A Virgo Environmental Survey Tracing Ionised Gas Emission (VESTIGE).IV.
  A tail of Ionised Gas in the Merger Remnant NGC 4424
  We have observed the late-type peculiar galaxy NGC 4424 during VESTIGE, a
blind narrow-band Halpha[NII] imaging survey of the Virgo cluster carried out
with MegaCam at the CFHT. The presence of a 110 kpc long HI tail in the S
direction indicates that this galaxy is undergoing a ram pressure stripping
event. The deep narrow-band image revealed the presence of a low surface
brightness ionised gas tail ~10 kpc long extending from the centre of the
galaxy to the NW direction, in the direction opposite to the HI tail. Chandra
and XMM X-rays data do not show any compact source in the nucleus nor the
presence of an extended tail of hot gas, while MUSE spectroscopy indicates that
the gas is photo-ionised in the inner regions and shock-ionised in the outer
parts. IFU spectroscopy confirms that the ionised gas is kinematically
decoupled from the stellar component and indicates the presence of two
kinematically distinct structures in the stellar disc. The analysis of the SED
of the galaxy indicates that the activity of star formation has been totally
quenched in the outer disc ~ 250-280 Myr ago, while only reduced by ~80% in the
central regions. All this observational evidence suggests that NGC 4424 is the
remnant of an unequal-mass merger occurred <= 500 Myr ago, when the galaxy was
already a member of the Virgo cluster, now undergoing a ram pressure stripping
event which has removed the gas and quenched the activity of star formation in
the outer disc. The tail of ionised gas probably results from the outflow
produced by a central starburst fed by the collapse of gas induced by the
merging episode. This outflow is sufficiently powerful to overcome the ram
pressure induced by the intracluster medium on the disc of the galaxy crossing
the cluster. This analysis thus suggests that feedback can participate in the
quenching process of galaxies in high-density regions.
