Webb Weighs an Early Twin of the Milky Way

What was the Milky Way like billions of years ago? One way we can find out is by looking at the most distant galaxies in the observable Universe. Seeing those far galaxies is one goal of the James Webb Space Telescope. It has revealed some surprising facts about early galaxies, and now it is starting to reveal the story of our own.

Most of the galaxies Webb has observed so far have been larger than we expected, which led to some speculation that perhaps the Big Bang was wrong, which isn’t the case. The bias toward large galaxies is partly because there are some surprisingly large ones in the early cosmos, but also because smaller galaxies are more difficult to see. But a chance alignment of a galaxy cluster has allowed us to see one small early galaxy that is quite similar to what the Milky Way may have appeared.

The galaxy has been nicknamed Firefly Sparkle, and we see it from a time when the Universe was just 600 million years old. Its light traveled for more than 13 billion years to reach us and would have been too dim even for Webb to see were it not for a trick of light. Since Firefly Sparkle is behind a large cluster of galaxies, its light is gravitationally lensed. Just as a glass lens can make an object appear larger and brighter than it actually is, so can a gravitational lens. In this case, the foreground galaxy cluster magnified the light of Firefly Sparkle making it bright enough for Webb to see.

Gravitational lensing also highly distorts our view of a distant galaxy, so astronomers have to trace the paths of light to reconstruct the true shape of the galaxy. Normally, that would be a problem, but in this case, the distortion was a surprise blessing. Rather than appearing as a single fuzzy blob, Firefly Sparkle appears as a string of glowing jewels. When viewed in the infrared, it gives us a kind of exploded view of the galaxy. Thanks to gravitational lensing the research team was able to show how Firefly Sparkle is in the early stages of becoming a true galaxy. They found clumps of active star-forming regions and that these regions are illuminated diffuse light from more mature stars. From the spectra of this galaxy, the team also found that star formation is happening in stages, not all at once. It gives us a rich view of early galaxies.

From the clumps of star-forming regions, the team could also estimate the overall mass of Firefly Sparkle, which is very similar to the hypothetical mass of the Milky Way at that age. The young galaxy even has a couple of companion dwarf galaxies, similar to the Magellanic clouds of the Milky Way. Overall, this gives us a much better understanding of how our galaxy might have formed.

Reference: Mowla, Lamiya, et al. “Formation of a low-mass galaxy from star clusters in a 600-million-year-old Universe.” Nature 636.8042 (2024): 332-336.