Based on MUSE data from the GASP survey , we study the H \alpha -emitting extraplanar tails of 16 cluster galaxies at z \sim 0.05 undergoing ram pressure stripping .
We demonstrate that the dominating ionization mechanism of this gas ( between 64 % and 94 % of the H \alpha emission in the tails depending on the diagnostic diagram used ) is photoionization by young massive stars due to ongoing star formation ( SF ) taking place in the stripped tails .
This SF occurs in dynamically quite cold HII clumps with a median H \alpha velocity dispersion \sigma = 27 km s ^ { -1 } .
We study the characteristics of over 500 star-forming clumps in the tails and find median values of H \alpha luminosity L _ { H \alpha } = 4 \times 10 ^ { 38 } erg s ^ { -1 } , dust extinction A _ { V } = 0.5 mag , star formation rate SFR = 0.003 M _ { \odot } yr ^ { -1 } , ionized gas density n _ { e } = 52 cm ^ { -3 } , ionized gas mass M _ { gas } = 4 \times 10 ^ { 4 } M _ { \odot } , and stellar mass M _ { * } = 3 \times 10 ^ { 6 } M _ { \odot } .
The tail clumps follow scaling relations ( M _ { gas } - M _ { * } , L _ { H \alpha } - \sigma , SFR- M _ { gas } ) similar to disk clumps , and their stellar masses are comparable to Ultra Compact Dwarfs and Globular Clusters .
The diffuse gas component in the tails is ionized by a combination of SF and composite/LINER-like emission likely due to thermal conduction or turbulence .
The stellar photoionization component of the diffuse gas can be due either to leakage of ionizing photons from the HII clumps with an average escape fraction of 18 % , or lower luminosity HII regions that we can not individually identify .