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

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6. Conclusions

We have characterized a sample of spirals selected as being isolated, in order to use it for comparisons with the analogous properties of spirals in different environmental situations. We caution that we have selected bright galaxies (brighter than MB = -18.5) and that the sample is too small to attempt the analysis by morphological types. We further notice that the sample is dominated by Sc type galaxies. Whether this is due to the small size of the sample or it reflects the nature of isolated spiral galaxies should be further analyzed with larger samples. We point out that similar results are obtained when existing larger samples of non-perturbed galaxies are examined, so the possibility that there is a preference for later types in poor environments should be explored.

We have used total parameters, as luminosity, color indices and mass, together with others describing their photometrical or kinematical behaviour in more detail, as the bulge/disk ratio, the color gradients or the shape of the rotation curve. The isolated spirals we have analyzed can be described as following:

  • The overall morphological aspect is quite symmetric and regular, as expected for galaxies supposed to be free of external influences in the last 109 years at least. On the other hand, the fraction of barred galaxies is similar to what is found for spirals irrespective of their environmental situation. Moreover, all of them show the presence of features indicative of non-axisymmetric components of the gravitational potential. Therefore, one is led to conclude that either the life time of these features is significantly over 109 years or that they can be spontaneously formed along the life of a galaxy.

  • The total and central color indices are well correlated: redder galaxies have also redder central regions. The outer parts of the disks are bluer and more similar than the central parts.

  • All of the galaxies in our sample without exception show Type I photometrical profiles. As we will show in Paper III, this is not the case for non-isolated spirals, that can present Type II profiles. The range of the disk scale lengths and effective surface brightness seems to be narrower for isolated spirals than for samples including galaxies in other situations. The disks of different isolated galaxies are bluer than their central regions, and much more alike that the bulges.

  • Their current star formation rates as given by total H[FORMULA] (or FIR) luminosities are within the range found for spirals classified as normal, i.e., without apparent peculiarities in their morphology or nucleus. The line ratios measured in the observed emission line regions indicate that they are photoionized by stars. For the cases for which the history of the star formation could be retraced, it is found compatible with a smooth, constant SFR along their lifetimes.

  • The overall shape of their rotation curves from Rmax to RM can be described as flat. The median value of the slope in that region is found to be 8[FORMULA].

  • We have applied the PCA to the data set we have constructed. In agreement with previous studies on larger samples including non isolated galaxies, we find that the isolated spirals in our sample constitute a family that can be described by two main dimensions given by size (either the luminosity, the optical size or the total mass) and form (either the inner gradient of the rotation curve, the G-parameter, or the bulge-to-disk luminosity ratio, B/D). The remaining variance could just be due to the errors in the parameters. We conclude that G or B/D are more robust and objective parameters than the morphological subtype or the color indices for the classification of spiral galaxies.

  • Previous bi-variate relations are confirmed, as those between the mass and the luminosity, the size or the angular momentum.

  • We have found for the first time a very tight correlation between G and B/D. B/D is a distance independent parameter, whereas G is not. We point out that the correlation G-B/D, constitutes a new distance indicator. We stress however that it would only be well suited for isolated systems since, as illustrated in Paper III, the scatter of the relation significantly increases when interacting galaxies are included.

  • The isolated spiral galaxies define very tight structural relations. The bulges define the Kormendy relation with a small scatter. A similar relation is found for the disks, with a lower scatter than in previous studies. The scatter in those relations, or in the G-B/D relation, increases significantly when interacting galaxies are included. Whether this is related to the inclusion of fainter galaxies or to the interaction status will be discussed in Paper III.

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© European Southern Observatory (ESO) 1999

Online publication: March 18, 1999
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