4.1. The Active Nucleus of NGC 1365
The nuclear region of NGC 1365 contains a number of bright `hot spots', i.e. condensations with strong emission lines (Fig. 5). The nucleus itself is not very prominent in the ultraviolet due to dust absorption but is strong in the red and infrared regions. In Fig. 16 the nuclear region is presented in the light of H + [N II], in the [O III] emission, and in the blue continuum within the B-band.
Figure 16. The nuclear region of NGC 1365 at three different wavelength regions. a. H + [N II] 6548, 6583. The most prominent star forming regions (`hot spots') have been labelled. b. [O III] 5007. c. Johnson B-band. The units on the axes are arcseconds offset from the nucleus. The upper right side (NW) is the near side of the galaxy. From Kristen et al. (1997).
Early spectra with the ESO 3.6-m telescope revealed a broad underlying component of the H line in the spectrum of the nucleus (Véron et al. 1980). This was confirmed by Alloin et al. (1981), who observed broad and narrow line components in the H and H lines. The FWHM of the broad component of the H line was found to be 1 800 km s-1. In addition, Edmunds and Pagel (1982) found very broad asymmetric extensions to the hydrogen lines in the nucleus and also presented spectra of the surrounding narrow-line region. A.S. Wilson (see M.M. Phillips & Frogel 1980) discovered that weak [Ne V] 3426 and He II 4686 are present in the nuclear spectrum, i.e. lines that cannot be produced in gas ionized only by O and B type stars, which indicates the presence of a significant non-thermal source.
Goerdt and Kollatschny (1998) performed a population and evolutionary synthesis of spectra along a slit through the nucleus of NGC 1365 and found the nuclear spectrum to be dominated by a non-thermal component that contributes 60% of the light at 5050 Å. A strong young stellar component was found around the nucleus, but a still stronger starburst region at about 10" distance from the nucleus.
In the classification scheme of Osterbrock NGC 1365 then hosts an active nucleus of type Seyfert 1.5. This nucleus is rather heavily absorbed by the dust lane that, coming in from the bar, penetrates the nuclear region and just touches the nucleus itself (cf. Sandqvist et al. 1988, Fig. 3a). Kristen et al. (1997) imaged the nucleus with the Faint Object Camera (FOC) on the HST. The filter used was an intermediate band F437M filter, centered at 4290 Å. This pre-refurbishment HST/FOC image was obtained in the F/96 mode and deconvolved with the point spread function. The point source at the Seyfert nucleus is still unresolved and has an effective radius < 3 pc. Its flux corresponds to B = 17m.0. The extinction in the nuclear region probably varies strongly over the area. From the Balmer decrement in the hot spots L2 and L3 (Fig. 16) Kristen et al. (1997) found the extinction AB = 2m.5. Goerdt and Kollatschny (1998) from their spectral synthesis derived an extinction corresponding to AB = 1m.6 for the non-thermal component in the nuclear spectrum and 1m.2 for the stellar component in the nuclear region.
According to the unified scheme the orientation and opening angle of an obscuring torus in the case of NGC 1365 must be such that the observer still can see the broad line region. In agreement with this, as we shall see below, an analysis of a cone-like region of high excitation gas indicates the existence of a confining torus with such an orientation that the line of sight to the nucleus falls within the unobscured sector.
Circumnuclear molecular disks have been revealed in a limited number of galaxies by `megamaser' emission in transitions of OH and H2O, where the luminosities of the masers are up to several order of magnitudes higher than the most luminous galactic masers. The OH megamasers are supposed to rest in a molecular disk around the nucleus of the galaxy, where the gas is pumped by far infrared radiation and the nuclear continuum radiation amplified. The H2O megamasers are located a few pc from the galactic nucleus supposedly in a dense disk of gas and dust. It is suggested that these masers are pumped by collisions when the gas is heated by X-ray radiation. NGC 1365 has been searched for maser radiation in OH (Norris et al. 1989), H2O (Nakai et al. 1995; Braatz et al. 1996), and also methanol (C.J. Phillips et al. 1998) but with no detections. A reason may be that the circumnuclear disk in NGC 1365 is seen at an inclination angle of i = 40° as judged from the nuclear outflow cone and radio jet (Section 4.3.) and not sufficiently edge on for the observer to catch the narrow cone of masering light.