Discovery of Carbon Radio Recombination Lines in M82

Leah K. Morabito, J. B. R. Oonk, Francisco Salgado, M. Carmen Toribio, H. J. A. Röttgering, A. G. G. M. Tielens, Rainer Beck, Björn Adebahr, Philip Best, Robert Beswick, Annalisa Bonafede, Gianfranco Brunetti, Marcus Brüggen, Krzysztof T. Chyży, J. E. Conway, Wim van Driel, Jonathan Gregson, Marijke Haverkorn, George Heald, Cathy HorellouAndreas Horneffer, Marco Iacobelli, Matt J. Jarvis, Ivan Marti-Vidal, George Miley, D. D. Mulcahy, Emanuela Orrú, Roberto Pizzo, A. M. M. Scaife, Eskil Varenius, Reinout J. van Weeren, Glenn J. White, Michael W. Wise

Research output: Contribution to journalArticlepeer-review


Carbon radio recombination lines (RRLs) at low frequencies (lsim 500 MHz) trace the cold, diffuse phase of the interstellar medium, which is otherwise difficult to observe. We present the detection of carbon RRLs in absorption in M82 with the Low Frequency Array in the frequency range of 48-64 MHz. This is the first extragalactic detection of RRLs from a species other than hydrogen, and below 1 GHz. Since the carbon RRLs are not detected individually, we cross-correlated the observed spectrum with a template spectrum of carbon RRLs to determine a radial velocity of 219 km s–1. Using this radial velocity, we stack 22 carbon-α transitions from quantum levels n = 468-508 to achieve an 8.5σ detection. The absorption line profile exhibits a narrow feature with peak optical depth of 3 × 10–3 and FWHM of 31 km s–1. Closer inspection suggests that the narrow feature is superimposed on a broad, shallow component. The total line profile appears to be correlated with the 21 cm H I line profile reconstructed from H I absorption in the direction of supernova remnants in the nucleus. The narrow width and centroid velocity of the feature suggests that it is associated with the nuclear starburst region. It is therefore likely that the carbon RRLs are associated with cold atomic gas in the direction of the nucleus of M82.
Original languageEnglish
Article numberL33
JournalAstrophysical Journal Letters
Issue number2
Publication statusPublished - 1 Nov 2014


  • galaxies: individual: M82
  • galaxies: ISM
  • ISM: general
  • radio lines: galaxies
  • radio lines: ISM


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