2019MNRAS.482.4466P

Based on MUSE data from the GASP survey, we study the Hα-emitting extraplanar tails of 16 cluster galaxies at z ∼ 0.05 undergoing ram pressure stripping. We demonstrate that the dominating ionization mechanism of this gas (between 64 per cent and 94 per cent of the Hαemission 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 H II clumps with a median Hαvelocity dispersion σ = 27 km s^–1. We study the characteristics of over 500 star-forming clumps in the tails and find median values of Hαluminosity L_ Hα_ = 4×10³⁸ erg s^–1, dust extinction A_V = 0.5 mag, star formation rate SFR=0.003 M_☉ yr^–1, ionized gas density n_e=52cm^–3, ionized gas mass M_ gas_ = 4×10^4M_☉, and stellar mass M_* = 3×10⁶M^_☉. The tail clumps follow scaling relations (M_gas - M_*, L_H α_ - σ, SFR-M_gas) similar to disc 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 H II clumps with an average escape fraction of 18 per cent, or lower luminosity H II regions that we cannot individually identify.