Carbon Recombination Lines from the Galactic Plane at 34.5 & 328 MHz

Abstract

We present results of a search for carbon recombination lines in the Galaxy at 34.5 MHz (C$575\alpha$) made using the dipole array at Gauribidanur near Bangalore. Observations made towards 32 directions, led to detections of lines in absorption at nine positions. Followup observations at 328 MHz (C$272\alpha$) using the Ooty Radio Telescope detected these lines in emission. A VLA D-array observation of one of the positions at 330 MHz yielded no detection implying a lower limit of 10' for the angular size of the line forming region. The longitude-velocity distribution of the observed carbon lines indicate that the line forming region are located mainly between 4 kpc and 7 kpc from the Galactic centre. Combining our results with published carbon recombination line data near 76 MHz (\nocite{erickson:95} Erickson \et 1995) we obtain constraints on the physical parameters of the line forming regions. We find that if the angular size of the line forming regions is $\ge 4^{\circ }$, then the range of parameters that fit the data are: \Te $=20-40$ K, \ne $\sim 0.1-0.3$ \cm3 and pathlengths $\sim 0.07-0.9$ pc which may correspond to thin photo-dissociated regions around molecular clouds. On the other hand, if the line forming regions are $\sim 2^{\circ }$ in extent, then warmer gas (\Te $\sim 60-300$ K) with lower electron densities (\ne $\sim 0.03-0.05$ \cm3) extending over several tens of parsecs along the line of sight and possibly associated with atomic \HI gas can fit the data. Based on the range of derived parameters, we suggest that the carbon line regions are most likely associated with photo-dissociation regions.

To appear in Journal of Astrophysics & Astronomy, March 2001

Cited by in corpus (10)

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• GMOSS: All-sky model of spectral radio brightness based on physical components and associated radiative processes
• 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
• Discovery of Carbon Radio Recombination Lines in absorption towards Cygnus~A
• Radio Recombination Lines at Decametre Wavelengths: Prospects for the Future
• Mapping low frequency carbon radio recombination lines towards Cassiopeia A at 340, 148, 54 and 43 MHz
• Exact Bound-Bound Gaunt Factor Values for Quantum Levels up to n=2000
• Low-Frequency Radio Recombination Lines Away From the Inner Galactic Plane