In the course of this review of the `prototype' barred galaxy NGC 1365 we may have gained the impression that this galaxy in several respects is rather unusual. Indeed, NGC 1365 is a supergiant galaxy falling at the very upper end of the infrared Tully-Fisher relation with a very strong bar. To what extent do its special properties have their origins in this bar?
The only spiral galaxy in our immediate neighbourhood that rivals NGC 1365 in size is the supergiant galaxy M 101 - a magnificent Sc galaxy with a prominent m = 1 asymmetry but where the faint trace of a bar seen in infrared light or in CO does not give the impression to be massive enough to have a decisive influence on the dynamics of the system. Why these galaxies are so different and why one of the two galaxies has a strong bar and the other almost none is only a subject for guess. Does M 101 possess a stabilizing halo, preventing the formation of a bar, that is missing in NGC 1365? To understand these differences is fundamental to our understanding of spiral galaxies.
A promising step forward in the search for the halo contribution in galaxies is the detection of ionized gas beyond the H I disk in the spiral galaxy NGC 253 (Bland-Hawthorn et al. 1997). This discovery is of profound importance as CO is not a good tracer of molecular hydrogen in metal poor surroundings. If hydrogen, ionized by some external radiation, can be detected beyond the H I cutoff, the rotation curve could be extended to give the total mass of the galaxy and the mass and extent of the halo. As the dominating mechanism for the ionization is suspected to be hot young stars in the inner region that are seen by a warped outer hydrogen disk, a search for such ionized hydrogen might successfully be undertaken in the warped galaxy NGC 1365.
We have seen how numerical simulations by successive approximations can lead to the determination of the distribution of mass within a galaxy. In further extensions these simulations should be made fully self-consistent with both stellar populations and interstellar matter gravitating and free to interact. A special problem to address is whether the gravitation from gaseous arms formed by the bar can prolong the life of stellar arms and help to drive the spiral arms through co-rotation. Another important problem to address is how to transport matter in the central region all the way into the nuclear accretion disk.
The relations between compact radio sources and compact super star clusters on one hand and between compact radio sources and infrared sources on the other, discovered in the nuclear region of NGC 1365, is remarkable. Further observations, for instance spectral observations at high spatial resolution and search for time variations in the radio domain, should contribute in an essential way to shed light on these strange classes of objects which are of fundamental importance for the understanding of star formation and stellar evolution in the nuclear region of a galaxy.
Many outflow cones from active nuclei have been presented in the literature. However, the methods of observation have differed. The draw-back of long slit measurements is the general lack of homogeneous spatial coverage and the great demand on observing time. The disadvantage of Fabry-Perot methods is the difficulty to detect faint emission, e.g. in extended line wings, and to compare line strengths. The modern 3-D spectrographs now developed for large telescopes should substantially improve the situation. Also, the modelling of these outflow cones has been very different from case to case. As an example we may mention the model worked out in detail for the high excitation gas in the Seyfert 2 galaxy NGC 7582 by Morris et al. (1985), where the approach differs fundamentally from the one applied for NGC 1365. It would be of great advantage for the understanding of these outflow high-excitation cones if the observations and the modelling were done in comparable ways for a number of galaxies.
We do not know for certain whether to classify the nucleus of NGC 1365 as a low luminosity AGN and to what degree it is obscured by intervening dust. It would be important to get a spectrum covering a large wavelength range with high spatial resolution of the nucleus itself to hopefully get the extinction, the luminosities in various emission lines, the total luminosity, and the shape of the continuum in order to compare this nucleus with other Seyfert nuclei.
The distribution of populations in NGC 1365 is not sufficiently known. Most CCD images in the optical region are not absolutely calibrated. Adequate photometry (performed with suitable focal reducer) would permit analysis of the nuclear bulge, lens or bar, population analysis over selected regions, improved mass/luminosity determinations and photometry of faint outer regions. It would be of interest to try to identify planetary nebulae and ordinary old globular clusters in the galaxy, not only to trace these populations but also to compare these distance indicators against distances given by Cepheids.
Finally, concentrating studies, involving observations over a wide range of wavelength bands, as well as simulations, on a single galaxy gives the possibility to relate different phenomena and reach a global scenario consistent with observations. Ideally, this should be undertaken in a systematic way for a number of selected galaxies.
My thanks are due to Rainer Beck and V. Shoutenkov for permission to publish the results shown in Fig. 7, to Massimo Tarenghi and Alan Moorwood for providing Figs. 5 and 29 respectively, to S. Jörsäter, M. Näslund and J.J. Hester for permission to publish Fig. 4, and to Claus Madsen and Hans Hermann Heyer for producing Figs. 1 and 2 from ESO photographic plates. Further, I am grateful to Claes Fransson, Maja Hjelm, Helmuth Kristen, Per Lindblad, Aage Sandqvist and Lodewijk Woltjer for critically reading the manuscript and for constructive discussions.