GASP XIII. Star formation in gas outside galaxies

Abstract

Based on MUSE data from the GASP survey, we study the Halpha-emitting\nextraplanar tails of 16 cluster galaxies at z~0.05 undergoing ram pressure\nstripping. We demonstrate that the dominating ionization mechanism of this gas\n(between 64% and 94% of the Halpha emission in the tails depending on the\ndiagnostic diagram used) is photoionization by young massive stars due to\nongoing star formation (SF) taking place in the stripped tails. This SF occurs\nin dynamically quite cold HII clumps with a median Halpha velocity dispersion\nsigma = 27 km s^-1. We study the characteristics of over 500 star-forming\nclumps in the tails and find median values of Halpha luminosity L_{Halpha} = 4\nX 10^38 erg s^-1, dust extinction A_V=0.5 mag, star formation rate SFR=0.003\nM_sun yr^-1, ionized gas density n_e =52 cm^-3, ionized gas mass M_gas = 4 X\n10^4 Msun, and stellar mass M_{*} = 3 X 10^6 Msun. The tail clumps follow\nscaling relations (M_gas-M_{*}, L_{Halpha} -sigma, SFR-M_gas) similar to disk\nclumps, and their stellar masses are comparable to Ultra Compact Dwarfs and\nGlobular Clusters.The diffuse gas component in the tails is ionized by a\ncombination of SF and composite/LINER-like emission likely due to thermal\nconduction or turbulence. The stellar photoionization component of the diffuse\ngas can be due either to leakage of ionizing photons from the HII clumps with\nan average escape fraction of 18%, or lower luminosity HII regions that we\ncannot individually identify.\n