A Virgo Environmental Survey Tracing Ionised Gas Emission (VESTIGE).III.
  Star formation in the stripped gas of NGC 4254
  During pilot observations of the Virgo Environmental Survey Tracing Galaxy
Evolution (VESTIGE), a blind narrow-band Halpha+[NII] imaging survey of the
Virgo cluster carried out with MegaCam at the CFHT, we have observed the spiral
galaxy NGC 4254 (M99). Deep Halpha+[NII] narrow-band and GALEX UV images
revealed the presence of 60 compact (70-500 pc radius) star forming regions up
to ~ 20 kpc outside the optical disc of the galaxy. These regions are located
along a tail of HI gas stripped from the disc of the galaxy after a rapid
gravitational encounter with another Virgo cluster member that simulations
indicate occurred 280-750 Myr ago. We have combined the VESTIGE data with
multifrequency data from the UV to the far-infrared to characterise the stellar
populations of these regions and study the star formation process in an extreme
environment such as the tails of stripped gas embedded in the hot intracluster
medium. The colour, spectral energy distribution (SED), and linear size
consistently indicate that these regions are coeval and have been formed after
a single burst of star formation that occurred ~< 100 Myr ago. These regions
might become free floating objects within the cluster potential well, and be
the local analogues of compact sources produced after the interaction of
gas-rich systems that occurred during the early formation of clusters.
