The SAURON project XVI: On the Sources of Ionisation for the Gas in
  Elliptical and Lenticular Galaxies
  Following our study on the incidence, morphology and kinematics of the
ionised gas in early-type galaxies we now address the question of what is
powering the observed nebular emission. To constrain the likely sources of gas
excitation, we resort to a variety of ancillary data, draw from complementary
information on the gas kinematics, stellar populations and galactic potential
from the SAURON data, and use the SAURON-specific diagnostic diagram
juxtaposing the [OIII]/Hb and [NI]/Hb line ratios. We find a tight correlation
between the stellar surface brightness and the flux of the Hb recombination
line across our sample, which points to a diffuse and old stellar source as the
main contributor of ionising photons in early-type galaxies, with
post-asymptotic giant branch (pAGB) stars being still the best candidate based
on ionising-balance arguments. Other ionising sources such as a central AGN,
OB-stars, shocks or the interaction between the hot and warm phases of the
interestellar medium are found to play only a limited or localised role in
powering the diffuse nebular emission observed in our sample galaxies. These
results lead us to investigate the relative importance of stellar and AGN
photoionisation in explaining the ionised-gas emission observed in early-type
galaxies by the Sloan Digital Sky Survey (SDSS). By simulating how our sample
galaxies would appear if placed at further distance and targeted by the SDSS,
we conclude that only in very few, if any, of the SDSS early-type galaxies that
display modest values for the equivalent width of the [OIII] line (less than
~2.4\AA) and LINER-like [OIII]/Hb values, the nebular emission is truly powered
by an AGN.
