Bar-driven Leading Spiral Arms in a Counter-rotating Dark Matter Halo

TL;DR

This study demonstrates through simulations that a barred galaxy in a retrograde dark matter halo can generate long-lived leading spiral arms, a phenomenon linked to rapid bar deceleration and halo resonance effects.

Contribution

It reveals a novel mechanism for leading spiral arm formation driven by bar-halo interactions in retrograde dark matter halos, supported by collisionless N-body simulations.

Findings

Leading spiral arms can last around 3 Gyr in such systems.

Rapid bar slowdown is caused by strong resonant coupling with the halo.

Dark matter wake is predicted to be perpendicular to the stellar bar.

Abstract

An overwhelming majority of galactic spiral arms trail with respect to the rotation of the galaxy, though a small sample of leading spiral arms has been observed. The formation of these leading spirals is not well understood. Here we show, using collisionless $N$-body simulations, that a barred disc galaxy in a retrograde dark matter halo can produce long-lived ($\sim3$ Gyr) leading spiral arms. Due to the strong resonant coupling of the disc to the halo, the bar slows rapidly and spiral perturbations are forced ahead of the bar. We predict that such a system, if observed, will also host a dark matter wake oriented perpendicular to the stellar bar. More generally, we propose that any mechanism that rapidly decelerates the stellar bar will allow leading spiral arms to flourish.