3.2. Magnetic fields and Spiral Arms (~ 15 kpc)
A quick survey of the literature from 1980 to 1995 shows the following: Of a dozen papers found on this subject, about 75% derived a spiral-type Milky Way with 4 spiral arms, and about 25% derived a spiral-type Milky Way with 2 spiral arms.
There is substantial non-thermal polarized emission from synchrotron radiating relativistic electrons trapped in the general interstellar magnetic field threading all spiral galaxies. Can the large scale magnetic field cross spiral arms in our Galaxy ? An early study of four nearby spiral arms suggested that the median pitch angle of the stellar arms (from O-type stars, and from HII regions there) was around 16° (rms = 6°), and the median pitch angle of the magnetic field lines in these arms (from quasars RM and radio galaxies RM, and from pulsars RM) was around 10° (rms = 6°) away inward from the circular tangent (e.g., Vallée 1988b). Both the magnetic field lines and the stellar arms follow a very similar spiral shape. Recent optical observations of nearby galaxies show that the magnetic field lines follow broadly the spiral arms, althought not exactly everywhere, such as in NGC1808 (Scarrott et al., 1993). Thus the determination of the magnetic field lines and directions should also give a determination of the spiral arms.
Many studies use only one type of spiral tracers. One such one-tracer study, by Beuermann et al. (1985), uses the galactic radio synchrotron emission at 408 MHz, with a spiral structure (4 arms, arm pitch angle taken as 13°). Another such one-tracer study, by Heiles (1996a), proposed a two-dimensional model of the stellar optical polarization data (data from stars not far from the sun), excluding stellar data in many regions (North Polar Spur; stars toward the galactic interior; Perseus and Taurus regions of star formation), and concluded that in the local neighbourhood of the sun the pitch angle was around -7° ± 4°. There are different bias in different spiral tracers.
A more recent study using all types of spiral tracers in the Milky Way (such as the dust, molecular gas, atomic gas, ionized gas, synchrotron gas, stars, and magnetic field), and weighting each result according to each tracer's limitations, found that the available evidence over the last 15 years indicated a mean spiral arm pitch angle of -12° ± 1° (radially inward), and that there are 4 main spiral arms (Vallée, 1995c).
Figure 7 shows the most likely locations of the spiral arms in the Milky Way, according to several different observational tests. The direction of the magnetic field lines is likely to follow the spiral arms, to a good approximation. The Sun is located in the local 'Orion spur', about halfway between the Perseus arm and the Sagittarius arm. A 12° pitch angle has been used (inward, starting from the circumference).
Figure 7. Face-on sketch of the Milky Way, showing 4 spiral arms drawn in logarithmic shapes. The pitch angle (12° inward) is obtained from a variety of different observational tests. Here all 4 arms start at a minimum rg = 3 kpc from the galactic center. Circular orbits around the galactic center are shown by dots at galactic radii rg = 1, 3, 5,..., 15 kpc. The position of the Sun at rg = 8 kpc is shown (circled dot) - the Sun is located in a local Orion spur (not shown). Little or nothing is known of the area behind the Galactic Center refered to here as our modern-times "Zona Galactica Incognita". See Vallée (1995c) for more details.
Historically, the observational search for the magnetic field strength and direction in and between various spiral arms in the Milky Way has progressed slowly, starting in 1949 via polarized dust observations at optical wavelengths. In more recent times, the most useful data are obtained at radio wavelengths, via Faraday rotation measure (RM) values (e.g., a review is given in Vallée 1983d).