Nebular emission and the Lyman continuum photon escape fraction in
  CALIFA early-type galaxies
  We use deep integral field spectroscopy data from the CALIFA survey to study
the warm interstellar medium (WIM) of 32 nearby early-type galaxies (ETGs). We
propose a tentative subdivision of our sample ETGs into two groups, according
to their Ha equivalent width (EW) and Lyman continuum (LyC) photon escape
fraction (PLF). Type i ETGs show nearly constant EWs and a PLF~0, suggesting
that photoionization by post-AGB stars is the main driver of their faint
extranuclear nebular emission. Type ii ETGs are characterized by very low,
outwardly increasing EWs, and a PLF as large as ~0.9 in their centers. Such
properties point to a low, and inwardly decreasing WIM density and/or volume
filling factor. We argue that, because of extensive LyC photon leakage,
emission-line luminosities and EWs are reduced in type ii ETG nuclei by at
least one order of magnitude. Consequently, the line weakness of these ETGs is
by itself no compelling evidence for their containing merely "weak"
(sub-Eddington accreting) active galactic nuclei (AGN). In fact, LyC photon
escape, which has heretofore not been considered, may constitute a key element
in understanding why many ETGs with prominent signatures of AGN activity in
radio continuum and/or X-ray wavelengths show only faint emission lines and
weak signatures of AGN activity in their optical spectra. The LyC photon
escape, in conjunction with dilution of nuclear EWs by line-of-sight
integration through a triaxial stellar host, can systematically impede
detection of AGN in gas-poor galaxy spheroids through optical emission-line
spectroscopy. We further find that type i and ii ETGs differ little (~0.4 dex)
in their mean BPT line ratios, which in both cases are characteristic of
LINERs. This potentially hints at a degeneracy of the projected,
luminosity-weighted BPT ratios for the specific 3D properties of the WIM in
ETGs. (abridged)
