Within the GASP survey, aimed at studying the effect of the ram-pressure stripping on the star formation quenching in cluster galaxies, we analyze here ALMA observations of the jellyfish galaxy JW100. We find an unexpected large amount of molecular gas (∼ 2.5 × 10 10 M ), 30% of which is located in the stripped gas tail out to ∼35 kpc from the galaxy center. The overall kinematics of molecular gas is similar to the one shown by the ionized gas, but for clear signatures of double components along the stripping direction detected only out to 2 kpc from the disk. The line ratio r 21 has a clumpy distribution and in the tail can reach large values (≥ 1), while its average value is low (0.58 with a 0.15 dispersion). All these evidence strongly suggest that the molecular gas in the tail is newly born from stripped HI gas or newly condensed from stripped diffuse molecular gas. The analysis of interferometric data at different scales reveals that a significant fraction (∼ 40%) of the molecular gas is extended over large scales (≥ 8 kpc) in the disk, and this fraction becomes predominant in the tail (∼ 70%). By comparing the molecular gas surface density with the star formation rate surface density derived from the Hα emission from MUSE data, we find that the depletion time on 1 kpc scale is particularly large (5 − 10 Gyr) both within the ram-pressure disturbed region in the stellar disk, and in the complexes along the tail.

We present VLA H I observations of JO206, a prototypical ram-pressure stripped galaxy in the GASP sample. This massive galaxy (M * = 8.5 × 10 10 M ) is located at a redshift of z = 0.0513, near the centre of the low-mass galaxy cluster, IIZw108 (σ ∼ 575 km s −1 ). JO206 is characterised by a long tail (≥90 kpc) of ionised gas stripped away by ram-pressure. We find a similarly long H I tail in the same direction as the ionised gas tail and measure a total H I mass of 3.2 × 10 9 M . This is about half the expected H I mass given the stellar mass and surface density of JO206. A total of 1.8 × 10 9 M (60%) of the detected H I 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 H I mass. On average we find a H I gas depletion time of ∼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 H I data. The star formation efficiency of the disc is about 10 times higher than that of the tail at fixed H I surface densities. Both the inner and outer parts of JO206 show an enhanced star formation compared to regions of similar H I surface density in field galaxies. The enhanced star formation is due to ram-pressure stripping during the galaxy's first infall into the cluster.

We present MeerKAT observations of neutral hydrogen gas (H i) in the nearby merger remnant NGC 1316 (Fornax A), the brightest member of a galaxy group which is falling into the Fornax cluster. We find H i on a variety of scales, from the galaxy centre to its large-scale environment. For the first time we detect H i at large radii (70 -150 kpc in projection), mostly distributed on two long tails associated with the galaxy. Gas in the tails dominates the H i mass of NGC 1316: 7 × 10 8 M -14 times more than in previous observations. The total H i mass is comparable to the amount of neutral gas found inside the stellar body, mostly in molecular form. The H i tails are associated with faint optical tidal features thought to be the remnant of a galaxy merger occurred a few billion years ago. They demonstrate that the merger was gas-rich. During the merger, tidal forces pulled some gas and stars out to large radii, where we now detect them in the form of optical tails and, thanks to our new data, H i tails; while torques caused the remaining gas to flow towards the centre of the remnant, where it was converted into molecular gas and fuelled the starburst revealed by the galaxy's stellar populations. Several of the observed properties of NGC 1316 can be reproduced by a ∼ 10:1 merger between a dominant, gas-poor early-type galaxy and a smaller, gas-rich spiral occurred 1 -3 Gyr ago, likely followed by subsequent accretion of satellite galaxies.