The NenuFAR Cosmic Dawn Key-Science project has recently made significant progress in exploring the early universe through the redshifted 21-cm transition line from neutral hydrogen. A recent study [1], using just a single night of observations of the North Celestial Pole deep field with the NenuFAR radio telescope, has set a new upper limits on the 21-cm power spectrum from the Cosmic Dawn at a redshift of z = 20.3. The best 2-σ upper limit of 2.4 × 107 mK2 at k = 0.041 h cMpc−1 at z = 20.3 was observed, the deepest yet in this redshift range.
NenuFAR, a low-frequency radio interferometer located at the Nançay Radio Observatory in France, stands out for its dense uv-coverage at short baselines, making it exceptionally sensitive to observe the 21-cm signal from the Cosmic Dawn. Observing the 21-cm signal from the Cosmic Dawn is particularly challenging due to the overwhelming presence of foreground emissions from our galaxy and other celestial sources, which significantly overpowers the faint signal from the early universe. This new analysis adopted a sophisticated approach involving several steps of foreground subtraction, with a particular emphasis on accurately removing the brightest radio sources in the sky through direction-dependent calibration. Following the subtraction of compact sources, residual foregrounds were addressed using the recently developed ML-GPR method [2]. The process included extensive and rigorous testing, such as the injection of mock 21-cm signals, to ensure robustness of the result.
This milestone represents a critical stride toward the eventual detection of the signal by NenuFAR. However, the journey ahead is still full of challenges. The team plans to continue investigating the sources of excess power and refining their analysis pipeline. The team also look forward towards upcoming NenuFAR Cosmic Dawn observations, which will focus on newly selected deep fields, carefully chosen to mitigate some systematic effects observed in the North Celestial Pole deep field.
[1] First upper limits on the 21-cm signal power spectrum from the Cosmic Dawn from one night of observations with NenuFAR (https://arxiv.org/abs/2311.05364)
[2] Retrieving the 21-cm signal from the Epoch of Reionization with learnt Gaussian process kernels (https://arxiv.org/abs/2307.13545)