Low-frequency carbon recombination lines in the central regions of the galaxy

Abstract

We have used the Parkes 64 m telescope to study low-frequency carbon recombination lines from the southern portion of the Galactic plane in a frequency band 4 MHz wide centered at 76.4 MHz. We have found a very large line-forming region that extends for approximately 40 in Galactic longitude from 1 = 340° to l = 20°. The region is several degrees wide in latitude. From the variation of radial velocity and line width with Galactic longitude we obtain kinematic distances to the line-forming region in the range 0.5-4 kpc. This places the line-forming region in the Sagittarius and/or Scutum arms. We also find the lines approximately tangent to the Scutum arm at 1 = 312 . By observing the C441α, C555β, and C635γ lines, which all occur in the analyzed band, we estimate the variation of line strength with principal quantum number. This variation appears to be similar to that of the well-observed Perseus arm absorption in the direction of Cas A and suggests that a model for the Perseus arm line-forming region may be at least partially applicable to the Sagittarius/Scutum arm regions. The most likely sites for the formation of these lines are the cold neutral H I concentrations in the interstellar medium. We have also searched the data for other lines and, in particular, we find no trace of hydrogen recombination lines.