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3. THE 10 KPC "RING" OF M31

3.1. Magnetic field structure

Radio continuum emission tells us about the distribution of magnetic fields and cosmic rays. Figure 1 shows the total and polarized intensities of M31 at lambda6 cm. The "ring" at about 10 kpc radius is well defined in both components. In total emission the similarity to the ring of CO emission (Nieten et al., this volume) and dust emission (Haas, this volume) is striking and gives further evidence that magnetic fields are anchored in gas clouds (Berkhuijsen et al., 1993).

Figure 1

Figure 1. Total intensity of M31 at lambda6.2 cm, observed with the Effelsberg 100-m telescope, smoothed to 3' beamsize. The lengths of the vectors are proportional to the polarized intensities, their orientations have been corrected for Faraday rotation by using the lambda11.1 cm Effelsberg data at 5' resolution (Berkhuijsen et al., in prep.)

The general coincidence of total and polarized emission regions in the ring is unusual. In all other spiral galaxies observed so far, polarized intensity is strongest between the spiral arms as traced by total emission, but weak in the arms due to field tangling by star-formation activity (Beck et al., 1996). Star formation activity is probably too weak to tangle M31's regular field significantly.

The variations of total nonthermal intensity (corrected for thermal emission) and polarized intensity with azimuthal angle (Fig. 2) reveal peaks near the minor axis and minima near the major axis, while the unpolarized nonthermal intensity shows almost no azimuthal variation. In polarized intensity, the peaks are roughly symmetric and the minima are deep, as expected from polarized emission from a strongly inclined toroidal field. Polarized synchrotron emission traces the component of the regular field perpendicular to the line of sight which is largest near the minor axis and smallest near the major axis of the galaxy's projected disk. Total synchrotron emission also depends on irregular fields which lead to unpolarized emission filling the minima of polarized emission near the major axis.

Polarized intensity varies as:

Equation 1

where NCRE is the number density of cosmic-ray electrons in the relevant energy range, alphant is the nonthermal spectral index and phi is the viewing angle between the field and the line of sight. In case of equipartition between cosmic rays and magnetic fields, NCRE propto Btot2.

For an axisymmetric spiral field with a pitch angle p:

Equation 2

where Theta is the azimuthal angle in the plane of the galaxy and i is the galaxy's inclination. For M31, alphant appeq -1 (see above), p appeq -12° (Fletcher et al., this volume) and i appeq 78°.

The variation of ln Ip with ln | sinphi | was computed between 8 and 12 kpc radius 1. In three quadrants the slopes are around 2, a flatter slope of 1.3 was found only in the NW quadrant. The slope of appeq 2 means that the variation of Ip can be described by the variation in phi due to geometry while NCRE and Breg are roughly constant along the ring. Locally in the ring, the nonthermal degree of polarization increases to appeq 50%. The magnetic field of M31 is exceptionally regular.

Figure 2a
Figure 2b

Figure 2. Variation of nonthermal intensity (a) and polarized intensity (b) at lambda6.2 cm (3' resolution) between 8 and 12 kpc radius with azimuthal angle, counted counter-clockwise in the plane of M31, starting from the northeastern major axis.

1 based on the "old" distance of 690 kpc, for better comparison with previous results. Back.

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