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Astron. Astrophys. 344, 421-432 (1999)

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3. The morphology of the isolated galaxies

The RC3 catalogue (de Vaucouleurs et al. 1991) contains detailed morphological information for many of the 22 galaxies in our sample. There is information on the subtype for all the galaxies, with the exception of NGC 6155, classified just as spiral. There is catalogue information on the presence or not of a bar for 16 galaxies, and we were able to add that information for other 5 galaxies (UGC 3511, UGC 3804, NGC 3835, NGC 6155 and NGC 6395, see notes on individual galaxies in Paper I 1), so the bar information does exist for all the 22 galaxies but one, namely NGC 4525, classified as Scd. The morphological information is completed with details on the presence of rings for 17 galaxies in the sample.

Looking at that information, the following picture emerges. First, only 5 out of the 21 galaxies with type information are earlier than Sc. This could be just an artifact due to the limited size of our sample, but we note that de Jong & van der Kruit (1994) find also a type distribution that peaks at Sc for their sample of 86 non-interacting, non-peculiar spirals.

Among the 21 galaxies with information on the presence of a bar, 8 are non barred (SA), 3 are barred (SB) and 10 are weakly barred (type SAB coded as SX in the RC3). The fraction of barred systems among the isolated galaxies amounts to 62%. In spite of the small size of our sample, we notice that this fraction is very similar to that found for a large population of spirals, without consideration of their environmental status (Moles et al. 1995).

Another interesting aspect is that all the non-barred (SA) galaxies in the sample are ringed spirals. Therefore, all the 21 galaxies with bar or ring information do show the presence of features indicative of the presence of non-axisymmetric components of the potential. Indeed, this kind of structures is easily explained as due to perturbations of the gravitational potential by companions (Simkim et al. 1980; Arsenault 1989; Elmegreen et al. 1990; Combes & Elmegreen 1993). But the selection criteria we have adopted to define the sample of isolated spirals was designed to select objects that wouldn't have experienced gravitational interaction in the last 109 years at least. The presence of those components in all of them should imply longer time scales for those features, unless the possibility of spontaneous formation of such structures along the galactic life is accepted.

The global aspect of those galaxies is however quite regular. For the 17 galaxies for which we could gather CCD broad band images, our analysis shows that all their disks are quite symmetric at their outskirts. We have calculated the decentering degree as the displacement of the center of the most external, recorded isophote with respect to the luminosity center, normalized to the last measured radius. The values we measured are always smaller than 5%, except for NGC 6155, for which we found 10%. The average decentering amounts to 2.4 [FORMULA] 2.7%. Concerning the spiral arm structure, it can be generally described as regular and symmetric in shape, although sometimes it is more intense in one of the hemispheres. Essentially all kind of structuring are encountered in our isolated spirals but, we cannot extract statistically significant results on their arm structure due to the limited number of systems we have in the sample.

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© European Southern Observatory (ESO) 1999

Online publication: March 18, 1999