Discovery of carbon radio recombination lines in absorption towards Cygnus A

J. B. R. Oonk 1 R. J. van Weeren 2 F. Salgado 3 L. K. Morabito 3 A. G. G. M. Tielens 3 H. J. A. Rottgering 3 A. Asgekar 1 G. J. White 4 A. Alexov 1 J. Anderson 5 I. M. Avruch 6 F. Batejat 5 R. Beck 7 M. E. Bell 8 I. van Bemmel 1 M. J. Bentum 1 G. Bernardi 9 P. Best 10 A. Bonafede 11 F. Breitling 12 M. Brentjens 1 J. Broderick 13 M. Bruggen 11 H. R. Butcher 1 B. Ciardi 5 J. E. Conway 14 A. Corstanje 15 F. de Gasperin 5 E. de Geus 16 M. de Vos 1 S. Duscha 1 J. Eisloffel 17 D. Engels 18 J. van Enst 1 H. Falcke 1 R. A. Fallows 1 R. Fender 13 C. Ferrari 19 W. Frieswijk 1 M. A. Garrett 1 Jean-Mathias Grießmeier 20 J. P. Hamaker 1 T. E. Hassall 21 G. Heald 1 J. W. T. Hessels 22 M. Hoeft 17 A. Horneffer 5 A. V. D. Horst 22 M. Iacobelli 3 N. J. Jackson 21 E. Juette 23 A. Karastergiou 24 W. Klijn 1 J. Kohler 25 V. I. Kondratiev 1 M. Kramer 7 M. Kuniyoshi 7 G. Kuper 1 J. van Leeuwen 22 P. Maat 1 G. Macario 19 G. Mann 12 S. Markoff 26 J. P. Mckean 1 M. Mevius 1 J. C. A. Miller-Jones 26 J. D. Mol 1 D. D. Mulcahy 5 H. Munk 1 M. J. Norden 1 E. Orru 1 H. Paas 1 M. Pandey-Pommier 27 V. N. Pandey 1 R. Pizzo 1 A. G. Polatidis 1 W. Reich 7 A. M. M. Scaife 13 A. Schoenmakers 1 D. Schwarz 28 A. Shulevski 29 J. Sluman 1 O. Smirnov 30 C. Sobey 7 B. W. Stappers 21 M. Steinmetz 12 J. Swinbank 26 Michel Tagger 20 Y. Tang 1 C. Tasse 30 S. T. Veen 15 S. Thoudam 15 C. Toribio 1 R. van Nieuwpoort 1 R. Vermeulen 1 C. Vocks 12 C. Vogt 1 R. A. M. J. Wijers 26 M. W. Wise 26 O. Wucknitz 31 S. Yatawatta 29 P. Zarka 32 A. Zensus 5
Abstract : We present the first detection of carbon radio recombination line absorption along the line of sight to Cygnus A. The observations were carried out with the Low Frequency Array in the 33-57 MHz range. These low-frequency radio observations provide us with a new line of sight to study the diffuse, neutral gas in our Galaxy. To our knowledge this is the first time that foreground Milky Way recombination line absorption has been observed against a bright extragalactic background source. By stacking 48 carbon α lines in the observed frequency range we detect carbon absorption with a signal-to-noise ratio of about 5. The average carbon absorption has a peak optical depth of 2 × 10-4, a line width of 10 km s-1 and a velocity of +4 km s-1 with respect to the local standard of rest. The associated gas is found to have an electron temperature Te ~ 110K and density ne~0.06 cm-3. These properties imply that the observed carbon α absorption likely arises in the cold neutral medium of the Orion arm of the Milky Way. Hydrogen and helium lines were not detected to a 3σ peak optical depth limit of 1.5 × 10-4 for a 4 km s-1 channel width. Radio recombination lines associated with Cygnus A itself were also searched for, but are not detected. We set a 3σ upper limit of 1.5 × 10-4 for the peak optical depth of these lines for a 4 km s-1 channel width.
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J. B. R. Oonk, R. J. van Weeren, F. Salgado, L. K. Morabito, A. G. G. M. Tielens, et al.. Discovery of carbon radio recombination lines in absorption towards Cygnus A. Monthly Notices of the Royal Astronomical Society, Oxford University Press (OUP): Policy P - Oxford Open Option A, 2014, 437 (4), pp.3506-3515. ⟨10.1093/mnras/stt2158⟩. ⟨insu-01284745⟩

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