A galaxy lurking in the far reaches of the Universe, a mere 600 million years after the Big Bang, is giving us our best glimpse yet at what our own infant Milky Way might have looked like.
Bursting with star formation, it's been given the nickname Firefly Sparkle. Yet having traveled 13.2 billion years to reach us, the galaxy's 'sparkly' light can only be seen thanks to a combination of the most powerful space telescope ever built and a quirk of gravity that bends space-time into a magnifying glass.
So detailed is the view of Firefly Sparkle that astronomers were able to identify 10 discrete star clusters, and determine the mass of the galaxy as it actively forms and grows in the darkness of the Cosmic Dawn.
"I didn't think it would be possible to resolve a galaxy that existed so early in the Universe into so many distinct components, let alone find that its mass is similar to our own galaxy's when it was in the process of forming," says astrophysicist Lamiya Mowla of Wellesley College the US.
"There is so much going on inside this tiny galaxy, including so many different phases of star formation."
Firefly Sparkle: a Growing Galaxy in The Early Universe Just Like a Baby Milky Way
The JWST observation in which Firefly Sparkle was found. (NASA, ESA, CSA, STScI, C. Willott/NRC-Canada, L. Mowla/Wellesley College, K. Iyer/Columbia)
The Universe's first billion years is of intense interest to anyone who studies the cosmos. This period is known as the Cosmic Dawn, and it contains the epochs in which all the matter in the Universe started to come together from the primordial fog of particles, turning into stars and galaxies and everything else that makes up the Universe around it.
Obviously, we want to know more about how the Universe was born; but the Cosmic Dawn is tremendously difficult to see. It's so far away that objects within it barely register as smudges of light to our most powerful telescopes.
This is where gravitational lensing comes in. When something massive is sitting in space, its immense gravity field causes space-time itself to warp – a bit like plopping a bowling ball on a trampoline stretches the mat. If you then roll a marble across the trampoline mat, it follows a curved path; so, too, does light traveling through a gravitational lens follow the curvature of the warped space-time.
When it comes out the other side, the light is often smeared, distorted, and replicated in multiple images; but, above all, it is magnified. Astronomers can take all that messed-up light and straighten it out to see the true nature of the source that produced it.
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Diagram illustrating gravitational lensing. (NASA, ESA & L. Calçada)
Thus it is with Firefly Sparkle. It's situated along our line of sight behind a giant foreground galaxy cluster whose light has traveled for 5.3 billion years to reach us. This cluster is known as a lensing source, and thus a prime candidate for JWST, whose infrared capabilities are perfect for studying far distant light stretched into redder wavelengths by the expansion of the Universe.
The light from Firefly Sparkle magnified by the galaxy cluster is stretched into a long, sparkling smear, along which clusters of stars can be seen glittering. The researchers reconstructed this light into a coherent galaxy resembling a teardrop blooming with the birth of new stars.
"Our reconstruction shows that clumps of actively forming stars are surrounded by diffuse light from other unresolved stars," says astrophysicist Kartheik Iyer of Columbia University in the US. "This galaxy is literally in the process of assembling."
Because the light of the galaxy was stretched into an arc, the researchers were easily able to identify individual star clusters and determine the mass of the galaxy. It's pretty lightweight, with a mass that we'd expect to see in a baby Milky Way. In addition, the star clusters show different colors across the spectrum.
Firefly Sparkle: a Growing Galaxy in The Early Universe Just Like a Baby Milky Way
An artist's reconstruction of Firefly Sparkle and the star clusters within. (NASA, ESA, CSA, Ralf Crawford/STScI)
This suggests that the formation of stars is not simultaneous, since different stars at different life stages emit different kinds of light. Each of the clusters is at a different phase of its evolution. Taken together, the clusters represent about half of Firefly Sparkle's entire mass. It's a rare insight into the galactic assembly process.
Another clue about the galaxy's growth is that other galaxies are very close by. Two galaxies are hanging out at distances from Firefly Sparkle of 6,500 light-years and 42,000 light-years, respectively.
That's close enough that the three galaxies are probably gravitationally bound in a three-way orbital dance – and could be the early stage of the cannibalistic way in which young galaxies grow. The Milky Way's growth, for example, was fueled by the absorption of multiple smaller galaxies.
"It has long been predicted that galaxies in the early Universe form through successive interactions and mergers with other tinier galaxies," says astrophysicist Yoshihisa Asada of Kyoto University in Japan. "We might be witnessing this process in action."
Firefly Sparkle: a Growing Galaxy in The Early Universe Just Like a Baby Milky Way
The Location of Firefly Sparkle and its two companion galaxies in the JWST image. (NASA, ESA, CSA, Ralf Crawford/STScI)
The tiny galaxy has given us one of the most detailed looks we've ever had at how baby galaxies grow, and represents a rare opportunity to understand our own galaxy better. But such opportunities are already becoming less rare.
"This is just the first of many such galaxies JWST will discover," says astronomer Maruša Bradač of the University of Ljubljana in Slovenia.
"Just like microscopes let us see pollen grains from plants, the incredible resolution of Webb and the magnifying power of gravitational lensing let us see the small pieces inside galaxies. Our team is now analysing all early galaxies, and the results are all pointing in the same direction: we have yet to learn much more about how those early galaxies formed."
The research has been published in Nature.