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First upper limits on the 21-cm signal power spectrum from the Cosmic Dawn from one night of observations with NenuFAR 

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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.

Spherical and cylindrical power spectra at some key stages of the analysis pipeline. The left-most panel shows the spherical power spectra after pre-processing (“Data”), after sky model subtraction (“Skymodel Sub”), and after GPR (“GPR Residual”), along with the thermal noise power spectrum (“Thermal Noise”). For the cylindrical power spectra (second panels), the ratio with respect to the thermal noise power spectrum is shown. The final spherical power spectra after noise bias subtraction and suppression factor correction are shown in the right-most pane.


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)

First NenuFAR Cosmic Dawn paper

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The first Nenufar CD KSP paper, Accurate modelling of the Lyman-α coupling for the 21-cm signal, observability with NenuFAR, and SKA (Semelin et al 2023), has been published in A&A.
The full text can be found here, and here is the abstract :

The measurement of the 21 cm signal from the Cosmic Dawn is a major goal for several existing and upcoming radio interferometers such as NenuFAR and SKA. During this era before the beginning of the Epoch of Reionisation, the signal is more difficult to observe due to brighter foregrounds, but it reveals additional information on the underlying astrophysical processes encoded in the spatial fluctuations of the spin temperature of hydrogen. To interpret future measurements, controlling the level of accuracy of the Lyman-α flux modelling is mandatory. In this work, we evaluate the impact of various approximations that exist in the main fast modelling approach compared to the results of a costly full radiative transfer simulation. The fast SPINTER code, presented in this work, computes the Lyman-α flux including the effect of wing scatterings for an inhomogeneous emissivity field, but assuming an otherwise homogeneous expanding universe. The LICORICE code computes the full radiative transfer in the Lyman-α line without any substantial approximation. We find that the difference between homogeneous and inhomogeneous gas density and temperature is very small for the computed flux. On the contrary, neglecting the effect of gas velocities produces a significant change in the computed flux. We identify the causes (mainly Doppler shifts due to velocity gradients) and quantify the magnitude of the effect in both an idealised setup and a realistic cosmological situation. We find that the amplitude of the effect, up to a factor of ∼2 on the 21 cm signal power spectrum on some scales (depending on both other model parameters and the redshift), can be easily discriminated with an SKA-like survey and can already be approached, particularly for exotic signals, by the ongoing NenuFAR Cosmic Dawn Key Science Program.