Misaligned Angular Momentum in Hydrodynamic Cosmological Simulations: Warps, Outer Discs, and Thick Discs

TL;DR

This study uses high-resolution cosmological simulations to analyze how misaligned hot gas halos cause warps and thick discs in galaxy outer regions, revealing insights into galaxy formation processes.

Contribution

It demonstrates that misaligned hot gas halos lead to warped outer discs and thick disc formation through gas accretion and angular momentum torques, a novel insight into galaxy evolution.

Findings

Outer disc warps result from misalignment with hot gas halos.

Star formation in warps creates low-metallicity thick discs.

Misaligned hot halos influence galaxy disc structures.

Abstract

We present a detailed analysis of a disc galaxy forming in a high-resolution fully cosmological simulation to investigate the nature of the outer regions of discs and their relevance for the disc formation process. Specifically, we focus on the phenomenon of misaligned disc components and find that the outer disc warp is a consequence of the misalignment between the inner disc and the surrounding hot gaseous halo. As the infalling cold gas sinks toward the centre of the galaxy, it is strongly torqued by the hot gas halo. By the time the fresh gas reaches the central disc-forming region its angular momentum is completely aligned with the spin of the hot gas halo. If the spin of the hot gas halo, in turn, is not aligned with that of the inner disc, a misaligned outer disc forms comprised of newly accreted material. The inner and outer components are misaligned with each other because they respond differently to infalling substructure and accretion. The warped disc feeds the main gas disc due to viscous angular momentum losses, but small amounts of star formation in the warp itself form a low-metallicity thick disc. We show that observations of resolved stellar populations in warped galaxies in the local universe could provide evidence for the presence of these processes and therefore indirectly reveal ongoing gas accretion and the existence of hot gas halos.