Our Milky Way formed at the beginning of the cosmos more than 13 billion years ago. But what she looked like and how she grew up back then is only partially clear. A distant galaxy that is very similar to an early form of the Milky Way could now provide more information. Astronomers observed this galaxy, dubbed “Firefly Sparkle,” with the James Webb Telescope and were able to find out more about its features and internal structure for the first time. The images reveal that the stars of this early galaxy are concentrated in ten massive globular clusters. In these clusters, new, massive stars are forming at an unusually high rate, the team reports. This could confirm assumptions that the Milky Way also emerged from such protogalactic star clusters.
The Milky Way has an eventful history behind it. They first began to form around 13 billion years ago, probably through the merger of several smaller precursors. Old, metal-poor stars from this first population are still preserved in our galaxy today. Many of them orbit the galactic center in two clusters each containing ten billion solar masses, as astronomers discovered in early 2024 using data from the Gaia space telescope. Based on the age and characteristics of these stellar populations, they could originate from the protogalactic precursors of the Milky Way. However, it is still unclear how exactly galaxies like the Milky Way formed in the early cosmos and what their low-mass original forms looked like. One of the reasons for this is that the light from low-mass young galaxies from that time is too weak to be visible even with powerful telescopes.
Globular clusters as mass centers
But in the case of the early twin of the Milky Way that is now being examined, a cosmic coincidence came to the aid of the astronomers: the galaxy, dubbed “Firefly Sparkle”, is made visible by the gravitational lensing effect despite its high redshift of z=8.3. In this effect, the enormous gravity of a galaxy cluster passing in the foreground deflects the light of the distant galaxy in such a way that its light is amplified and its structures appear enlarged. Lamiya Mowla from Wellesley College in the USA and her colleagues took advantage of this fortunate circumstance to examine Firefly Sparkle in more detail for the first time using the James Webb Telescope’s high-resolution NIRSpec spectrometer. “These observations provide us with the first spectroscopic views of a typical early-stage galaxy in the Universe, which is only 600 million years old,” writes the team of astronomers. The images show for the first time what the internal structure of such a Milky Way precursor might have looked like.
“Firefly Sparkle shows the features of a young, gas-rich galaxy in its early stages of formation,” report Mowla and her team. As they determined, this galaxy contained around ten million solar masses of stars at the time – making it one of the lowest-mass galaxies ever observed in the early universe and a model for the early Milky Way: “In terms of its stellar mass, it is similar to the precursor of the Milky Way at z = 8,” explain the astronomers. The images also revealed that the majority of the Firefly Galaxy’s stars are concentrated in ten globular clusters. These are similar in size and mass to the globular star clusters of today’s Milky Way, but are significantly poorer in metal. In addition, the density of the Firefly globular clusters and the temperature of the gases they contain are significantly higher than that of old or young globular clusters in our home galaxy, as the team determined.
Model for the early Milky Way?
According to astronomers, these features of Firefly Sparkle provide valuable clues about how galaxies evolved in the early cosmos. Accordingly, globular star clusters could have played an important role as extremely efficient star formation “factories”. “With high-mass, surface-dense star clusters, low metallicity, high electron temperature, and evidence of a top-heavy initial mass function, Firefly Sparkle shows the hallmarks of star formation under extreme conditions,” write Mowla and her colleagues. “Firefly Sparkle suggests that early galaxies could also have evolved through such dense star clusters.” However, what happens to such “founding clusters” in the further course of galaxy evolution is still unclear. “They could have survived to this day or they could have been torn apart by tidal forces in the galaxy’s emerging star disk and only survived as a core cluster in the galaxy center,” the astronomers explain. They hope to find out more about Firefly Sparkle and its contemporaries through further observations with the James Webb Telescope.
Source: Lamiya Mowla (Wellesley College, Massachusetts) et al., Nature, doi: 10.1038/s41586-024-08293-0