  The morphological, spectroscopic and kinematical properties of the warm
interstellar medium (wim) in early-type galaxies (ETGs) hold key observational
constraints to nuclear activity and the buildup history of these massive
quiescent systems. High-quality integral field spectroscopy (IFS) data with a
wide spectral and spatial coverage, such as those from the CALIFA survey, offer
a precious opportunity for advancing our understanding in this respect. We use
deep IFS data from CALIFA (califa.caha.es) to study the wim over the entire
extent and optical spectral range of 32 nearby ETGs. We find that all ETGs in
our sample show faint (H\alpha\ equivalent width EW~0.5...2 {\AA}) extranuclear
nebular emission extending out to >= 2 Petrosian_50 radii. Confirming and
strengthening our conclusions in Papaderos et al. (2013) we argue that ETGs
span a broad continuous sequence with regard to the properties of their wim,
and they can be roughly subdivided into two characteristic classes. The first
one (type i) comprises ETGs with a nearly constant EW~1-3 {\AA} in their
extranuclear component, in quantitative agreement with (even though, no proof
for) the hypothesis of photoionization by pAGB stars. The second class (type
ii) consists of virtually wim-evacuated ETGs with a large Lyman continuum (Lyc)
photon escape fraction and a very low (<= 0.5 {\AA}) EW in their nuclear zone.
These two classes appear indistinguishable from one another by their
LINER-specific emission-line ratios. Additionally, here we extend the
classification by the class i+ which stands for a subset of type i ETGs with
low-level star-fomation in contiguous spiral-arm like features in their
outermost periphery. These faint features, together with traces of localized
star formation in several type i&i+ systems point to a non-negligible
contribution from young massive stars to the global ionizing photon budget in
ETGs.
