Discovery of a nonthermal galactic center filament (G358.85+0.47) parallel to the galactic plane

摘要

We report the discovery of a new nonthermal filament, G358.85+0.47, the ``Pelican,'' located ~225 pc in projection from Sagittarius A and oriented parallel to the Galactic plane. VLA continuum observations at 20 cm reveal that this 7' (17.5 pc) structure bends at its northern extension and is comprised of parallel strands, which are most apparent at its ends. Observations at 6 and 3.6 cm reveal that the Pelican is a synchrotron-emitting source and is strongly linearly polarized over much of its extent. The spectral index of the filament changes from alpha_20/6=-0.8 to alpha_6/3.6=-1.5. The rotation measures exhibit a smooth gradient, with values ranging from -1000 to 500 rad m^-2. The intrinsic magnetic field is well aligned along the length of the filament. Based on these properties, we classify the Pelican as one of the nonthermal filaments unique to the Galactic center. Since these filaments (most of which are oriented perpendicular to the Galactic plane) are believed to trace the overall magnetic field in the inner Galaxy, the Pelican is the first detection of a component of this field parallel to the plane. The Pelican may thus mark a transition region of the magnetic field orientation in the inner 1 kpc of the Galaxy.