and colleagues write in a paper in the journal Nature.

Nicknamed the Wolfe Disc after the late astronomer Arthur M Wolfe, it is estimated to have a mass 72 billion times that of the Sun, with the disc spinning at around 272 kilometres per second. 

It was first observed in 2017 using the ESO’s Atacama Large Millimeter/submillimeter Array (ALMA) in Chile.

“While previous studies hinted at the existence of these early rotating gas-rich disc galaxies, thanks to ALMA we now have unambiguous evidence that they occur…” says Neeleman, from Germany’s Max Planck Institute for Astronomy. 

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 suggests that the infalling gas is heated, resulting in a spherical structure that can only support the formation of a disc once the central region cools.

However, the discovery of DLA0817g, when the Universe was so young, indicates that other growth processes must have dominated, Neeleman and colleagues say.

“The existence of such a massive, rotationally supported, cold disc galaxy when the Universe was only 1.5 billion years old favours formation through either cold-mode accretion or mergers, although its large rotational velocity and large content of cold gas remain challenging to reproduce with most numerical simulations,” they write in their paper.

However, in an accompanying News & Views article in Nature, Alfred Tiley from the University of Western Australia emphasises that this finding is based on a single galaxy, and similar observations of larger numbers of galaxies are needed to determine whether cold-mode accretion was a common mode of galaxy formation.

Neeleman’s team found the galaxy when they examined the light from a more distant quasar. The light was absorbed as it passed through a massive reservoir of hydrogen gas surrounding the galaxy, which then revealed itself. 

Rather than looking for direct light from extremely bright but rarer galaxies, astronomers use this “absorption” method to find fainter, more normal galaxies in the early Universe.

“The fact that we found the Wolfe Disc using this method, tells us that it belongs to the normal population of galaxies present at early times,” says Neeleman. 

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

The team also used the Very Large Array (VLA) in New Mexico and the NASA/ESA Hubble Space Telescope to learn more about star formation in the Wolfe Disc. 

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. “The star formation rate in the Wolfe Disc is at least 10 times higher than in our own galaxy,” says co-author J Xavier Prochaska, from the University of California, US. “It must be one of the most productive disc galaxies in the early Universe.”