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, 31) , 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 (32) , S. Yatawatta (29) , P. Zarka (33, 31) , A. Zensus (5)

1 ASTRON - Netherlands Institute for Radio Astronomy
2 CfA - Harvard-Smithsonian Center for Astrophysics
3 Leiden Observatory [Leiden]
4 Department of Physics and Astronomy [Milton Keynes]
5 Max Planck Institute for Astrophysics
6 SRON - SRON Netherlands Institute for Space Research
7 MPIFR - Max-Planck-Institut für Radioastronomie
8 SIfA - Sydney Institute for Astronomy
9 Department of Physics-Electronics
10 University of Edinburgh
11 UHH - Universität Hamburg
12 AIP - Leibniz-Institut für Astrophysik Potsdam
13 School of Physics and Astronomy [Southampton]
14 OSO - Onsala Space Observatory
15 Radboud University [Nijmegen]
16 Medstar Research Institute
17 TLS - Thüringer Landessternwarte Tautenburg
18 Hamburger Sternwarte/Hamburg Observatory
19 LAGRANGE - Joseph Louis LAGRANGE
20 LPC2E - Laboratoire de Physique et Chimie de l'Environnement et de l'Espace
21 Jodrell Bank Centre for Astrophysics
22 UvA - University of Amsterdam [Amsterdam]
23 Ruhr-Universität Bochum [Bochum]
24 Oxford Astrophysics
25 KIT - Karlsruhe Institute of Technology
26 AI PANNEKOEK - Astronomical Institute Anton Pannekoek
27 CRAL - Centre de Recherche Astrophysique de Lyon
28 Universität Bielefeld
29 Kapteyn Astronomical Institute [Groningen]
30 Rhodes University, Grahamstown
31 LESIA - Laboratoire d'études spatiales et d'instrumentation en astrophysique
32 AlfA - Argelander-Institut für Astronomie
33 OP - Observatoire de Paris - Site de Paris

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.

Keywords: ISM: general ISM: structure local interstellar matter radio lines: ISM

Domains: Cosmology and Extra-Galactic Astrophysics [astro-ph.CO]

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, 2014, 437 (4), pp.3506-3515. ⟨10.1093/mnras/stt2158⟩. ⟨insu-01284745⟩