The large scattering disk of NGC 6334B

At centimeter wavelengths, the angular size of NGC 6334B increases as the square of the wavelength suggesting that plasma scattering broadens the image. This source, which has a size of 3" at 20 cm wavelength, has the largest known scattering disk. We present spectral line observations of neutral hydrogen and continuum observations at 20, 6, and 2 cm wavelength made with the VLA that improve our knowledge of this unusual source and its scattering agent. We obtained the following results. (1) The measurements of H I in absorption indicate that NGC 6334B is much more distant than the NGC 6334 H II region-molecular cloud complex. NGC 6334B is probably an extragalactic source whose line of sight passes through the star-forming region. (2) The continuum flux density measurements confirm the time variability of the source, which suggests that the intrinsic angular size of the source is much smaller than its observed angular size. (3) The fringe visibility as a function of baseline length at 6 cm wavelength is accurate1y modeled by a Gaussian function, as expected for scattering by a turbulent plasma with a Gaussian spectrum or with a power-law spectrum and spectral index of 4 or an arbitrary index with an inner length scale greater than 35 km. (4) The northern lobe of the bipolar H II region NGC 6334A, which is centered about 2' south of NGC 6334B, covers the line of sight to NGC 6334B and may be responsible for some or all of the scattering.