Magnetic fields in galaxies can be traced by radio polarization measurements.
We present a high-resolution study of the magnetic field structure, strength,
and energy density in the Scd galaxy M33 using the linearly polarized
intensity and polarization angle data at 3.6, 6.2 and 20cm along with a 3D
modeling of the regular magnetic field.
The average total and regular magnetic field strengths in M33 are ~6.4 and 2.5
micro Gauss, respectively. Under the assumption that the disk of M33 is flat,
the regular magnetic field consists of both horizontal and vertical
components: however the inferred vertical field may be partly due to a
galactic warp. The horizontal field is represented by an axisymmetric (m=0)
mode from 1 to 3kpc radius and a superposition of axisymmetric and
bisymmetric (m=0+1) modes from 3 to 5 kpc radius. The presence of a m=0 mode
of the regular magnetic field in each ring suggests that a galactic dynamo is
operating in M33.
The energy densities of the magnetic field and turbulence are about the same,
confirming the theory of generation of interstellar magnetic fields from
turbulent gas motions. Furthermore, the ISM in M33 can be characterized by a
low-beta plasma and dominated by a supersonic turbulence, as the energy
densities of the magnetic field and turbulence are both higher than the
thermal energy density.
It is also shown that the north-south asymmetry in the observed wavelength
dependent depolarization can be explained by an excess of differential
Faraday rotation in the southern half of M33 together with strong Faraday
dispersion in the southern spiral arms.