Astronomers from the Cosmic Dawn Center have revealed the formation of galaxies in the primordial universe with images captured by the James Webb Space Telescope. In an article published in Astronomy & Astrophysics (ref.), they depicted a protogalaxy similar to the Milky Way, seen in a moment when it is forming from smaller galaxies. The discovery amplifies our understanding of how galaxies form.

From protogalaxies to current galaxies

According to the current model of structure formation in the universe, galaxies are born in a hierarchical manner. Starting from structures formed in the primordial universe, they subsequently merge to form larger structures. This theoretical prediction based on computer simulations is verified by observations of galaxies in various eras of the universe. To observe the assembly of these structures, we must look as far back in time as possible.

But these sources are both very small and very weak, and their detection requires very advanced technologies. The article focused on the ancestor of what is today probably a massive galaxy, the size of the Milky Way. This group of smaller galaxies, nicknamed CGG-z5, was found through the CEERS observing program with the James Webb Space Telescope.

“I developed the software during my studies to detect this type of structure, and now we have applied it to the CEERS data” says Nikolaj Sillassen. “It’s fantastic to see how useful my software is becoming.” “The other members of the group are both small and weak. Without the sensitivity and spatial resolution of James Webb, we simply wouldn’t be able to study the formation of these galaxies” explains Shuowen Jin, the lead author of the study.

Four snapshots taken from the computer simulation of the evolution of a proto-galaxy that resembles the observed CGG-z5 group. The brightness shows the density of stars in the galaxies. Credit: A. Vijiayan and S. Jin
Four snapshots taken from the computer simulation of the evolution of a proto-galaxy that resembles the observed CGG-z5 group. The brightness shows the density of stars in the galaxies. Credit: A. Vijiayan and S. Jin

Computer simulations

What the exact future of the CGG-z5 galaxy group will be is obviously unknown. Instead of forming a single galaxy, it may be that the group has evolved into a large cluster of galaxies. Or there is a possibility that the components are not as dense as they appear, but instead are part of a filamentary structure.

To distinguish between these scenarios, more precise observations involving spectroscopy are needed. But in the meantime, computer simulations can help us. “To better understand the nature and evolution of CGG-z5, we searched for similar structures in large-scale hydrodynamic simulations” says Aswin Vijiayan. “We found 14 structures that closely match the physical properties of the CGG-z5 group. We then traced the evolution of these structures over time in the simulations, from the primordial universe to the present day”.

Well, although the exact evolution of these 14 structures is different, they all shared the same fate about 1 billion years later, merging to form a single galaxy with masses comparable to our Milky Way. Given the predictions of the simulations, it is therefore possible to hypothesize that the CGG-z5 system will also follow a similar evolutionary path. “We have thus captured the process of small galaxies assembling into a massive galaxy” says Shuowen Jin.

Stefano Gallotta
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