3.1.2. Radio continuum
NGC 1365 was observed in the radio continuum with the Parkes 64-m single dish telescope at the wavelengths of 21 and 74 cm by Mathewson and Rome (1963). It has since been observed at a number of radio wavelengths from 2 cm to 1.87 m (see Harnett 1987 for references). A low-resolution map obtained with the Very Large Array (VLA) of the National Radio Astronomy Observatory at 20 cm has been presented by Condon (1987).
The nucleus of the galaxy was mapped with the VLA at 20, 6 and 2 cm by Sandqvist et al. (1982, 1995), at 20 and 6 cm by Saikia et al. (1994) and with the Australia Telescope Compact Array at 6 and 3 cm by Forbes and Norris (1998). A number of nuclear sources were detected in this region. We will return to those observations in Section 4.4. Bosma et al. (1983) report faint continuum emission extending over the bar region at a wavelength of 21 cm.
Beck and Shoutenkov (1999) have recently observed NGC 1365 in the 3.5-cm continuum with the VLA CnD-array and report the detection of polarized radio emission. Their map is reproduced in Fig. 7. The polarization arises in non-thermal synchrotron radiation due to relativistic electrons circulating in an ordered large-scale interstellar magnetic field. As Faraday rotation is very small at this wavelength, the B-vectors are almost perfectly aligned with the magnetic field.
Figure 7. Map of NGC 1365 in the radio continuum at a wavelength of 3.5 cm. The contour level n corresponds to 0.03 x 2n-1 mJy/beam. For the polarization vectors (B-vectors) a length of 1' corresponds to a polarized intensity of 0.3 mJy/beam. The B-vectors are closely aligned with the magnetic field. Courtesy R. Beck and V. Shoutenkov.
The radio arms are seen to follow closely the optical spiral arms with intensity maxima coinciding with prominent H II regions, while the polarization seems to be more associated with the dust lanes. Beck and Shoutenkov point out that in typical normal galaxies there are generally regular magnetic fields associated with dust lanes, but the dominating turbulent field (and thus the total synchrotron emission) is concentrated in regions of high gas density and star formation. The authors estimate the spectral index in the spiral arms to be -0.8 and conclude that most of the radio emission is nonthermal. They note as an interesting feature that the strongest regular field (outside the nuclear region) occurs at 1' SE of the nucleus, well inside the optical and total 3.5-cm intensity spiral arm, which according to the authors indicates that the magnetic field may be strong and regular also in regions with low gas density. This region is upstream of the shock front in the bar and the authors suggest that the feature may be the result of fast inflow of gas. The magnetic field is here closely aligned with the faint dust lanes one finds across the bar and throughout the central body of the galaxy. When running into the shock front at the leading edge of the bar the B-vectors turn by almost 90° following the pattern of the dust lanes and the flow lines of the gas as shown by the numerical simulations (Section 3.4). The observed properties of the magnetic field are similar to, but less pronounced, than the case of the barred galaxy NGC 1097 (Beck et al. 1999).
An unresolved point source, which does not correspond to any optical feature, is seen 1'.6 NE of the nucleus. This source was interpreted by Sandqvist et al. (1982) as a backgound source and the new observations confirm its non-thermal spectral index.