Observations of carbon radio recombination lines with the NenuFAR telescope

Context . Carbon radio recombination lines (CRRLs) at decametre wavelengths trace the diffuse phase of the interstellar medium (ISM) of the Galaxy. Observations of these lines allow for physical parameters of this phase to be measured. Aims . We observed CRRLs with the recently commissioned New Extension in Nançay Upgrading LOFAR (NenuFAR) telescope towards two of the brightest sources at low-frequency (10–85 MHz): Cassiopeia A and Cygnus A (hereafter, Cas A and Cyg A, respectively). We then measured the density, n e , and temperature, T e , of the electrons in line-of-sight clouds. Methods . We used NenuFAR’s beam-forming mode and integrated several tens of hours on each source. The nominal spectral resolution was 95.4 Hz. We developed a reduction pipeline primarily aimed at removing the radio frequency interference (RFI) contamination and correcting the baselines. We then performed a first fitting of the spectral lines observed in absorption associated with the line-of-sight clouds. Results . Cas A is the brightest source in the sky at low frequencies and represents an appropriate test bench for this new telescope. On this source, we detected 398 Cα lines between the principal quantum numbers n = 426 and n = 826. Cyg A is also a bright source, however, the Cα lines were observed to be fainter. We stacked the signal by groups of a few tens of lines to improve the quality of our fitting process. For both sources, we reached a significantly higher signal-to-noise ratio (S/N) and spectral resolution than the most recent detections by the LOw Frequency ARray (LOFAR). The variation of the spectral line widths with the electronic quantum number provides constraints on the physical properties of the clouds: T e , n e , and the temperature, T 0 , of the radiation field, the mean turbulent velocity, ν t , and the typical size of the cloud. Conclusions . Our final constraints differ from those inferred from LOFAR results, with ∼50% lower T e , ∼35% lower n e , and from 10 to 80% higher ν t , on average. The NenuFAR observations sample a larger space volume than LOFAR’s towards the same sources due to the differences in instrumental beam sizes. These discrepancies highlight the sensitivity of low-frequency CRRLs as probes of the diffuse ISM, paving the way towards large area surveys of CRRLs in our Galaxy.

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Journal
Astronomy and Astrophysics
Published
2025-08-11
DOI
https://doi.org/10.1051/0004-6361/202555085
Primary Topic
Stellar, planetary, and galactic studies
Type
article
Field-Weighted Citation Impact
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article

Observations of carbon radio recombination lines with the NenuFAR telescope

M. Tagger, Alan Loh, Maryvonne Gérin, В. В. Захаренко et al.
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Astronomy and Astrophysics
Stellar, planetary, and galactic studies
article

Observations of carbon radio recombination lines with the NenuFAR telescope

M. Tagger, Alan Loh, Maryvonne Gérin, В. В. Захаренко, P. Salas, A. Bracco, S. Corbel, Florent Mertens, P. L. Tokarsky, F. Motte, P. Salomé, Jonathan Freundlich, A. G. G. M. Tielens, Antoine Marchal, J.‐M. Grießmeier, P. Lesaffre, O. M. Ulyanov, G. Theureau, I. A. Grenier, A. Gusdorf, Martin Houde, Sergiy Stepkin, M. I. R. Alves, Alexandre Marcowith, Oleksandr Konovalenko, P. Zarka, F. Boulanger, R. Javier Goicoechea, Lucie Cros
article en

Abstract

Context . Carbon radio recombination lines (CRRLs) at decametre wavelengths trace the diffuse phase of the interstellar medium (ISM) of the Galaxy. Observations of these lines allow for physical parameters of this phase to be measured. Aims . We observed CRRLs with the recently commissioned New Extension in Nançay Upgrading LOFAR (NenuFAR) telescope towards two of the brightest sources at low-frequency (10–85 MHz): Cassiopeia A and Cygnus A (hereafter, Cas A and Cyg A, respectively). We then measured the density, n e , and temperature, T e , of the electrons in line-of-sight clouds. Methods . We used NenuFAR’s beam-forming mode and integrated several tens of hours on each source. The nominal spectral resolution was 95.4 Hz. We developed a reduction pipeline primarily aimed at removing the radio frequency interference (RFI) contamination and correcting the baselines. We then performed a first fitting of the spectral lines observed in absorption associated with the line-of-sight clouds. Results . Cas A is the brightest source in the sky at low frequencies and represents an appropriate test bench for this new telescope. On this source, we detected 398 Cα lines between the principal quantum numbers n = 426 and n = 826. Cyg A is also a bright source, however, the Cα lines were observed to be fainter. We stacked the signal by groups of a few tens of lines to improve the quality of our fitting process. For both sources, we reached a significantly higher signal-to-noise ratio (S/N) and spectral resolution than the most recent detections by the LOw Frequency ARray (LOFAR). The variation of the spectral line widths with the electronic quantum number provides constraints on the physical properties of the clouds: T e , n e , and the temperature, T 0 , of the radiation field, the mean turbulent velocity, ν t , and the typical size of the cloud. Conclusions . Our final constraints differ from those inferred from LOFAR results, with ∼50% lower T e , ∼35% lower n e , and from 10 to 80% higher ν t , on average. The NenuFAR observations sample a larger space volume than LOFAR’s towards the same sources due to the differences in instrumental beam sizes. These discrepancies highlight the sensitivity of low-frequency CRRLs as probes of the diffuse ISM, paving the way towards large area surveys of CRRLs in our Galaxy.

Astronomy and AstrophysicsVol. 701
Australian National University (AU), Leiden University (NL), Université d'Orléans (FR), Western University (CA), Centre National de la Recherche Scientifique (FR), University of Groningen (NL), Université de Montpellier (FR), Université Paris Cité (FR), Commissariat à l'Énergie Atomique et aux Énergies Alternatives (FR), Université Paris Sciences et Lettres (FR), Université Paris-Saclay (FR), Arcetri Astrophysical Observatory (IT), Observatoire de Paris (FR), Observatoire astronomique de Strasbourg (FR), École Normale Supérieure - PSL (FR), Mount Stromlo Observatory (AU), Sorbonne Université (FR), Astrophysique, Instrumentation et Modélisation (FR), Sorbonne Paris Cité (FR), Instituto de Física Fundamental (ES), Laboratoire Univers et Particules de Montpellier (FR), Institute of Radio Astronomy (UA), CEA Paris-Saclay (FR), Institut National de Physique Nucléaire et de Physique des Particules (FR), Laboratoire de Physique et Chimie de l’Environnement et de l’Espace (FR), Université de Strasbourg (FR), Université Grenoble Alpes (FR)
Agence Nationale de la Recherche, Centre National d’Etudes Spatiales, Centre National de la Recherche Scientifique, Université d'Orléans, Observatoire de Paris, Université de Recherche Paris Sciences et Lettres
Openalex Percentile: Top 85%
Stellar, planetary, and galactic studies
DOIOpen Access PDFOpenAlex
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