Discovery of Carbon Radio Recombination Lines in absorption towards Cygnus~A

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 to 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 $\alpha$ 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$\times$10${}^{-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 $T_e\sim$ 110 K and density $n_e\sim$ 0.06 cm${}^{-3}$. These properties imply that the observed carbon $\alpha$ 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 3$\sigma$ peak optical depth limit of 1.5$\times$10${}^{-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 3$\sigma$ upper limit of 1.5$\times$10${}^{-4}$ for the peak optical depth of these lines for a 4 km s${}^{-1}$ channel width.

In press

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Cited by in corpus (14)

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• Low Frequency Carbon Radio Recombination Lines I: Calculations of Departure Coefficients
• Discovery of Carbon Radio Recombination Lines in M82
• LOFAR observations of decameter carbon radio recombination lines towards Cassiopeia A
• Low Frequency Carbon Radio Recombination Lines II: The Diffuse Interstellar Medium
• Mapping low frequency carbon radio recombination lines towards Cassiopeia A at 340, 148, 54 and 43 MHz
• The first detection of radio recombination lines at cosmological distances
• Searching for the largest bound atoms in space
• Pipeline Collector: gathering performance data for distributed astronomical pipelines
• Spectroscopy with the Engineering Development Array: cold H${}^{+}$ at 63 MHz towards the Galactic Centre
• Evolution of the cold gas fraction and the star formation history: Prospects with current and future radio facilities
• Observations of Carbon Radio Recombination Lines with the NenuFAR telescope. I. Cassiopeia A and Cygnus A