Astronomy: Early disk galaxy puts formation models in a spin

A massive rotating disk galaxy that formed 1.5 billion years after the Big Bang is described in Nature this week. This is much earlier than predicted by traditional models of galaxy formation, and adds to an ongoing debate about when and how disk galaxies, such as the Milky Way, form.

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 observation of an early disk galaxy reported by Marcel Neeleman and colleagues supports an alternative hypothesis, the so-called cold-mode accretion. They present evidence for the presence of a cold, dusty, rotating disk detected in a galaxy from around 12.5 billion years ago. This 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 272 kilometres 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. However, he emphasizes 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.