High-resolution radio continuum survey of M 33 - I. The radio maps

Aims. We study the exponential scale length of total radio emission, the spectral index distribution, and the linear radio polarization in the Scd galaxy M 33. Methods. Observations were carried out using the 3.6 cm dual channel and the 6.2 cm four-channel receivers of the 100-m Effelsberg telescope along with the L-band VLA D-array at 20 cm. Results. High spatial resolution and sensitivity in both total and linearly polarized radio continuum emission from M 33 were achieved. We found considerable extended emission, not only from the main arms I S and IN, but also from the weaker arms. The large-scale magnetic field exhibits well-ordered spiral structure with almost the same orientation as that of the optical spiral arms; however, it does not show a clear structural correlation or anti-correlation with the optical arms. There is a north-south asymmetry in polarization that is frequency-dependent. We found that the ring mean spectral index versus radius increases faster beyond R = 4 kpc. At each wavelength, the exponential scale length is larger inside than outside R = 4 kpc. Conclusions. From the larger scale lengths at R < 4 kpc, we conclude that star-forming regions are mainly spread over the region R < 4 kpc without a dominant nuclear concentration. Furthermore, at R < 4 kpc, a spatial correlation between cosmic rays and star-forming regions may exist. From the behavior of the mean spectral indices obtained from different pairs of the radio continuum data at 3.6, 6.2, and 20 cm, we confirm that a decrease in the thermal fraction causes an increase in the spectral index. The frequency-dependent asymmetry in the polarization hints at asymmetry in Faraday depolarization.