Firefly Sparkle galaxy offers a taste of the infant Milky Way

By Will Dunham

NASA's James Webb Space Telescope has spotted a young galaxy dating to the early universe - called Firefly Sparkle because its gleaming star clusters resemble the bioluminescent bugs - in a discovery that is giving astronomers a peek at what our Milky Way may have looked like in its infancy.

Researchers said the galaxy, still in the process of assembling, dates to when the universe was roughly 5% of its current age, about 600 million years after the Big Bang event that initiated the cosmos. It has a mass equal to about 10 million stars the size of our sun, and has two other relatively small galaxies as neighbors, dubbed Firefly-Best Friend and Firefly-New Best Friend.

It is comprised of 10 densely packed star clusters embedded in a diffuse arc of stars, eight in its central region and two along its extended arm. Its main visible portion spans about 1,000 light-years across. A light year is the distance light travels in a year, 5.9 trillion miles (9.5 trillion km).

The galaxy is estimated to have formed 100-400 million years before its evolutionary stage that was observed by Webb, the most capable space telescope ever deployed.

"The Milky Way began forming very early in the universe's history, likely around the same time as Firefly Sparkle," said astronomer Lamiya Mowla of Wellesley College in Massachusetts and the Center for Astronomy, Space Science and Astrophysics in Bangladesh, co-lead author of the study published on Wednesday in the journal Nature.

"This observation gives us a direct look at what galaxies like our Milky Way might have looked like in their infancy. Using simulations and statistical methods, we find that the Firefly Sparkle's mass is consistent with what we'd expect for a Milky Way ancestor at that early time. At this stage, the galaxy was about 10,000 times less massive than our current Milky Way, which is typical of galaxies at that epoch," Mowla added.

So how did the firefly name come about?

"A group of fireflies - which is what this galaxy looks like - is called a sparkle," Mowla said.

Firefly Sparkle is located about 6,500 light-years from Firefly-Best Friend and about 42,000 light-years from Firefly-New Best Friend. All three would fit neatly inside the present-day Milky Way, which measures about 100,000 light-years across.

"The Milky Way continued to grow and evolve over billions of years through mergers with other galaxies and continued star formation, while we're seeing Firefly Sparkle in its initial formation stages," said astronomer and study co-lead author Kartheik Iyer, a NASA Hubble Fellow at Columbia University in New York.

"Early galaxies like Firefly Sparkle appear to form through the collapse of dense gas clouds in the early universe. Current theory and simulations suggest several possible mechanisms for how stars form from gas in the extreme conditions of the early universe. Our analysis suggests that these galaxies can build up through the formation of massive star clusters in regions of extreme pressure and density, which then merge or grow together over time," Iyer added.

Its star clusters display slightly different colors, indicating they did not all form simultaneously. For instance, hotter, younger stars look more blue, while older stars are more red.

Webb has been able to look across vast distances - thus back in history because of the time it takes light to travel - to see galaxies dating to the first few hundred million years after the Big Bang roughly 13.8 billion years ago, an age called cosmic dawn.

Firefly Sparkle is one of the earliest low-mass galaxies currently known, observed with the help of a phenomenon called gravitational lensing. This occurs when a massive object with tremendous gravitational strength - in this case, a galaxy cluster - bends light from more distant objects behind it from the vantage point of Earth, acting like a natural magnifying glass.

"In this study, the foreground galaxy cluster magnified the light from Firefly Sparkle by about 16-26 times, making it possible to see details that would otherwise be too faint to observe," Iyer said.