Astronomers have taken a remarkable step into the early Universe.
They probed the molecular gas content and interstellar medium (ISM) conditions of REBELS-25, a massive star-forming galaxy that existed only a few hundred million years after the Big Bang.
Using deep Very Large Array (VLA) and Atacama Large Millimeter/submillimeter Array (ALMA) observations, an international team of researchers discovered a significant amount of cold molecular gas inside REBELS-25.
These values generally match scaling relations extrapolated from field local galaxies, suggesting that the star-forming systems of the early Universe followed relatively familiar evolutionary paths.
It also explored the feasibility of fine-structure emission as a tracer of molecular gas at high redshifts.
Astronomers have taken a remarkable step into the early Universe. They probed the molecular gas content and interstellar medium (ISM) conditions of REBELS-25, a massive star-forming galaxy that existed only a few hundred million years after the Big Bang.
Using deep Very Large Array (VLA) and Atacama Large Millimeter/submillimeter Array (ALMA) observations, an international team of researchers discovered a significant amount of cold molecular gas inside REBELS-25. This is the highest-redshift detection of a low-J CO transition to date.
Andrea Pallottini of the Department of Physics at the University of Pisa said, “For the first time, we have direct evidence that some galaxies in the early Universe were extremely rich in molecular gas, the fuel from which stars are born. Understanding the physical properties of this material is essential for reconstructing the processes that led to the formation of the first galaxies and understanding the evolution of the Universe during its first few billion years.”
The derived molecular gas mass from the CMB-corrected CO(3−2) flux corroborates the presence of a large (~~1011 M ⊙ ⊙ ) reservoir of gas within REBELS-25. Another example is a massive galaxy observed at redshift 8, which is located in the epoch of reionization, highly gas-rich with a high gas-to-dust ratio and a depletion timescale of a few hundred million years.
These values generally match scaling relations extrapolated from field local galaxies, suggesting that the star-forming systems of the early Universe followed relatively familiar evolutionary paths.
The team used the radiative transfer code TUNER to interpret the observational data. This allowed them to model the CO and dust continuum emission consistently, even in the presence of a high cosmic microwave background. This method imposed tight constraints on ISM properties, extracting substantial relevant information while avoiding the usual assumptions about uncertain dust temperature and gas excitation.
It also explored the feasibility of fine-structure emission as a tracer of molecular gas at high redshifts. The researchers combined the CO and [Cii] measurements to derive an empirical [Cii]-to-H₂ conversion factor, supporting the theory that [Cii] remains a reliable tracer of molecular gas in this epoch of reionization.
These results demonstrate that low-J CO emission can be detected even at ultra-high redshift, establishing the most basic relation required by next-generation facilities to map the molecular gas content of early galaxies. As a remarkable case study, REBELS-25 has confirmed that large, gas-rich systems assembled in only the first billion years of cosmic history – rarely have galaxies formed and evolved with such speed as this period demonstrates.
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