  We obtained using MegaCam at the CFHT a deep narrow band Halpha+[NII] wide
field image of NGC 4569, the brightest late-type galaxy in the Virgo cluster.
The image reveals the presence of long tails of diffuse ionised gas without any
associated stellar component extending from the disc of the galaxy up to ~ 80
kpc (projected distance) with a typical surface brightness of a few 10^-18 erg
s-1 cm-2 arcsec-2. These features provide direct evidence that NGC 4569 is
undergoing a ram presure stripping event. The image also shows a prominent 8
kpc spur of ionised gas associated to the nucleus that spectroscopic data
identify as an outflow. With some assumptions on the 3D distribution of the
gas, we use the Halpha surface brightness of these extended low surface
brightness features to derive the density and the mass of the gas stripped
during the interaction of the galaxy with the ICM. The comparison with ad-hoc
chemo-spectrophotometric models of galaxy evolution indicates that the mass of
the Halpha emitting gas in the tail is comparable to that of the cold phase
stripped from the disc, suggesting that the gas is ionised within the tail
during the stripping process. The lack of star forming regions suggests that
mechanisms other than photoionisation are responsible for the excitation of the
gas (shocks, heat conduction, magneto hydrodynamic waves). This analysis
indicates that ram pressure stripping is efficient in massive (M_star ~ 10^10.5
Mo) galaxies located in intermediate mass (~ 10^14 Mo) clusters under
formation. It also shows that the mass of gas expelled by the nuclear outflow
is ~ 1 % than that removed during the ram pressure stripping event. All
together these results indicate that ram pressure stripping, rather than
starvation through nuclear feedback, can be the dominant mechanism responsible
for the quenching of the star formation activity of galaxies in high density
environments.
