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

The redshifted 21-cm signal from neutral hydrogen is a direct probe of the physics of the early universe and has been an important science driver of many present and upcoming radio interferometers. In this study, we use a single night of observations with the New Extension in Nan\c{c}ay Upgrading LOFAR (NenuFAR) to place upper limits on the 21-cm power spectrum from the Cosmic Dawn at a redshift of $z$ = 20.3. NenuFAR is a new low-frequency radio interferometer, operating in the 10-85 MHz frequency range, currently under construction at the Nan\c{c}ay Radio Observatory in France. It is a phased array instrument with a very dense uv-coverage at short baselines, making it one of the most sensitive instruments for 21-cm cosmology analyses at these frequencies. Our analysis adopts the foreground subtraction approach, in which sky sources are modeled and subtracted through calibration, and residual foregrounds are subsequently removed using Gaussian process regression (GPR). The final power spectra are constructed from the gridded residual data cubes in the uv-plane. Signal injection tests are performed at each step of the analysis pipeline, and the relevant pipeline settings are optimized to ensure minimal signal loss, and any signal suppression is accounted for through a bias correction on our final upper limits. We obtain a best 2$\sigma$ upper limit of $2.4\times 10^7$ $\text{mK}^{2}$ at $z$ = 20.3 and $k$ = 0.041 $h\,\text{cMpc}^{-1}$. We see a strong excess power in the data, making our upper limits two orders of magnitude higher than the thermal noise limit. We investigate the origin and nature of this excess power and discuss further improvements in the analysis pipeline, which can potentially mitigate it and consequently allow us to reach the thermal noise sensitivity when multiple nights of observations are processed in the future.
Comment: 27 pages, 21 figures, and 6 tables; accepted for publication in Astronomy and Astrophysics (A&A)