5 Correlations between parameters

We plot in Figs. 15 and 16 the relationships between absolute B magnitude and the size of the galaxy (24.5 isophote diameter) and also between M_B and the distribution of light (concentration index c₃₁). Information about morphological classification is also shown.

There is a tight correlation between M_B and D_24.5. A least-square fit to our data leads to:

$${\rm log}\,D_{24.5}=(-2.65\pm 0.16)+(-0.20\pm 0.01)\,M_B.$$ (10)

The slope is very similar to the value expected for a constant luminosity-area ratio ( ${\rm log}\,D=C-0.2\,M$). Therefore, despite there is a great variety in morphological and spectroscopic types, a uniformity in the mean surface brightness is exhibited, as was also proved with the red data (Vitores et al. 1996b found a slope value of $-0.21\pm 0.01$). The fit gives a mean surface brightness value of -13.3 magkpc^-2.

In Fig. 16 a general trend between the concentration index c₃₁, the absolute B magnitude M_B and the morphological type is apparent. Early-type galaxies show medium-high magnitudes and high concentration indices. If we move downwards to the zone of low concentration index we find spirals, from Sa to late-type. Finally, BCDs have c₃₁ values typical for spirals but are fainter than normal galaxies.

Figure 17 shows the segregation in morphological type in a $c_{\rm in}$ versus A diagram. In this plot and the next, median values for the different morphological types are plotted with a black dot; ellipse semi-axes are the $\sigma$ of each parameter. There is a clear trend from left to right in decreasing Hubble type. S0 galaxies are placed in the high symmetry-high $c_{\rm in}$ zone. BCDs also appear as highly symmetrical objects. On the other hand, irregulars are shown as highly asymmetrical objects in the top-left zone of the plot and interactive systems are located among the most asymmetrical galaxies. A trend can be also remarked in the spiral sequence: early-type galaxies are more symmetrical than late-type ones (due to the presence of more HII regions, for example).

Figure 14 showed that there is a clear correlation between the concentration indices and Hubble type. This trend is also observed with the asymmetry coefficient. Table 7 presents the mean values of the bulge-to-disk ratio, mean effective surface brightness, concentration indices and asymmetry coefficient of each Hubble type. The statistics of A have been split into barred and non-barred objects; barred galaxies are more asymmetrical than non-barred ones.

Figure 15: Relationship between the size of the UCM galaxies represented by the diameter of the 24.5 isophote D24.5 and the total B luminosity of the object MB. A least-square fit to the data is also plotted

Figure 16: Concentration index c31 versus absolute magnitude MB. Different symbols stand for distinct morphological types

Figure 18 depicts the absolute B magnitude of the UCM objects versus the mean effective surface brightness. Early-type galaxies appear as bright, high surface brightness objects. Late-type spirals have lower $\mu_{\rm e}$, although no significant difference in M_B is present. BCDs are clearly segregated due to their faintness. Irregulars and interactive systems show also a distinctive surface brightness.