A Virgo Environmental Survey Tracing Ionised Gas Emission (VESTIGE).VI.
  Environmental quenching on HII region scales
  The Virgo Environmental Survey Tracing Ionised Gas Emission (VESTIGE) is a
blind narrow-band Halpha+[NII] imaging survey of the Virgo cluster carried out
with MegaCam at the Canada-French-Hawaii telescope (CFHT). We use a new set of
data extracted from VESTIGE to study the impact of the hostile cluster
environment on the star formation process down to the scale of HII regions (~
50 pc). HII regions are identified and their parameters measured using the
HIIphot code on a sample of 114 late-type galaxies spanning a wide range in
morphological type (Sa-Sd, Im, BCD), stellar mass (10^6.5 <= M_star <= 10^11
Mo), and star formation activity (10^-3 <= SFR <= 10 Mo yr^-1). Owing to the
exquisite average resolution of the VESTIGE data (0.65 arcsec), we detect 11302
HII regions with an Halpha luminosity L(Halpha) >= 10^37 erg s^-1. We show that
the typical number of HII regions in gas-stripped objects is significantly
lower than in healthy late-types of similar stellar mass. We also show that in
these gas-stripped galaxies the number of HII regions significantly drops
outside the effective radius, suggesting that the quenching process occurs
outside-in, in agreement with other multifrequency observations. These new
results consistently confirm that the main mechanism responsible for the
decrease of the star formation activity observed in cluster galaxies is ram
pressure, allowing us to discard other milder processes such as starvation or
strangulation unable to reproduce the observed radially truncated profiles.
