Dark Matter Trapping by Stellar Bars: The Shadow Bar

TL;DR

This paper introduces the concept of a shadow bar, a dark matter trapping feature formed during stellar bar evolution, which influences angular momentum transfer and could be observable through density, velocity signatures, and dark matter detection.

Contribution

It reveals the formation of a shadow bar trapping dark matter during stellar bar evolution, affecting angular momentum transfer and bar dynamics in galaxy simulations.

Findings

Shadow bar traps dark matter near the stellar bar.

Shadow bar reduces angular momentum transfer rate by over three times.

Shadow bar may be observable via density and velocity signatures.

Abstract

We investigate the complex interactions between the stellar disc and the dark-matter halo during bar formation and evolution using N-body simulations with fine temporal resolution and optimally chosen spatial resolution. We find that the forming stellar bar traps dark matter in the vicinity of the stellar bar into bar-supporting orbits. We call this feature the shadow bar. The shadow bar modifies both the location and magnitude of the angular momentum transfer between the disc and dark matter halo and adds 10 per cent to the mass of the stellar bar over 4 Gyr. The shadow bar is potentially observable by its density and velocity signature in spheroid stars and by direct dark matter detection experiments. Numerical tests demonstrate that the shadow bar can diminish the rate of angular momentum transport from the bar to the dark matter halo by more than a factor of three over the rate predicted by dynamical friction with an untrapped dark halo, and thus provides a possible physical explanation for the observed prevalence of fast bars in nature.