7 Summary and conclusions

We have obtained long slit spectra along the major axes of a sample of spiral galaxies selected to be either isolated or in isolated pairs, with similar intermediate-scale environment and with a recognizable and well defined spiral morphology. We have further investigated their environmental status and reclassified them, what allows us to define a sample of isolated objects, to be later compared with mildly interacting spirals (with small satellites and/or companions of similar size). The main results we have obtained are the following:

• We have confirmed previous results (Paper II) that isolated galaxies tend to be of later Hubble types and lower luminosity than the interacting galaxies.
• the outer parts of the rotation curves of isolated galaxies tend to be flatter than in interacting galaxies. They show similar relations between global parameters. The scatter of the Tully-Fisher relation defined by isolated galaxies appears to be significantly lower than that of interacting galaxies.
• there is a clear trend between the metallicity of the HII-like nuclei and the morphological type of the galaxy, the earlier types showing larger Z values. Extrapolation of the Z-trend in the disk to the central position gives consistent results with the direct measurement of the nuclear HII region. No trend with the interaction status was found.
• we report here for the first time the existence of a tight correlation between Z and the gradient of the inner rigid solid rotation part of the rotation curve, G.
• the Z-gradient of the disks depends on the type, being almost flat for early spirals, and increasing for later types.
• [NII]/H$\alpha$ ratios appear to be larger for disk HII regions interacting galaxies. This could be simply due to the fact that early types are more frequent among them. (A similar result is obtained for AGNs, but we cannot further test it given the small size of our sample.) On the other hand, the H$\alpha$ EW present similar values in all kind of galaxies. At face value those results would indicate that mildly interacting galaxies (such as those in our sample) have different histories form normal galaxies. This difference however has no marked episodes (bursts) of star formation, but only small cumulative effects that result in more metallic (and redder) disks and nuclei.

Acknowledgements

We are very grateful to the anonymous referee, whose comments and suggestions helped us improve the presentation. We thank Prof. G. Paturel who kindly made available to us prior to publication the Galaxy Catalog we have used to determine the interaction status of our sample galaxies. We also thank L. Cariggi for her careful reading of the manuscript and valuable comments. I. Márquez acknowledges financial support from the Spanish Ministerio de Ciencia y Tecnología and the IAA. D. Jesús Varela acknowledges a scholarship from the Ministerio de Ciencia y Tecnología. This work is financed by DGICyT grants PB93-0139, PB96-0921, PB98-0521, PB98-0684, ESP98-1351, AYA2001-2089 and the Junta de Andalucía. This research has made use of the NASA/IPAC extragalactic database (NED), which is operated by the Jet Propulsion Laboratory under contract with the National Aeronautics and Space Administration.