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