First upper limits on the 21 cm signal power spectrum from cosmic dawn from one night of observations with NenuFAR

¹ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
e-mail: munshi@astro.rug.nl
² LERMA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, 75014 Paris, France
³ ASTRON, PO Box 2, 7990 AA Dwingeloo, The Netherlands
⁴ Université de Strasbourg, CNRS, Observatoire astronomique de Strasbourg (ObAS), 11 rue de l’Université, 67000 Strasbourg, France
⁵ School of Physics and Astronomy, Tel Aviv University, Tel Aviv 69978, Israel
⁶ Institute for Advanced Study, 1 Einstein Drive, Princeton, New Jersey 08540, USA
⁷ Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064, USA
⁸ Laboratoire de Physique de l’Ecole Normale Supérieure, ENS, Université PSL, CNRS, Sorbonne Université, Université de Paris, 75005 Paris, France
⁹ LESIA, Observatoire de Paris, CNRS, PSL, Sorbonne Université, Université Paris Cité, 92195 Meudon, France
¹⁰ ORN, Observatoire de Paris, CNRS, PSL, Université d’Orléans, 18330 Nançay, France
¹¹ Kavli Institute for Cosmology, Madingley Road, Cambridge CB3 0HA, UK
¹² Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
¹³ ARCO (Astrophysics Research Center), Department of Natural Sciences, The Open University of Israel, 1 University Road, PO Box 808, Ra’anana 4353701, Israel
¹⁴ AIM, CEA, CNRS, Université Paris Cité, Université Paris-Saclay, 91191 Gif-sur-Yvette, France
¹⁵ LPC2E, Université d’Orléans, CNRS, 45071 Orléans, France
¹⁶ LUTh, Observatoire de Paris, Université PSL, Université de Paris Cité, CNRS, 92190 Meudon, France

Received: 19 October 2023
Accepted: 3 November 2023

Abstract

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 Nançay Upgrading LOFAR (NenuFAR) to place upper limits on the 21 cm power spectrum from 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 Nançay 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. 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, 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 2σ upper limit of 2.4 × 10⁷ mK² at z = 20.3 and k = 0.041 h 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 to the analysis pipeline that can potentially mitigate it and consequently allow us to reach thermal noise sensitivity when multiple nights of observations are processed in the future.