Astronomers have found a galaxy with a disk that formed just 1.5 billion years after the Big Bang – early enough to challenge current ideas about galaxy formation.
Massive disk galaxies, like our Milky Way, were expected to have formed around 3 or 4 billion years after the Big Bang. However, Marcel Neeleman at the Max Planck Institute for Astronomy in Germany and his colleagues have used a powerful radio telescope, the Atacama Large Millimeter/submillimeter Array (ALMA) in northern Chile, to detect a galaxy with a flat rotating disk that formed much earlier.
Light from distant galaxies takes longer to reach us here on Earth than those close by. This means that astronomers are often looking back in time to observe galaxies as they appeared in the past, which in this case is over 12 billion years ago.
The existence of Galaxy DLA0817g, which has been nicknamed the Wolfe Disk after the late astronomer Arthur M. Wolfe, suggests that disk galaxies can grow relatively quickly, thus challenging many prior galaxy formation simulations.
Current cosmology theories indicate that dark matter ‘haloes’ were the earliest large-scale structures in our Universe. Stars and galaxies were formed when surrounding gas fell into these haloes, and haloes and galaxies continued to grow by merging (hierarchical assembly) and through further gas accretion. Merging is thought to be well understood, whereas there is still debate among astronomers surrounding gas accretion and how it eventually leads to galactic structures.
Gas accretion is thought to take place in one of two modes – hot or cold. The main difference is, perhaps not surprisingly, the temperature of the gas as it falls onto a galaxy. Hot mode accretion would lead to late-forming galaxy disks because of the significant amount of time needed for the gas to cool and settle into a disk. A gentler alternative, cold mode accretion, would allow faster disk-formation because the gas is already cooler as it falls into the centre. Recent simulations have indicated that, with the help of cold accretion, massive disk galaxies could have formed as early as a billion years after the beginning of the Universe.
These new results provide some of the first observational evidence of cold gas disks in massive galaxies this soon after the Big Bang. It could indicate that galaxies like our own may have formed far earlier than previously thought.
More galaxies like this one will need to be observed to determine whether this process is common, but astronomers are excited about this new discovery.
Written by Anna Purdue and edited by Ailie McWhinnie.
