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
|
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.
20 cm in