Astron. Astrophys. 343, 367-372 (1999)

5. The molecular torus and other nuclear structure

A schematic overview of the different components in the nuclear region of NGC 1365 is presented in Fig. 7, which may facilitate the interpretation of the CO map from Figs. 5 and 6. The increased resolution of the CO MEM map strengthens the appearance of the circumnuclear molecular torus. The position of the coincident super star cluster SSC:10 and radio supernova A, mentioned in Sect. 1, lies very close to the southwestern molecular torus peak. This region thus contains three strong signatures of an intense starburst activity. Also the northeastern torus peak has suggested radio supernovae (D and G) in its immediate neighbourhood, although the identification with SSC:s is not as obvious due to the high optical extinction in this region.

Fig. 7. Schematic diagram showing the different components in the nuclear region of NGC 1365. The optical nucleus - cross, CO distribution - solid contours, H II -regions and hot spots - hatched areas, radio supernovae and super star clusters - solid triangles, radio jet - wide solid contours, high-excitation [O III ] cone - dashed contour, Outer Inner Lindblad Resonance (OILR) - large dotted ellipse, Inner Inner Lindblad Resonance (IILR) estimate - small dotted ellipse

Various CO extensions lead out from the torus into the two dominant eastern and western dust lanes of the bar, as well as into the H II -regions north and northeast of the nucleus. Distinct CO emission enhancement occurs near the positions of the two H II -regions L12 and L4 (Sandqvist et al. 1982), about north and northeast of the nucleus, respectively. Although it may be fortuitous, it is worthwhile pointing out that there is a secondary CO minimum near the positions of the radio jet and the conical shell of ionized [O III ] gas, southeast of the nucleus. Outside the ends of the jet and cone there is a secondary maximum of CO.

From hydrodynamical simulations of NGC 1365, Lindblad et al. (1996) have found that the radius of the Outer Inner Lindblad Resonance (OILR) in this galaxy is . The molecular torus peaks lie at an angular distance of only 6:005 ( pc) from the nucleus, as measured on the MEM map, and the torus thus exists well inside the OILR, in fact in the vicinity of the expected Inner Inner Lindblad Resonance (IILR). Some velocity indications of orbit crowding of the molecular gas as it crosses the OILR have been presented by Sandqvist (1996).

In the past few years, interferometric CO observations have begun to yield a better understanding of the distribution of molecular gas in the nuclei of spiral galaxies. These results show that the molecular gas in the central regions of barred galaxies sometimes displays a "twin-peak" structure (e.g. Kenney 1996) located near the Inner Lindblad Resonances on the scale of 500-1000 pc. But also rings or partial rings, filled exponential disks, or small spirals are revealed in these regions for different galaxies. However, new surveys of normal galaxies indicate that bar-induced streaming is not the only mechanisms which can create a nuclear gas concentration (Sakamoto et al. 1998).

In a number of cases of barred spiral galaxies, the inward gas flow along the bar is predicted to be slowed down significantly near the Inner Lindblad Resonance resulting in a build-up of molecular gas concentrations in the region between the OILR and IILR (Combes 1988; Shlosman 1996). This effect seems to be present in NGC 1365, where the torus reaches its maximum concentration just outside the estimated IILR. However, in order to properly study the kinematic build-up of molecular gas concentrations between the OILR and IILR, we really need sensitive interferometric observations of the central region of this southern galaxy with such instruments as the proposed LSA/MMA.

© European Southern Observatory (ESO) 1999

Online publication: March 1, 1999
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