Discovery of Carbon Radio Recombination Lines in M82

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-α 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.

6 pages, 5 figures, accepted for publication in ApJL

References in corpus (5)

• The LOFAR radio environment
• Radio recombination lines from the largest bound atoms in space
• LOFAR detections of low-frequency radio recombination lines towards Cassiopeia A
• Detection of the H92alpha recombination line from the starbursts in the Circinus galaxy and NGC 1808
• High-velocity-resolution observations of OH main line lasers in the M82 starburst

Cited by in corpus (14)

• The LOFAR Two-metre Sky Survey - I. Survey Description and Preliminary Data Release
• A Space-Based Observational Strategy for Characterizing the First Stars and Galaxies Using the Redshifted 21-cm Global Spectrum
• Subarcsecond international LOFAR radio images of the M82 nucleus at 118 MHz and 154 MHz
• Compression of interferometric radio-astronomical data
• Science with the Murchison Widefield Array: Phase I Results and Phase II Opportunities
• Low Frequency Carbon Radio Recombination Lines I: Calculations of Departure Coefficients
• LOFAR observations of decameter carbon radio recombination lines towards Cassiopeia A
• Carbon radio recombination lines from gigahertz to megahertz frequencies towards Orion A
• Low Frequency Carbon Radio Recombination Lines II: The Diffuse Interstellar Medium
• PDRs4All VII. The 3.3 $\mu$m aromatic infrared band as a tracer of physical properties of the ISM in galaxies
• H-, He-like recombination spectra III: $n$-changing collisions in highly-excited Rydberg states and their impact on the radio, IR and optical recombination lines
• Searching for the largest bound atoms in space
• H, He-like recombination spectra IV; H, He-like recombination spectra IV: clarification and refinement of methodology for $l$-changing collisions
• Observations of Carbon Radio Recombination Lines with the NenuFAR telescope. I. Cassiopeia A and Cygnus A