Anisotropic Radio Scattering of NGC 6334B

Abstract

The extragalactic radio source NGC 6334B is the most strongly scattered object known, with an angular size of 3" at 20 cm wavelength. Its angular size scales as the square of the observing wavelength, and its phase structure function has a power-law dependence on projected baseline length. These properties suggest that NGC 6334B is a point source, probably a quasar, scatter broadened by an intervening region of turbulent plasma. The line of sight to the source intercepts the northern lobe of the bipolar H II region NGC 6334A, which is probably the source of the scattering. We discuss observations of NGC 6334B with the VLA at 1.4, 2, 6, 20, and 90 cm, and with the VLBA at 1.4, 3.6, and 6 cm. The VLA observations indicate that the scattering disk of NGC 6334B is anisotropic, with an axial ratio of ~1.2 at 20 and 6 cm, and ~1.5 at 2 cm. This anisotropy is probably due to the effects of a magnetic field in the scattering medium. The position angle of the scattering disk implies that the mean projected magnetic field direction is parallel to the outflow from NGC 6334A. The position angle of the scattering disk rotates slightly between 6 and 2 cm; we use this rotation to place an upper limit on the outer scale of turbulence, rout <3 sigma ) fringes in interferometric data. Detections of low-level excess visibility on long VLBA baselines at 3.6 cm place an upper limit on the inner dissipative scale of turbulence rin of 1011-12 cm, while the baseline scaling of fringe visibility amplitude and the wavelength scaling of scattering-disk size place a lower limit on rin of 5 x 106 cm. Nearby high-velocity (-80 km s-1) H2O maser features show little sign of broadening, implying that they are in front of, or less than 100 pc behind, the scattering region.