Discovery of Carbon Radio Recombination Lines in M82

TL;DR

This paper reports the first extragalactic detection of carbon radio recombination lines in M82 at low frequencies, revealing cold atomic gas in the galaxy's nucleus through radio observations with LOFAR.

Contribution

It presents the first detection of carbon RRLs outside the Milky Way at frequencies below 1 GHz, using a novel stacking method to identify faint signals.

Findings

Detected carbon RRLs in M82 at 48-64 MHz.

Determined a radial velocity of 219 km/s for the lines.

Associated the lines with cold atomic gas in M82's nucleus.

Abstract

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.