The relative efficiencies of bars and clumps in driving disc stars to retrograde motion

TL;DR

This study compares how bars and clumps in disc galaxies generate retrograde stellar orbits, finding both can produce significant retrograde populations, but with different spatial distributions and limitations regarding counter-rotating discs.

Contribution

It provides a comparative analysis of the efficiencies of bars and clumps in producing retrograde stars using N-body+SPH simulations, highlighting their distinct roles and spatial characteristics.

Findings

Both bars and clumps can generate ~10% retrograde stars.

Clump-driven retrograde stars are found at large radii, bar-driven near the bar.

Neither mechanism produces counter-rotating discs.

Abstract

The presence of stars on retrograde orbits in disc galaxies is usually attributed to accretion events, both via direct accretion, as well as through the heating of the disc stars. Recent studies have shown that retrograde orbits can also be produced via scattering by dense clumps, which are often present in the early stages of a galaxy's evolution. However, so far it has been unclear whether other internally-driven mechanisms, such as bars, are also capable of driving retrograde motion. Therefore, in this paper, we investigate the efficiencies with which bars and clumps produce retrograde orbits in disc galaxies. We do this by comparing the retrograde fractions and the spatial distributions of the retrograde populations in four $N$-body$+$smooth particle hydrodynamics (SPH) simulations of isolated disc galaxies spanning a range of evolutionary behaviours. We find that both bars and clumps are capable of generating significant retrograde populations of order $\sim 10\%$ of all stars. We also find that while clump-driven retrograde stars may be found at large galactocentric radii, bar-driven retrograde stars remain in the vicinity of the bar, even if the bar dissolves. Consequently, we find that retrograde stars in the Solar Neighbourhood in the clumpy models are exclusively clump-driven, but this is a trace population, constituting $0.01-0.04\%$ of the total stellar population in this region. Finally, we find that neither bars (including dissolving ones) nor clumps in the models are able to produce rotationally supported counter-rotating discs.