Idealized models for galactic disk formation and evolution in 'realistic' LambdaCDM haloes

TL;DR

This study uses idealized models to explore how initial conditions and angular momentum orientations affect the formation and evolution of galactic disks within realistic LambdaCDM haloes, revealing complex dependencies and diverse outcomes.

Contribution

It introduces a controlled simulation framework to systematically analyze the impact of initial gas conditions and angular momentum orientation on disk galaxy formation in cosmological haloes.

Findings

Disk-to-bulge ratio depends on cooling time and angular momentum orientation.

Orientation parallel to halo minor axis yields more stable disks.

Significant misalignments and diverse structures can develop over time.

Abstract

We study the dynamics of galactic disk formation and evolution in 'realistic' LambdaCDM haloes with idealized baryonic initial conditions. We add rotating spheres of hot gas at z=1.3 to two fully cosmological dark-matter-only halo (re)simulations. The gas cools according to an artificial and adjustable cooling function to form a rotationally supported galaxy. The simulations evolve in the full cosmological context until z=0. We vary the angular momentum and density profiles of the initial gas sphere, the cooling time and the orientation of the angular momentum vector to study the effects on the evolution of the disk. The final disks show realistic structural and kinematic properties. The slower the cooling/accretion processes, the higher the kinematic disk-to-bulge ratio D/B of the resulting system. We find that the initial orientation of the gas angular momentum with respect to the halo has a major effect on the resulting D/B. The most stable systems result from orientations parallel to the halo minor axis, but the sign of the spin can have a strong effect. Despite the spherical and coherently rotating initial gas distribution, the orientation of the central disk and of the outer gas components and the relative angle between the components can all change by more than 90 degrees over several billion years. Disks can form from initial conditions oriented parallel to the major axis, but there is often strong misalignment between inner and outer material. The more the orientation of the baryonic angular momentum changes during the evolution, the lower the final D/B. The behaviour varies strongly from halo to halo. Even our simple initial conditions can lead to strong bars, dominant bulges, massive, misaligned rings and counter-rotating components. We discuss how our results may relate to the failure or success of fully cosmological disk formation simulations. (abridged)