To understand the emergence of protogalaxies, we must go back to the very early history of the Universe. Shortly after the Big Bang, about 13.8 billion years ago, matter was hot, dense, and uniformly distributed. As the Universe expanded, it cooled, allowing the first hydrogen and helium atoms to form.
Tiny density variations, visible today in the cosmic microwave background, served as gravitational seeds. Under their own weight, these slightly denser regions slowly attracted surrounding gas, forming halos where dark matter played a central role. It is within these gravitational wells that gas began to collapse, giving rise to the very first stars.
The James Webb Space Telescope allows us to go back to this key period, sometimes called the "cosmic dawn." Thanks to its infrared sensitivity, it captures light from galaxies that appeared less than 400 million years after the Big Bang. By studying their size, brightness, and composition, astronomers can test models describing how matter organized itself into increasingly vast structures, leading to the galaxies we observe today.
To determine if a fuzzy patch in the sky is truly a protogalaxy, astronomers combine several techniques. The James Webb Telescope (JWST) uses its infrared instruments to detect light emitted by the very first stars, which has been "reddened" by the expansion of the Universe. By analyzing this redshift, they can estimate the era in which this object existed.
This observation is complemented by measuring the brightness and distribution of stars within the patch. If the structure contains a lot of gas but few already-formed stars, it is likely a protogalaxy assembling its first stellar populations. Researchers then compare this data with galactic formation models to confirm the nature of the object.
Finally, JWST observations are cross-referenced with those from other telescopes, as Edwin Hubble (1889-1953) had anticipated by linking distance and expansion velocity. This synergy allows for a more complete history: how gas clouds became, over time, the first large cosmic structures.
Once a protogalaxy is detected, astronomers seek to understand its physical properties: how many stars it contains, how quickly they form, and its gas and dust content. To do this, they measure the light received in different colors, a method called SED (Spectral Energy Distribution). Each color, or wavelength, provides clues about the temperature and mass of the stars.
By comparing the observed light with models of forming galaxies, we can estimate:
These measurements allow us to create an overall portrait of the protogalaxy. For example, a very bright but low-mass protogalaxy might be rapidly forming its first stars, while another, more massive but dimmer, may have already begun to stabilize. Thanks to these diagnostics, scientists better understand how the very first galaxies evolved and contributed to cosmic reionization.
N.B.: After the Big Bang, the Universe was filled with neutral gas (mainly hydrogen). When the first stars and galaxies formed, they emitted ultraviolet light energetic enough to strip electrons from these hydrogen atoms. This process, called reionization, transformed neutral gas into ionized gas, making the Universe transparent to light.
Protogalaxies emit light that reaches us today highly "reddened" by the expansion of the Universe. By analyzing this light with a spectrograph, astronomers can detect certain characteristic wavelengths, called spectral lines. For example, the He II line at 1640 Å or the [O III] line at 5007 Å provide information about the temperature of the stars and the chemical composition of the gas.
These signals also help assess the amount of photons capable of reionizing the Universe. Shortly after the Big Bang, the Universe was filled with neutral gas. The first stars produced light energetic enough to ionize this gas, making the Universe transparent to light. By studying the intensity and presence of these lines, JWST helps astronomers determine which galaxies contributed most to this process.
Thus, spectral signatures serve as "tracers" of stellar activity and reionization. They allow us to link the appearance of the first stars to the major stages of cosmic evolution, providing a direct glimpse into the formation of the first structures in the Universe.
Since the arrival of the James Webb, astronomers have been surprised by several aspects of the first galaxies detected. Some objects already appear massive and bright less than 400 million years after the Big Bang, seemingly contradicting classical hierarchical formation models where galaxies gradually assemble from small matter blocks.
Moreover, the observed diversity is astonishing: some protogalaxies are compact, others already extended; some show intense star formation, others more moderate. These rapid and marked differences indicate that physical processes, such as gas accretion, halo merging, or the impact of radiation from massive stars, can act very early and more efficiently than expected.
The obtained spectra sometimes reveal a chemical composition already enriched in heavy elements, a sign that stellar nucleosynthesis occurred faster than standard models had anticipated. These observations are prompting astrophysicists to reconsider the timeline and complexity of the cosmic dawn, and they could lead to significant adjustments in galaxy formation simulations.
In summary, the "cosmic dawn" appears richer, more varied, and faster than previously imagined, pushing the scientific community to revise some foundations of our understanding of the early Universe.
| Source | Redshift \(z\) | Stellar Mass (M⊙) | SFR (M⊙/yr) | Comment |
|---|---|---|---|---|
| GN-z11 | 11.1 | 1×109 | 25 | One of the first galaxies detected at high redshift, confirmed by spectroscopy. Very red light due to the expansion of the Universe. |
| CEERS-93316 | 16 (candidate) | 3×108 | 5 | Very young source, detected by infrared photometry. Star formation has just begun. |
| GLASS-z12 | 12.4 | 5×108 | 15 | Observed as part of the GLASS program, shows sustained stellar activity. Interesting candidate for studying reionization. |
| JADES-GS-z13-0 | 13.2 | 4×108 | 20 | Discovered by the JADES program, shows intense star formation and a spectrum strongly redshifted. |
| CEERS-1749 | 14.1 | 2×108 | 8 | Very small candidate galaxy, detected only by infrared photometry. May represent an early stage of formation. |
| NGDEEP-z15 | 15.3 | 3×108 | 12 | Candidate from the NGDEEP program. Modest mass but active star formation, useful for studying the diversity of primitive galaxies. |
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