The twenty gravitationally connected galaxies extend through the early Universe for millions of light years.

Ever since it began sending data back to Earth in the summer of 2022, the impact of the James Webb Space Telescope on our understanding of the early Universe has been almost immeasurable. Now, one of the latest discoveries made by astronomers using the $10 billion USD telescope could be its biggest yet.

 A team of researchers from the Cosmic Dawn Center (DAWN) in Denmark, including team leader and post-doctoral scientist Shuowen Jin, have discovered a massive chain or “cosmic vine” of 20 closely packed and gravitationally bound galaxies. Massive, densely packed structures such as this are believed to be the seeds that grow to form galaxy clusters, the largest gravitationally bound structures in the known Universe.

Thus, the discovery of this megastructure, detailed in a paper set to be published in the journal Astronomy & Astrophysics, available as a pre-peer-reviewed paper published on the research repository website arXiv, could help scientists discover how the largest structures in the Universe were formed, and how they have evolved over 13.8 billion years of cosmic history.

A 13-million light year-long thread through the early Universe

The astronomers wrote that they discovered this huge cosmic vine while studying an area of the Universe called the Extended Groth Strip, a region of space located in the constellation of Ursa Major. 

They found this chain of galaxies extends for around 13 million light years, with a width of 650,000 light years and a mass equivalent to around 260 billion Suns. 

It seems like the structure is not in equilibrium  —  or is not viralized  —  meaning it could be on its way to transforming into a cluster of galaxies, according to the paper’s authors. This is remarkable because other proto-galactic clusters seen at this time in the early Universe have been much smaller.

The team of scientists was also able to determine that this chain of galaxies is between 11 and 12 billion years old, meaning the JWST saw these galaxies as they were when the Universe was around 2 billion years old.

This age estimation was possible thanks to a phenomenon called a redshift, which occurs to light as it travels to us after being omitted in the early Universe. The further away an emitting object is, the longer its light has traveled to reach us, and as it travels, the expansion of the Universe causes the wavelength of light to be “stretched”, meaning the light moves down the electromagnetic spectrum to its red end. 

The longer light has traveled, the more extreme this “redshift” is. That is why the JWST has been designed to observe the Universe in infrared, as the light that left the earliest galaxies in the visible region of the electromagnetic spectrum has been converted to infrared light after traveling for billions of years and across billions of light years. 

The redshift of the cosmic vine is z = 3.44, indicating its light had traveled for around 12 billion years before it was observed by the JWST.

How do galaxies die on the cosmic vine?

Of particular interest in this cosmic vine are two galaxies labeled by the team as Galaxy A and Galaxy E. These are not only the two most massive galaxies in the chain, but they have shapes dominated by large central bulges so have structures similar to more “modern” galaxies.

Another remarkable thing about these large early galaxies is the fact that they appear to have slowed their rate of star formation significantly. As the JWST saw them, they were only forming stellar bodies at a rate equivalent to just half a Sun each year. 

Galaxies with such slow rates of star formation are referred to as being “quiescent”. The cessation of star formation usually occurs as galaxies run low on gas and dust that form the building blocks of stellar bodies, with our galaxy, the Milky Way, being an example of a modern quiescent galaxy.

Such “quenched” galaxies are rare finds in the early Universe, and the team suspects that Galaxy A and Galaxy E may have become quiescent as a result of collisions and mergers with other galaxies around 500 million years prior to reaching the state they are in now. These mergers would have triggered a massive explosion in star formation — or a starburst — that quickly burned through gas and dust. 

The discovery of Galaxy A and Galaxy E in this cosmic vine indicates that massive quiescent galaxies can form in large cosmic structures that themselves go on to birth galactic clusters. 

The team wrote in the study that they now intend to conduct an investigation of other quiescent galaxies in clusters found in the early Universe, further improving our understanding of the growth of galactic structures.

Reference: S. Jin, et al., Cosmic Vine: A z=3.44 Large-Scale Structure Hosting Massive Quiescent Galaxies, arXiv, (2023)[https://arxiv.org/pdf/2311.04867.pdf]

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