A Virgo Environmental Survey Tracing Ionised Gas Emission (VESTIGE).XII.
  Ionised gas emission in the inner regions of lenticular galaxies
  As part of the VESTIGE survey, a blind narrow-band Ha+[NII] imaging survey of
the Virgo cluster carried out with MegaCam at the CFHT, we discovered 8 massive
lenticular galaxies with prominent ionised gas emission features in their inner
(few kpc) regions. These features are either ionised gas filaments similar to
those observed in cooling flows (2 gal), or thin discs with sizes 0.7<R(Ha)<2.0
kpc (6 gal), thus significantly smaller than those of the stellar disc. These
discs have morphological properties similar to those of the dust seen in
absorption in high-resolution HST images. Using a unique set of multifrequency
data we show that while the gas located within these inner discs is
photoionised by young stars, signaling ongoing star formation, the gas in the
filamentary structures is shock-ionised. These discs have a star formation
surface brightness similar to those observed in late-type galaxies. Because of
their reduced size, however, these lenticular galaxies are located below the
main sequence of unperturbed or cluster star-forming systems. By comparing the
dust masses measured from absorption maps in optical images, from the Balmer
decrement, or estimated by fitting the UV-to-far-IR spectral energy
distribution of the target galaxies, we confirm that those derived from optical
attenuation maps are heavily underestimated because of geometrical effects due
to the relative distribution of the absorbing dust and the emitting stars. We
have also shown that these galaxies have gas-to-dust ratios of G/D~80, and that
the star formation within these discs follows the Schmidt relation, albeit with
an efficiency reduced by a factor of ~ 2.5. Using our unique set of
multifrequency data, we discuss the possible origin of the ionised gas in these
objects, which suggests multiple and complex formation scenarios for massive
lenticular galaxies in clusters.
