The Large-Scale Magnetic Field Structure Near the Galactic Centre

Abstract

ABSTRACT.High frequency polarization observations reveal the existence of a poloidal magnetic field structure in the Galactic Centre region on scales of about 200 pc. At lower frequencies large non-thermal spurs are seen tracing the magnetic field up to kpc distances from the Galactic Centre.The well known mixture of thermal and non-thermal emission structures close to the Galactic Centre region requires high frequency observations to trace the magnetic field structure. Fig. 1 shows a preliminary 32 GHz map observed with the Effelsberg 100-m telescope of the "arc" region. This is a bar-like structure running almost perpendicular to the Galactic plane at a distance of about 0*2 from Sgr A. 32 GHz is a frequency high enough that rotation measures of several thousand rad m~2 are required to cause a significant rotation of the observed polarization angles from the intrinsic magnetic field direction. In Fig. 1 a very regular magnetic field structure is seen running along the arc with polarization percentages close to 50% (for -5' ^ b ^ -10'). This percentage polarization is near to the intrinsic value of about 60% for the non-thermal flat spectrum emission seen in this region (Reich et al., 1988a). The equipartition magnetic field strength is about 135 ßG. Strong thermal filaments crossing the arc at b = -2* (Yusef-Zadeh et al., 1989) cause complete depolarization. North of this region the percentage polarization is about 30%. This implies the existence of a non-thermal component with a fraction of at least 60% of the total emission, beside a weaker thermal component as indicated by recombination line observations. A rotation measure of about -10000 rad m" 2 is expected for this region for the case that the magnetic field direction continues to run parallel to the arc structure.The arc connects two large highly polarized plumes north and south of the Galactic plane. These have been observed with the 100-m telescope at 10.7 GHz (Seiradakis et al., 1985; as shown in Fig. 1. From two rotation measure determinations (Sofue et al., 1987;Tsuboi et al., 1986) the magnetic field direction was found to run perpendicular to the Galactic plane. This has been taken as evidence for the existence of a poloidal magnetic field component in the Galactic Centre region on