The MUSE view of ram pressure stripped galaxies in clusters: The GASP sample

We present the full sample of 76 galaxies in 39 galaxy cluster fields at z = 0.04 to 0.07 observed with VLT/MUSE by the GAs Stripping Phenomena in galaxies (GASP) survey. Most of them (64) were observed as possible ram pressure stripped galaxies (stripping candidates) based on optical B-band images, while the remaining 12 were a control sample of both star-forming and passive galaxies. Based on spatially resolved ionized gas and stellar kinematics, we assessed the physical origin of the gas asymmetries and find that 89% of the stripping candidates are confirmed by the VLT/MUSE data. In addition, three of the four star-forming galaxies in the control sample also show signs of ram pressure. These control galaxies display a ring of unusual emission line ratios, which we observe in field galaxies as well, possibly originating from the interaction with a hotter surrounding medium. The stripped galaxies are classified into various classes corresponding to different degrees of stripping, from the weakest to the strongest and most extreme (jellyfish galaxies) stripping, as well as truncated gas disks with gas remaining only in the galaxy center. Our results show that selecting cluster stripping candidates based on optical imaging yields a sample that is indeed largely dominated by galaxies affected by ram pressure at different stages and stripping strength, though some contamination is present mostly due to tidal processes. Strong ram pressure cases are found in galaxies over the whole range of stellar masses studied (109-1011.5 M⊙), both in low-mass and high-mass clusters (cluster velocity dispersions σ = 500-1100 km s-1). We examine the possible connection between the progressive stages of stripping, up to the phase of a truncated gas disk, and the subsequent complete stripping of gas. We discuss the incompleteness intrinsic to this and other methods of selection to obtain a complete census of ram pressure stripping in clusters.