GASP XVII. HI imaging of the jellyfish galaxy JO206: gas stripping and
  enhanced star formation
  We present VLA HI observations of JO206, a prototypical ram-pressure stripped
galaxy in the GASP sample. This massive galaxy (M$_{\ast} =$ 8.5 $\times$
10$^{10}$ M$_{\odot}$) is located at a redshift of $z =$ 0.0513, near the
centre of the low-mass galaxy cluster, IIZw108 ($\sigma \sim575$ km/s). JO206
is characterised by a long tail ($\geq$90 kpc) of ionised gas stripped away by
ram-pressure. We find a similarly long HI tail in the same direction as the
ionised gas tail and measure a total HI mass of $3.2 \times 10^{9}$
M$_{\odot}$. This is about half the expected HI mass given the stellar mass and
surface density of JO206. A total of $1.8 \times 10^{9}$ M$_{\odot}$ (60%) of
the detected HI is in the gas stripped tail. An analysis of the star formation
rate shows that the galaxy is forming more stars compared to galaxies with the
same stellar and HI mass. On average we find a HI gas depletion time of
$\sim$0.5 Gyr which is about four times shorter than that of "normal" spiral
galaxies. We performed a spatially resolved analysis of the relation between
star formation rate density and gas density in the disc and tail of the galaxy
at the resolution of our HI data. The star formation efficiency of the disc is
about 10 times higher than that of the tail at fixed HI surface densities. Both
the inner and outer parts of JO206 show an enhanced star formation compared to
regions of similar HI surface density in field galaxies. The enhanced star
formation is due to ram-pressure stripping during the galaxy's first infall
into the cluster.
