numerical classes: 3=Sa, 4=Sab ... 7=Sc, 8=Irr/Pec. To avoid superposition of points, we added to each class a random number between -0.3 and 0.3. The majority of galaxies in our sample have small angular size (1-2 arcmin), thus the classification error is probably up to 2 bin in Hubble type. In particular, due to the adopted classification scheme (see column 12 of Table 1) the Pec objects might in fact belong to any of the earlier-type bins. Nevertheless, in spite of the uncertainty on our classification, Fig. 5a shows a definite trend of $\rm H\alpha$ E.W. with type. In fact the average $\rm H\alpha$ E.W. among types Sa is 5.3 $\pm$ 1.9 $\AA$, significantly lower than 24.5 $\pm$ 3.3 $\AA$ found among types Sc (see also Roberts & Haynes 1994 and KTC94).

To further investigate the reasons for the residual scatter found within each individual morphological class (see Fig. 5a), we analyze the dependence of the $\rm H\alpha$ E.W. on the galaxy’s mass, a quantity carrying a more direct physical meaning than the Hubble type. Following GPB96, we use the H band luminosity as a tracer of the dynamical mass of disk galaxies ($\rm Log~M_{dyn}=Log~L_H+0.66$, solar units) to show in 