3.3.3. Strength of Breg
How strong is the galactic magnetic field, and how does it change with increasing galactic radius ? Observations of B(r) already exist, and can be compared with magnetic field models. The observational data from the radio RM observations of QSO and galaxies (e.g., Vallée 1991b) and those from the RM observations of pulsars (e.g., Rand & Lyne 1994) show a non-periodic, non-cosine variation of B(r) with radius, between 5 and 10 kpc.
Figure 8 here shows the run of B(r) versus the galactic radius. The radio RM observations are shown by open squares (from QSO & galaxies) and stars (from pulsars). The dynamo theories adapted to the Milky Way (e.g., Fig. 4a in Poezd et al. 1993) confirm a non-periodic variation of B(r) with radius, similar to the observations. As can be seen, there is a clear agreement from the two sets of data (QSO and pulsars). Rand (1994) has shown that RM analysis of pulsar data have confirmed well (but later on) many of the results of the RM analyses of QSO and galaxies data. There have been recent claims that the pulsar RM data are not the best indicator for the large-scale galactic magnetic field (Heiles, 1996b; Heiles, 1995).
Figure 8. The run of Bu(r), the uniform component of the galactic magnetic field, as a function of the galactic radius rgal, in the Milky Way. The Sun is shown at rgal = 8 kpc. Open circles: B from RM of quasars and galaxies (from Vallée, 1991b). Stars: B from RM of pulsars (from Rand & Lyne, 1994).
Recent claims were made by Nelson (1988), Battaner et al. (1992), Battaner (1993), and Binney (1992) that there exists a strong magnetic field which could speed up the interstellar gas (but not the stars), to the point of reproducing the flat curve of rotational velocity against galactic radius. These claims were investigated and found unsupportable. Observationally, Vallée (1994c) found that in the Milky Way (and in M31) the required magnetic field strength for magnetic support of the gas was too high by at least a factor 2, as compared to the weaker observed magnetic field strengths. Also, when the newly-born stars have formed from the interstellar gas and clouds (moving at 260 km/s), they will magnetically decouple from the interstellar medium and clouds, and will behave balistically with their initial input velocities (= cloud velocities). These initial balistic velocities of newly-formed stars (about 260 km/s) are thus much higher than those of the older stars (alleged to be moving at 160 km/s, following a Keplerian curve). Such a 100 km/s deceleration has not been properly worked out, via stellar encounters on a short time scale or otherwise, in the magnetic-support theory. Other investigators also found problems for the magnetic-support theory, as pointed out by Melrose (1995), Katz (1994), and Jokipii & Levy (1993).
Figure 9 shows the directions of the regular magnetic field over the Milky Way. For simplicity, the magnetic field deviations due to the superbubbles have been omitted.
Figure 9. Face-on view of the whole area of the Milky Way disk. Arrows show the direction of the galactic magnetic field lines. For simplicity, the nearby superbubbles have been omitted. Four spiral arms of stars are shown. The Sun is shown (circled dot). Going radially inward from the Sun to the Galactic Centre, one encompasses two magnetic field reversals (shown with dashes). Little or nothing is known of the area behind the Galactic Center ("Zona Galactica Incognita"). See Vallée (1996) for more details.