SDSS IV MaNGA - Spatially resolved diagnostic diagrams: A proof that
  many galaxies are LIERs
  We study the spatially resolved excitation properties of the ionised gas in a
sample of 646 galaxies using integral field spectroscopy data from SDSS-IV
MaNGA. Making use of Baldwin-Philips-Terlevich diagnostic diagrams we
demonstrate the ubiquitous presence of extended (kpc scale) low ionisation
emission-line regions (LIERs) in both star forming and quiescent galaxies. In
star forming galaxies LIER emission can be associated with diffuse ionised gas,
most evident as extra-planar emission in edge-on systems. In addition, we
identify two main classes of galaxies displaying LIER emission: `central LIER'
(cLIER) galaxies, where central LIER emission is spatially extended, but
accompanied by star formation at larger galactocentric distances, and `extended
LIER' (eLIER) galaxies, where LIER emission is extended throughout the whole
galaxy. In eLIER and cLIER galaxies, LIER emission is associated with radially
flat, low H$\alpha$ equivalent width of line emission ($<$ 3 \AA) and stellar
population indices demonstrating the lack of young stellar populations,
implying that line emission follows tightly the continuum due to the underlying
old stellar population. The H$\alpha$ surface brightness radial profiles are
always shallower than $\rm 1/r^{2}$ and the line ratio
[OIII]$\lambda$5007/[OII]$\lambda$3727,29 (a tracer of the ionisation parameter
of the gas) shows a flat gradient. This combined evidence strongly supports the
scenario in which LIER emission is not due to a central point source but to
diffuse stellar sources, the most likely candidates being hot, evolved
(post-asymptotic giant branch) stars. Shocks are observed to play a significant
role in the ionisation of the gas only in rare merging and interacting systems.
