4 Discussion and conclusion

The possible link between bar component and nuclear activity has a long history either in the observational and in the theoretical field (e.g. Shlosman 1994). Recent observations have lead Ho et al. (1997) to disagree with this idea and to conclude that, even though the presence of a bar can enhance star formation in the bulge region, it does not affect significantly the active nucleus. Our data for NGC 6221 support this interpretation, because the physical properties of the central line emitting region indicate clearly that ionization comes from the presence of stellar sources. An AGN and its observational signatures might be obscured by gas and dust, but this is, of course, speculation.

The asymmetries of the velocity curves outside the bar region may be due to a tidal distorsion caused by NGC 6215. Koribalski (1996a) discovered an $\rm H \;{\scriptsize I}$ bridge between NGC 6221 and NGC 6215 and later (Koribalski 1996b) suggested a possible interaction of NGC 6221 with the two newly discovered low-surface brightness galaxies nearby, namely BK1 and BK2.

Elmegreen & Elmegreen (1989) and later Keel (1996) proposed a dichotomy of bar properties between early-type (Sbc and earlier) and late-type galaxies (Sc and later) based on observational properties such as bar size, density profile of the bar, star formation, and so on. Early-type bars tend to be a dominant component in the galaxy with a flat density profile extending out to the corotation radius, where they drive symmetric spiral arms (Combes & Elmegreen 1993). On the contrary, bars in late-type galaxies are small with exponentially decreasing density profiles, which extend only to the ILR radius. These kind of bars are unable to influence the wave pattern of the stellar disk. This view is also supported by the model of Combes & Elmegreen (1993).

NGC 6221 is found to exhibit intermediate properties between early-type and late-type barred spiral galaxies, as discussed below:

i) Late-type features/ :

The surface brightness profile shows an exponential decrease; the star formation, i.e. the H$\alpha$ emission, is detected along all the bar; the arm structure is typical of a late-type.

ii) Early-type features/ :

The edge of bar is located between the outer ILR radius and the corotation radius beyond the rising part of the velocity curve; the strong gradient of gas velocity curve in the central 2 kpc; the dust-lane pattern is similar to that of the prototype SBb galaxies (see NGC 1300 in Panels S8 and 154 in the CAG as example); the radial profiles of the fluxes ratio of the H$\alpha$ and [$\rm N \;{\scriptsize II}$] emission lines reveal the presence of a ring structure associated with an ILR; moreover NGC 6221 is interacting with NGC 6215, and Elmegreen et al. (1990) have demonstrated the presence of bars in paired early-type spirals.

Indeed the case of NGC 6221 is analogous to that of certain other galaxies, notably NGC 1300 (Combes & Elmegreen 1993). It has been classified as a late-type barred spiral because of the presence of gas, star formation and spiral arms, but it could be considered as an early-type due to the dynamical properties of its bar.

Pfenniger (1992) has suggested that a major effect due to the presence of a bar could be that the galaxy evolves towards earlier morphological types. Our study indicates that NGC 6221 is a typical case of a late-type barred spiral evolving to an earlier type. If our interpretation is correct, it is understandable that there is so difficult to reach agreement about its morphological classification which is Sbc(s) in RSA and SBc(s) in RC3. Sometimes the criteria of classification in the Hubble sequence cannot describe exactly the complicated scenario of the evolution of galaxies, as appears to be the case for NGC 6221.

Acknowledgements

The DENIS team and in particular the operations team at La Silla is warmly thanked for making this work possible. The DENIS project is supported by the SCIENCE and the Human Capital and Mobility plans of the European Commission under grants CT920791 and CT940627, the European Southern Observatory, in France by the Institut National des Sciences de l' Univers, the Education Ministery and the Centre National de la Recherche Scientifique, in Germany by the State of Baden-Wuerttenberg, in Spain by the DGICYT, in Italy by the Consiglio Nazionale delle Ricerche, in Austria by the Science Fund (P8700-PHY, P10036-PHY) and Federal Ministry of Science, Transport and the Arts, in Brazil by the Foundation for the development of Scientific Research of the State of Sao Paulo (FADESP). JCV acknowledges the support by a grant of the Telescopio Nazionale Galileo and Osservatorio Astronomico di Padova. WWZ acknowledges the support of the Jubiläumsfonds der Österreichischen Nationalbank (grant 6323). The research of MS is supported by the Austrian Science Fund projects P9638-AST and S7308. EMC acknowledges the head of IAC for hospitality during the preparation of this paper. This work was partially supported by grant PB94-1107 of the Spanish DGICYT. The authors thank S. García Burrillo for his valuable comments about bars, and B. Koribalski for her useful information about NGC 6221 interaction with its companions.