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2.2. Magnetic field strengths

The average and maximum nonthermal and polarized intensities at lambda6 cm and at 3' resolution were used to compute the average and maximum equipartition strengths of the total field and its regular component, assuming a ratio of cosmic-ray protons to electrons of 100 and pathlengths through the emitting regions of 3 kpc and 2 kpc (taking into account the different inclinations). Equipartition is probably valid on average over the ring of M31 but invalid within the ring (Sect. 3.2), so the maximum field strengths are lower limits. The results are summarized in Table 3. The two galaxies are similar in most of their radio continuum properties. The main differences between M31 and M33 are the morphology (the "ring" in M31, a diffuse disk in M33) and the degree of regularity of the magnetic field which is much higher in M31. Corrected for thermal contribution, the nonthermal degree of polarization at lambda6 cm at 3' resolution becomes 33% in M31, only 10% in M33. No other spiral galaxy reveals a degree of polarization as high as M31, even when observed with higher spatial resolution.

Table 3.. Average radio continuum properties

M31 M33

Spectral index of the integrated emission 0.76 ± 0.04 0.75 ± 0.10
Nonthermal spectral index 1.0 ± 0.1 0.9 ± 0.1
Thermal fraction at lambda6 cm appeq 35% appeq 30%
Average total field strength 6 ±2 µG 6 ± 2 µG
Maximum total field strength geq 8 ± 2 µG 9 ± 2 µG
Average regular field strength 4 ± 1 µG 2 ± 1 µG
Maximum regular field strength geq 6 ± 1 µG 4 ± 1 µG
Scale length of the disk at lambda6 cm 3.3 ± 0.1 kpc* 2.3 ± 0.1 kpc

*beyond 10 kpc radius

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