  We investigate the temperature evolution of the electronic structure of
ferromagnetic CaB$_6$ using ultra-high resolution photoemission spectroscopy;
electronic structure of paramagnetic LaB$_6$ is used as a reference. High
resolution spectra of CaB$_6$ reveal finite density of states at the Fermi
level, $\epsilon_F$ at all the temperatures and evidence of impurity induced
localized features in the vicinity of $\epsilon_F$, which are absent in the
spectra of LaB$_6$. Analysis of the high resolution spectra suggests that
disorder in B-sublattice inducing partial localization in the mobile electrons
and low electron density at $\epsilon_F$ is important to achieve ferromagnetism
in these systems.
