4.4. Nuclear Radio Sources
NGC 1365 has been observed in the radio continuum at 20, 6 and 2 cm wavelength with the VLA at a number of configurations (Sandqvist et al. 1982, 1995; Saikia et al. 1994). It was observed by Forbes and Norris (1998) with the Australia Telescope Compact Array at 6 and 3 cm. The agreement between the maps presented by these different authors is very good. The 20-cm map by Sandqvist et al. (1995) of the nuclear region is reproduced in Fig. 24. As can be seen, the nucleus is surrounded by a ring of resolved and unresolved radio sources. According to Sandqvist et al. (1995) these sources have estimated fluxes at 20 cm up to 4.6 mJy, while the Seyfert nucleus itself is not a particularly strong source with a flux of 2.50 mJy. Sources labeled A, D and E in Fig. 24 have rather flat non-thermal spectra with spectral indices in the 20 to 6 cm interval of = -0.2 to -0.6 (the spectral index here being defined such that the spectral power is proportional to the frequency ). Sources G and H are of a more thermal character and have spectral indices around = 0. Source F, on the other hand, shows a steeper non-thermal spectrum with = -0.94 similar to the Seyfert nucleus where = -0.87. Three of the nuclear sources (A, D and G) could be detected at 2 cm wavelength with the higher resolution of 0".25 x 0".10. They are still unresolved at that resolution which means that their sizes should be less than 9 pc.
Figure 24. Map of the radio continuum emission at 20 cm in NGC 1365. The size of the synthesized beam is 2".3 x 1".0. The cross marks the position of the optical nucleus. From Sandqvist et al. (1995).
The positions of the nuclear radio sources are to some extent correlated with the hot spots but there is not an exact overlap. The radio source A in Fig. 24 lies for instance 1".6 east of the maximum continuum of the hot spot L 3 in Fig. 16. In Section 4.6 we will see that the explanation for this may carry some considerable importance.
The source F has the steepest non-thermal spectrum of all and seems to be connected with the nucleus in a position angle of 125°, which deviates by only 5° from the apparent minor axis of the main body of the galaxy. Sandqvist et al. suggest that this 5" long feature is a radio jet emanating from the nucleus and seen projected against the far side of the galactic disk.
On account of the high luminosities, Sandqvist et al. (1982) proposed that the unresolved circumnuclear radio sources could be `radio supernovae' of a similar kind as SN 1979C (Weiler et al. 1981).
The objects known as radio supernovae (Weiler et al. 1986; Weiler & Sramek 1988; Fransson 1994; Weiler et al. 1998) are characterized by being very luminous at radio wavelengths with typical luminosities more than 100 times that of the powerful galactic supernova remnant Cas A. The progenitors are young stars with mass > 10 M with a high rate of mass loss. The radio emission arises as a result of the interaction of the shock wave from the supernova with the dense material deposited by the progenitor. The radio light curves show a slow, wavelength-dependent rise, typical of free-free absorption by the circumstellar material. For the longest observed radio supernovae the emission is known to persist for several decades. The optical spectrum contains strong H and [O III] in emission. Also this optical emission lasts as long as the circumstellar interaction and can be constant for several decades (Fesen & Matonick 1994, their Figure 4). One of the brightest radio supernovae is SN 1986J in the nearby spiral galaxy NGC 891. Its peak flux density at 6 cm, occuring three years after the outburst, corresponded to a radio luminosity 3 000 times that of the galactic SNR Cas A. Two years after the outburst its 20-6 cm spectral index had decreased and seemed to flatten out at = -0.3, while the 6-2 cm spectral index stayed around = -0.7 (Weiler et al. 1990).
The radio source NGC 1365:A had a brightness at the time of observation corresponding to 0.04 times the peak brightness of SN 1986J or 120 times the luminosity of Cas A. The 20-6 cm spectral index was = -0.36 and the 6-2 cm index = -0.27.