  Recently gathered observational data on a sample of Type Ia Supernovae (SNe
Ia) reveal a wide distribution of expansion velocities of the Fe cores,
measured from the width of the nebular lines. Moreover, the velocity appears to
correlate with the luminosity decline rate after maximum light, $\Delta
m_{15}(B)$. Since it has been shown that for SNe Ia $\Delta m_{15}(B)$
correlates with the absolute magnitude at maximum, this then implies a relation
between the expansion velocity of the Fe nebula and the luminosity at maximum.
Physically, the maximum luminosity is related to the mass of synthesized
$^{56}$Ni, whereas the FWHM of the lines is related to the kinetic energy of
the ejecta. Our finding constitutes observational proof of the theoretical
prediction that the two quantities have to be related.
