Discovery of Carbon Radio Recombination Lines in M82
  Carbon radio recombination lines (RRLs) at low frequencies (<=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 LOFAR 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 +- 9 km/s . Using this radial velocity, we stack 22
carbon-{\alpha} transitions from quantum levels n = 468-508 to achieve an 8.5
sigma detection. The absorption line profile exhibits a narrow feature with
peak optical depth of 0.003 and FWHM of 31 km/s. 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.
