Modern disk galaxies often have a dual structure consisting of a thick outer disk and a thin disk “embedded” within it. For example, the thick disk of our Milky Way galaxy is about 3,000 light-years high, while its thin disk is approximately 1,000 light-years high.
But how is this dual disc structure formed? To answer this question, a group of astronomers used the James Webb Space Telescope (JWST). They analyzed a sample of 11 disk galaxies that are edge-on to Earth. The most distant of them is located 11 billion light years away from us.
The analysis shows that galaxies first form a thick disk and then a thin one. The time when this occurs depends on the mass of the galaxy: high-mass galaxies with a single disk transitioned to a two-disk structure about 8 billion years ago. In contrast, low-mass galaxies with a single disk formed their embedded thin disks later, around 4 billion years ago.
To explain this transition from a single disk to a two-disk structure, as well as the difference in timing for high- and low-mass galaxies, the team went beyond their original sample of galaxies and studied data showing gas motion obtained with ALMA and ground-based observations.
Taking into account the motion of the galaxies’ gas disks, the team found that their results were consistent with the “turbulent gas disk” scenario, one of three main hypotheses proposed to explain the formation of thick and thin disks. In this scenario, a turbulent gas disk in the early Universe triggers intense star formation, forming a thick stellar disk. As stars form, they stabilize the gas disk, which becomes less turbulent and, as a result, thinner.
Since massive galaxies can convert gas into stars more efficiently, they settle down earlier than their low-mass counterparts, leading to the earlier formation of thin disks. The formation of thick and thin disks is not an isolated phenomenon: thick disks continue to grow as the galaxy evolves, albeit at a slower rate than thin disks. Researchers also note that the transition period from a single-disk to a two-disk structure roughly coincides with the formation of the Milky Way’s thin disk.
Earlier, we reported on how James Webb discovered a population of galaxies that played a key role in the reionization of the Universe.
According to NASA
