Galactic Origins: Unveiling the Inside-Out Growth in the Universe’s Earliest Galaxies

In the early Universe, around 700 million years after the Big Bang, galaxies were beginning to form, and one such galaxy, JADES-GS+53.18343−27.79097, offers profound insights into the process of galaxy growth. Observed as part of the JWST Advanced Deep Extragalactic Survey (JADES), this galaxy shows a unique characteristic of "inside-out" growth, where a dense stellar core forms first, followed by a star-forming disc. This discovery challenges our understanding of early galaxy formation. According to Dr. Sandro Tacchella from the University of Cambridge, "You expect galaxies to start small as gas clouds collapse under their gravity, forming very dense cores." Remarkably, the core of JADES-GS+53.18343−27.79097 is less than 100 parsecs in radius yet possesses a stellar density comparable to massive elliptical galaxies of today, even though the galaxy itself contains 1,000 times fewer stars.

Fig 1. Color-composite image of JADES-GS+53.18343−27.79097 showing the core and disc components

One of the most striking findings is that while the core formed earlier and is now less active, the surrounding disc—extending up to 400 parsecs—is experiencing rapid star formation. This raises important questions about the nature of star formation in such early systems. Could these galaxies grow in bursts rather than through continuous formation? JWST’s advanced capabilities allowed researchers to observe the galaxy’s star formation history in detail, confirming that the outer regions are rapidly expanding and forming stars. As William Baker, a co-author of the study, noted, "One of the many reasons that Webb is so transformational to us as astronomers is that we are now able to observe what had previously been predicted through modeling." These observations validate theoretical models, showing how galaxies like JADES-GS+53.18343−27.79097 grow from the inside out, with the outer disc doubling its mass every 10 million years—a rate far greater than that of the Milky Way, which doubles every 10 billion years.

Fig 2. NIRSpec spectrum of the galaxy showing emission lines

But why does this rapid star formation occur mainly in the galaxy’s outskirts, while the core has slowed down? Data from JWST indicates that the outer regions are rich in gas, fueling star formation at an accelerated pace. The central core, while dense, has used up most of its gas, causing star formation to slow down. This pattern suggests that galaxies like JADES-GS+53.18343−27.79097 may follow a two-phase growth: first, a compact core forms in a burst of star formation, and later, the outer disc expands as gas is accreted from larger distances. As Dr. Tacchella explains, “It’s like a spinning figure skater—galaxies pull in gas from their surroundings, speeding up star formation in the outer regions as they evolve.”

This discovery also raises an important question about the future of such galaxies. Will JADES-GS+53.18343−27.79097 evolve into a massive elliptical galaxy like those observed today? The galaxy's central stellar mass density suggests that it might be on this evolutionary path. As mergers and interactions with other galaxies occur over cosmic time, this early system could continue to grow, eventually resembling the giant ellipticals we observe in the present-day Universe. Researchers are now analyzing similar data from other early galaxies to determine whether this pattern of inside-out growth is common. Dr. Tacchella remarked, “By looking at different galaxies across cosmic time, we may be able to reconstruct the growth cycle and demonstrate how galaxies grow to their eventual size today.”

In conclusion, JADES-GS+53.18343−27.79097 represents a critical piece of the cosmic puzzle, demonstrating how galaxies in the early Universe may have grown through inside-out mechanisms. This discovery not only affirms long-held theoretical predictions but also opens new avenues of inquiry into the processes that shaped galaxies during the first billion years of cosmic history. As JWST continues to reveal the hidden details of early galaxies, our understanding of the Universe’s formation and evolution will continue to deepen, reshaping our models of how the cosmos came to be.

Reference: 

https://www.azoquantum.com/News.aspx?newsID=10540#:~:text=Astronomers%20have%20utilized%20the%20NASA,years%20after%20the%20Big%20Bang.&text=Despite%20being%20only%20one%2Dhundredth,for%20such%20an%20early%20stage.

Image Credits: 

Baker, W. M., et al. (2024) A core in a star-forming disc is evidence of inside-out growth in the early Universe. Nature Astronomy. doi.org/10.1038/s41550-024-02384-8.