, Volume 157, Issue 1-2, pp 321-329

V-R diagram for spiral galaxies. New insights into the hubble sequence

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Abstract

We examine the relation between the global parameters of spiral galaxies and their morphological type. We have represented the rotational velocityV at a radius of 25 mag arc sec−2 against this radiusR for a sample of 56 galaxies. Different types of galaxies appear separated on this diagram. We have plotted above theV-R diagram isolines of total mass, angular momentum and ‘mean surface density’ for isolated galaxies with circular motion in centrifugal equilibrium showing: (1) That the parameter that represents the morphological type is the ‘mean surface density’ or its observational parameterV 2/R. (2) From aV-R diagram we can easily see what kinds of correlations we expect to find between the global properties of galaxies and their morphological type. (3) For every morphological type of galaxies there is a critical mass above which there is a change in its dynamical behaviour. Finally, consideringV 2/R as representative of the morphological type, we can deduce theoretically the empirical Tully-Fisher relation and the dependence of the slope and the zero point on the morphological type. The separation in ‘logV’ that we expect to find for different types of galaxies for a constant luminosity is the same order as the experimental errors in the determination of ‘logV’. This explains the tight power-law relationship observed between this and the luminosity.