Exploring Lenticular Galaxy Formation in Field Environments Using NewHorizon: Evidence for Counterrotating Gas Accretion as a Formation Channel

The formation pathways of lenticular galaxies (S0s) in field environments remain a matter of debate. We utilize the cosmological hydrodynamic simulation, NewHorizon, to investigate the issue. We select two massive star formation quenched S0s as our main sample. By closely tracing their physical and morphological evolution, we identify two primary formation channels: mergers and counterrotating gas accretion. The former induces central gas inflow due to gravitational and hydrodynamic torques, triggering active central star formation, which quickly depletes the gas of the galaxy. Counterrotating gas accretion overall has a similar outcome but more exclusively through hydrodynamic collisions between the preexisting and newly accreted gas. Both channels lead to S0 morphology, with gas angular momentum cancellation being a crucial mechanism. These formation pathways quench star formation on a short timescale (<Gyr) compared to the timescales of environmental effects. We also discuss how counterrotating gas accretion may explain the origin of S0s with ongoing star formation and the frequently observed gas-star misaligned kinematics in S0s.