2.5. The Center of the Milky Way Galaxy's disk (~ 8 kpc away)
The center of the Milky Way is roughly 8 kpc away from the Sun. Within 500 pc of the galactic center (i.e., within an angular radius of about 3.6 degrees), Spergel & Blitz (1992) predicted that there exist a large volume of space with high gas pressure, and a magnetic field strength close to 130 µGauss (magnetic pressure in equilibrium with the gas pressure).
Within 100 pc of the galactic center (i.e., within a radius of about 40 arc min), large and highly organized radio filaments or 'arcs' are found (Yusef-Zadeh et al., 1984; Yusef-Zadeh & Wardle, 1993; Tsuboi et al., 1995). The radio emission shows that the nonthermal emitting gas is arranged along magnetic filaments (poloidal component). The magnetic field lines are parallel to the long axis of the vertical filaments, and perpendicular to the circumnuclear disk, with a strength of order 1 mG (i.e., 1000 µGauss). Morris (1996) argued that the magnetic field of about 1000 µGauss cannot be localized to the filaments unless it can be confined by external pressure, and prefers a model where the filamentary structure is caused by a locally intermittent population of relativistic electrons in a large volume filled with a strong magnetic field.
An alternative model for the filaments is that the magnetic field is localized near the filaments over a small volume of space, and B is negligible outside. Uchida & Güsten (1995) have also studied several molecular clouds within 100 pc of the Galactic center, using the Zeeman effect in the OH line. They put an upper limit (rms) of 100 µGauss (outside of the filaments). They then argue that the magnetized filaments are lo calized and contained in bundles of magnetic fields over a small volume of space, and that the rest of the volume is at best pervaded by a diffuse magnetic field below 100 µGauss within 100 pc of the galactic center. A third model is where the filaments lie in a compressed, cylindrical ring surrouunding the galactic center over a small volume of space (e.g., Davidson 1996). Future trends: Clearly more data, not more theories, are most needed at this point, to find out if strong B values are found in a large or a small volume of space.
A similar magnetized filament or giant lobe has been discovered in the nucleus of the galaxy M 81, using the Very Large Array in New Mexico to observe the Stokes parameter at wavelengths of 6 cm and 20 cm. Kaufman et al. (1996) found a highly polarized non-thermal arc with a length of about 1 kpc and a width of about 0.2 kpc, located about 0.8 kpc northeast of the galactic center. Its minimum-energy magnetic field strength is 10-20 µGauss, and the magnetic field lines are aligned along the arc, It may be a large scale version of the filament or giant lobe found in our Milky Way galaxy.
Within 3 pc of the galactic center, there is a 2'-diameter circumstellar molecular ring around the center of the galaxy. Using Far IR dust polarimetry, Hildebrand et al. (1990; 1993) have found a magnetic field running along the circular ring, or at some small angle, drawn in the azimuthal/toroidal direction by differential rotation. HI Zeeman observations of this 2' ring have given conflicting results: less than 500 µGauss in both the South and North parts (Marshall et al., 1995); less than 1500 µGauss in the S but 500 µGauss in the N (Schwarz & Lasenby, 1990); ~ 600 µGauss in the N (Plante et al., 1995), ~ 2000 µGauss in maser spots (Yusef-Zadeh et al., 1996). In addition, some infalling gas (ionized streamers) seen near the N part of the circumnuclear ring has a magnetic field strength ~ 3 mGauss (Plante et al., 1995).
Within 1 pc of the galactic center, there is a 0.5' mini-spiral of gas with several arms, close to Sagittarius A*. Upper limits of 8 mGauss have been put on the magnetic field strength in the mini-spiral's northern area (e.g., Roberts & Goss 1993; Roberts et al. 1991). For a recent review of the galactic center environment, see Morris & Serabyn (1996).