GASP XXVI. HI Gas in Jellyfish Galaxies: The case of JO201 and JO206
  We present HI observations of the jellyfish galaxy, JO201. This massive
galaxy (M$_{\ast} = 3.5 \times 10^{10}$ M$_\odot$) is falling along the
line-of-sight towards the centre of a rich cluster (M$_{200} \sim 1.6 \times
10^{15}$ M$_\odot$, $\sigma_{cl} \sim 982$ km/s) at a high velocity $\geq$3363
km/s. Its H$\alpha$ emission shows a $\sim$40 kpc tail confined closely to its
stellar disc and a $\sim$100 kpc tail extending further out. We find HI
emission coinciding only with the shorter clumpy H$\alpha$ tail. In total, we
measure an HI mass of M$_{\rm HI} = 1.65 \times 10^{9}$ M$_\odot$, which is
about 60% lower than expected based on its stellar mass and stellar surface
density. We compared JO201 to another jellyfish in the GASP sample, JO206 (of
similar mass but residing in a 10$\times$ less massive cluster), and find that
they are similarly HI-deficient. Of the total HI mass in JO201, about 30% lies
outside the galaxy disc in projection. This HI fraction is probably a lower
limit since most of the HI is redshifted relative to the stellar disc and could
be outside the disc. The global star formation rate (SFR) analysis of JO201
suggests that its observed SFR would be expected if it had 10$\times$ its
current HI mass. The disc is the main contributor of the high star formation
efficiency at a given HI gas density for both galaxies, but their tails also
show higher star formation efficiencies compared to the outer regions of field
galaxies. Generally, we find that JO201 and JO206 are similar based on their HI
content, stellar mass and star formation rate. This finding is unexpected
considering their different environments. A toy model comparing the ram
pressure of the ICM versus the restoring forces of these galaxies suggests that
the ram pressure strength exerted on them could be comparable if we consider
their 3D orbital velocities and radial distances relative to the clusters.
