4 The continuum

The continuum image has been made by averaging the channels free of line emission. This image has been cleaned to below the noise level and smoothed to a $30\arcsec \times 30\arcsec$ resolution. Even then the emission level at the position of the galaxy is low; the peak emission is at six times the noise level of 0.18 K. The image is displayed in Fig. 7 in greyscale and in contours superposed on the optical image. Despite the low level of emission a few associations between continuum and the optical picture can be made. There is a continuum enhancement at the position of the bulge. Furthermore, continuum is associated with spiral arms especially those at the South West side. A strong unresolved source with a flux of 1.5 mJy can be seen in the South East on a spiral arm. This source might originate from a star formation region though an association with an optical counterpart is not obvious. It is certainly too bright to be produced by a single supernova remnant. An alternative explanation is a background source. The total continuum flux of the galaxy amounts to 43.2 mJy being quite normal for galaxies of the size of NGC 3992.

Figure 7: Cleaned continuum map of NGC 3992 at a resolution of $30\arcsec \times 30\arcsec$. The continuum emission is at a low level and consequently has a noisy appearance. Top: a contour and greyscale map. Contour levels are at 1.5, 3, 4.5, and 6 times the rms noise level of 0.18 K. Bottom: as a contour map superposed on the optical image. The total continuum flux of NGC 3992 amounts to 43.2 mJy.