The origin and stability of strong vertical magnetic fields in the central 150 pc of the Galaxy.

1 Benjamin D. G. Chandran

1 Department of Physics & Astronomy, University of Iowa, Iowa City, IA 52242

E-mail contact:

See: ApJ, 562, 737 (2001)

Observations of narrow radio-emitting filaments near the Galactic center may indicate a pervasive vertical (i.e. to the Galactic plane) milliGauss magnetic field in the central pc of the Galaxy. This presentation describes how a field of this orientation and strength can arise from inflow of plasma in the Galactic disk, and also how such a strong magnetic field could be confined. A simple cylindrically symmetric model for the equilibrium in this central region is proposed in which horizontal (i.e. to the Galactic plane) magnetic fields embedded in an annular band of partially ionized molecular material of radius pc are wrapped around vertical magnetic fields threading low-density hot plasma. The central vertical magnetic field, which has a pressure that significantly exceeds the thermal pressure of the medium, is confined by the weight of the molecular material. The stability of this equilibrium to a class of modes that perturb the interface between the high and low-density regions is studied, and it is found that the model equilibrium is stable provided , where is the effective gravity, is the angular velocity (assumed for simplicity to be uniform to allow easier treatment of the Rayleigh-Taylor dynamics of the interface), a is the sound speed in the dense plasma, and r is distance from the spin axis of the galaxy. Since the effective gravity is determined by the degree to which magnetic (and to a lesser degree pressure) forces support the dense plasma, the stability criterion gives an upper limit on the strength of the axial magnetic field, which is  mG for Galactic-center parameters.

Tom Geballe