The Halo Shape and Evolution of Polar Disc Galaxies

TL;DR

This study uses simulations to analyze the shape and evolution of a polar disc galaxy, revealing the dark matter halo's oblate shape, the galaxy's formation via a major merger, and the role of filamentary gas infall.

Contribution

It provides new insights into the halo shape, formation history, and gas accretion processes specific to polar disc galaxies through detailed simulation analysis.

Findings

Dark matter halo is an oblate ellipsoid flattened towards the polar disc.

The galaxy's shape profile shows a twist, with inner flattening perpendicular to the old disc.

The galaxy likely formed from a major merger with orthogonal angular momentum, followed by coherent gas infall.

Abstract

We examine the properties and evolution of a simulated polar disc galaxy. This galaxy is comprised of two orthogonal discs, one of which contains old stars (old stellar disc), and the other, containing both younger stars and the cold gas (polar disc) of the galaxy. By exploring the shape of the inner region of the dark matter halo, we are able to confirm that the halo shape is a oblate ellipsoid flattened in the direction of the polar disc. We also note that there is a twist in the shape profile, where the innermost 3 kpc of the halo flattens in the direction perpendicular to the old disc, and then aligns with the polar disc out until the virial radius. This result is then compared to the halo shape inferred from the circular velocities of the two discs. We also use the temporal information of the simulation to track the system's evolution, and identify the processes which give rise to this unusual galaxy type. We confirm the proposal that the polar disc galaxy is the result of the last major merger, where the angular moment of the interaction is orthogonal to the angle of the infalling gas. This merger is followed by the resumption of coherent gas infall. We emphasise that the disc is rapidly restored after the major merger and that after this event the galaxy begins to tilt. A significant proportion of the infalling gas comes from filaments. This infalling gas from the filament gives the gas its angular momentum, and, in the case of the polar disc galaxy, the direction of the gas filament does not change before or after the last major merger.