The giant galaxies we see in the universe today, including our own Milky Way galaxy, were originally much smaller. Mergers over the universe’s 13.7 billion years have gradually brought together modern giant galaxies. But they may have started out as ordinary star clusters. To understand the oldest galaxies, JWST studied their ancient light for clues about how they became so large.

JWST can effectively see into the past, when the universe was only 5 percent older than it is now. In that distant past, structures that later became as large as the Milky Way or even larger were only 1/10,000th the mass today. What clues could the powerful infrared space telescope reveal to show us how galaxies got so big?

A new paper presents JWST observations of a galaxy at redshift z~8.3. During this redshift, light traveled over 13 billion years and began its journey only 600 million years after the Big Bang. One galaxy, called Firefly Spark, contains a network of massive star clusters that provide evidence of how galaxies grow.

The article is titled “Firefly Glow: The Earliest Stages of the Merger of a Milky Way-Type Galaxy in the 600 Myr Old Universe.” The article is being prepared for publication and has not yet been peer-reviewed.

“Firefly Glow provides an unprecedented applied study of the early stages of formation of a Milky Way-like galaxy in a universe only 600 million years old,” from Mowla et al. year 2024

Despite the power of JWST, this distant ancient galaxy can only be seen through the gravitational lens of the massive foreground galaxy cluster. Firefly Sparkle looks like a bow thanks to its lenses. There are two more galaxies nearby called Firefly BF (Best Friend) and Firefly NBF (New Best Friend).

“The Firefly Spark shows signatures expected from a future Milky Way-type galaxy captured in the earliest and most gas-rich phase of its formation,” the authors write.

The mass of the young galaxy is concentrated in 10 clusters ranging from approximately 200,000 solar masses to 630,000 solar masses. According to the authors, these clusters “lie at the boundary between low-mass galaxies and high-mass globular clusters.”

These clusters are important because they are key to the growth of the galaxy. The researchers were able to estimate the clusters’ age and star formation history. They found that they experienced a burst of star formation at about the same time.

“The ages of the clusters indicate that they are gravitationally associated with a star formation history that indicates a recent burst of starfall triggered by interaction with a companion galaxy located at the same redshift, at an estimated distance of ~2 kpc from Firefly.”

There are two candidates for the galaxy interaction site: Firefly’s Best Friend (BF) and Firefly’s New Best Friend (NBF). But NBF is about 13 kilometers per second and BF is about two kilometers per second, making BF a likely interactor.

“Weak elements with low surface brightness can be seen at the corners of the arc close to the neighbor, indicating a possible interaction between the two galaxies, which may have triggered a burst of star formation in both galaxies,” the researchers explain.

The researchers paid particular attention to the central cluster. They found that the temperature was extremely high, around 40,000 Kelvin (40,000 C; 72,000 F). It also has an initial upper gravity mass function, indicating that it formed in a very low-metallic environment.

These observations and other evidence indicate that Firefly Sparkle is likely the ancestor of galaxies like ours. For these reasons, “… Firefly Sparkle provides an unprecedented hands-on study of the early stages of the formation of a Milky Way-like galaxy in a universe only 600 million years old,” the authors write.

Fortunately, the researchers behind these results have a powerful supercomputer simulation with which they can compare the observations. It’s called Illustris TNG. It is a large-scale cosmological magnetohydrodynamic simulation based on a comprehensive physical model of the universe.

Illustris ran three tests called TNG, TNG50, TNG 100 and TNG 300. The researchers compared their results with TNG 50.

Finding these ancient star clusters is intriguing, but we cannot assume they will survive unscathed. There are inflow and evaporation forces. The authors investigated the stability of individual star clusters and how they would perform over time.

“Most of these star clusters are expected to persist in the modern Universe, expanding and then breaking apart to form the stellar disk and halo of the galaxy,” the authors explain.

“The only way to survive is to launch them long distances from the dense tidal field of the galaxy.” What is expelled can be stored in spherical clusters.

One of JWST’s main scientific goals is to study how galaxies formed and evolved in the early universe. The space telescope achieves its goal by finding a cluster where clusters are still forming.

“Firefly Spark is one of the first JWST spectrophotometric observations of an hyperlensing galaxy clustering at high redshifts, with clusters in the process of forming rather than at later periods,” the authors write.