The astronomical understanding of how massive galaxies form and evolve is being revisited after the recent discovery of a pancake-shaped disc galaxy that stopped forming stars just a few billion years after the Big Bang.
This ‘dead’ galaxy – so-called for its lack of star formation – was discovered by Sune Toft from the Niels Bohr Institute in Denmark and his colleagues using gravitational lensing and NASA’s Hubble telescope. Ancient disc galaxies are normally too far away to examine in detail, but gravitational lensing offered the researchers a magnified view of this one.
When Toft and team examined the galaxy, known as MACS 2129-1, they initially expected to see a chaotic ball of stars that had formed from the merging of different galaxies.
Surprisingly, however, they found evidence in the photographs taken by the Hubble Telescope that the galaxy’s stars were born in a flattened disc formation.
Their findings, published in the journal Nature, appear to conflict with observations that elliptical galaxies are generally comprised of older stars and spiral galaxies are usually the domain of younger ones.
Toft and his team suggest that the current rotation of MACS 2129-1 indicates that it must have begun life as a flattened disc and only later changed its shape to become more elliptical.
Toft hypothesises that such a metamorphosis could be caused by a series of mergers with other galaxies from a variety of angles, which would eventually randomise the orbits of stars into what can be seen today.
As Toft points out, this research is invaluable because it is forcing astronomers to re-evaluate their theories of how galaxies burn out early on and evolve over time.
“Perhaps we have been blind to the fact that early ‘dead’ galaxies could in fact be discs, simply because we haven’t been able to resolve them,” he says.