An international team of astronomers has identified and characterized the oldest disc in our galaxy, the Milky Way, which they have named PanGu.
PanGu, whose name comes from Chinese mythology and literally means “ancient disc”, was formed more than 13.5 billion years ago, just a few hundred million years after the Big Bang. This discovery pushes the birth of our galaxy even further back in time than previously thought.
According to the researchers, it’s like finding the Milky Way’s childhood photo album, as PanGu shows what our galaxy was like in its early days and helps us understand how it has grown and evolved since then.
The team discovered that PanGu currently has a mass of approximately 2 billion times that of the Sun. While this might seem enormous, it is only a small fraction of the Milky Way’s total mass today. However, at that early time, PanGu was likely the dominant component of our nascent galaxy.
The researchers also reconstructed the star formation history of the galaxy. They found that the rate of star formation started modestly, with around 2 solar masses forming each year (the Solar mass is an astronomical measurement equal to approximately 2 x 10^30 kg). This rate gradually increased, reaching a peak of 11 solar masses per year about 11 billion years ago, before rapidly declining. It’s as if the Milky Way went through a growth spurt in its adolescence, producing stars at a frantic pace during that period.
One of the most surprising findings of the study is how the shape of the galactic disc has changed over time. In its early days, the disc was much thicker in comparison to its diameter. As time passed, it gradually flattened to reach the shape we see today.
The scientists also compared their findings with computer simulations of galaxies similar to the Milky Way. Surprisingly, they discovered that our galaxy is quite unusual. Only about one in six simulated galaxies had such a well-preserved ancient disc like ours.
The discovery suggests that galaxies may begin forming orderly discs much earlier than previously thought, which is consistent with recent observations of distant galaxies made by the James Webb Space Telescope.
SOURCES
Xiang, M., Rix, HW., Yang, H. et al. The formation and survival of the Milky Way’s oldest stellar disk. Nat Astron (2024). doi.org/10.1038/s41550-024-02382-w
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