Low Frequency Carbon Recombination Lines in the Southern Galaxy

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Session 29 -- General Interstellar Medium
Display presentation, Tuesday, 31, 1994, 9:20-6:30

[29.09] Low Frequency Carbon Recombination Lines in the Southern Galaxy

W.C.Erickson (UMD), D.McConnell (ATNF), K.R.Anantharamaiah (RRI)

We have used the Parkes 64m radio telescope at 75 MHz to study Carbon recombination lines in a number of directions towards the inner regions of the Galaxy. The Parkes 16384-channel spectrometer allowed seven adjacent recombination-line transitions from $438\alpha$ to $444\alpha$ to be observed simultaneously in two polarizations, greatly increasing the sensitivity through long effective integrations. At this frequency the beamwidth of the telescope is $4\deg$ and we made observations along the Galactic Plane at approximately one beamwidth intervals from l = $348\deg$ to l = $16\deg$. Absorption lines were detected in all of these directions. Both the $\alpha$ and $\beta$ transitions were detected and the radial velocities given by the two transitions agree with each other in every case. Effects of differential galactic rotation are observed and yield kinematic distances of approximately 2 kpc to the line-forming regions, which are presumably cold clouds in the Sagittarius arm. The line profiles are much narrower than HI profiles in the same directions but much wider than higher frequency Carbon emission lines which are generated partially ionized regions adjacent to HII regions. The linewidths increase smoothly from 6 km/s at l = $348\deg$ to 23 km/s at l = $16\deg$. These observations essentially double the number of directions in which low frequency recombination lines have been detected. They suggest that the lines would be detectable in other directions along the Galactic Plane if there existed strong enough background radiation to illuminate the foreground clouds and if sufficient sensitivity could be obtained through long effective integrations. We made observations in a number of other directions; in some of them the background emission was weaker and in others the data were seriously degraded by interference, so no lines were detected in these other directions. Further observations are scheduled in April, 1994.

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