75 Kiloparsec Trails of Ionized Gas behind Two Irregular Galaxies in A1367

Abstract

In a 6 hr Halpha exposure of the northwest region of the cluster of galaxies A1367, we discovered a 75 kpc cometary emission of ionized gas trailing behind two Irr galaxies. The Halpha trails correspond in position and length with tails of syncrotron radiation. At the galaxy side opposite to the tails, the two galaxies show bright H II regions aligned along arcs, where the star formation takes place at the prodigious rate of ~1 M solar yr -1. From the morphology of the galaxies and of the trailing material, we infer that the two galaxies are suffering from ram pressure due to their high-velocity motion through the cluster intergalactic medium. We estimate that ~10 9 M solar of gas, probably ionized in the giant H II regions, is swept out, forming the tails. The tails cross each other at some 100 kpc from the present galaxy location, indicating that a major tidal event occurred some ~5×10 7 yr ago. We exclude that mutual harassment produced the observed morphology, and we show with numerical simulations that it could have marginally aided ram pressure stripping by loosening the potential well of the galaxies.