Decoupled gas kinematics in isolated S0 galaxies

A sample of completely isolated S0 galaxies has been studied by means of long-slit spectroscopy at the Russian 6-m telescope. Seven of 12 galaxies have revealed the presence of extended ionized-gas discs whose rotation is mostly decoupled from the stellar kinematics. Five of these seven (71 ± 17 per cent) galaxies show a visible counterrotation of the ionized-gas component with respect to the stellar component. The emission-line diagnostics demonstrates a wide range of gas excitation mechanisms, although pure excitation by young stars is rare. We conclude that in all cases the extended gaseous discs in our sample S0s are of external origin, despite the visible isolation of the galaxies. Possible sources of external accretion, such as systems of dwarf gas-rich satellites or cosmological cold-gas filaments, are discussed.