ar X iv : a st ro - p h / 96 04 06 2 v 1 1 2 A pr 1 99 6 Radio Continuum and Molecular Gas in the Galactic Center

Nonthermal radio emission in the galactic center reveals a number of vertical structures across the galactic plane, which are attributed to poloidal magnetic field and/or energetic outflow. Thermal radio emission comprises star forming regions distributed in a thin, dense thermal gas disk. The thermal region is associated with dense molecular gas disk, in which the majority of gas is concentrated in a rotating molecular ring. Outflow structures like the radio lobe is associated with rotating molecular gas at high speed, consistent with a twisted magnetic cylinder driven by accretion of a rotating gas disk. 1 1 Radio Continuum Emission 1.1 Flat Radio Spectra The radio emission from the Galactic Center is a mixture of thermal and nonthermal emissions. The conventional method to investigate the emission mechanism is to study the spectral index, either flat (thermal) or steep (nonthermal). Howerver, the spectral index in the central 3 region has been found to be almost everywhere flat (Sofue 1985), even in regions where strong linear polarization has been detected. Therefore, a flat spectrum observed near the galactic center can no longer be taken as an indicator of thermal emission. 1.2 Infrared-to-Radio Intensity Ratio Separation of thermal and nonthermal radio emission can be done by comparing far-IR (e.g. 60 μm) and radio intensities (both in Jy/str): thermal (HII) regions have high IR-to-radio ratio, R = IFIR/IR ≃ 10 , while nonthermal regions have small IR-to-radio ratio, R < 0 ∼ 300. Using this method, thermal and nonthermal emission regions have been distinguished in a wide area (Reich et al 1987). The region near the galactic plane is dominated by thermal emission and many To appear in proceedings of Nobel Symposium 98: “Barred galaxies and circumnuclear activity”, Saltsjvbaden, Stockholm Obs., 30 Nov 3 Dec 1995, ed. Aa.Sandqvist