Exploring lenticular galaxy formation in field environments using NewHorizon: evidence for counter-rotating gas accretion as a formation channel

TL;DR

This study uses the NewHorizon simulation to explore how lenticular galaxies form in isolated environments, highlighting counter-rotating gas accretion as a key formation channel that explains certain observed features.

Contribution

It identifies and characterizes two main formation pathways for S0 galaxies in the field, emphasizing the role of counter-rotating gas accretion, which was less understood before.

Findings

Counter-rotating gas accretion can produce S0 galaxies with ongoing star formation.

Both formation channels lead to S0 morphology through angular momentum cancellation.

Star formation quenching occurs rapidly, within less than a billion years.

Abstract

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 counter-rotating 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. Counter-rotating gas accretion overall has a similar outcome but more exclusively through hydrodynamic collisions between the pre-existing 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 counter-rotating gas accretion may explain the origin of S0s with ongoing star formation and the frequently observed gas-star misaligned kinematics in S0s.