Dense Gas Formation via Collision-induced Magnetic Reconnection in a Disk Galaxy with a BiSymmetric Spiral Magnetic Field

TL;DR

This study explores whether collision-induced magnetic reconnection (CMR) can generate dense gas in galactic disks with spiral magnetic fields, finding it feasible under certain conditions and emphasizing the importance of high-resolution observations.

Contribution

It extends the CMR mechanism to galactic scales, demonstrating its potential role in dense gas formation in spiral arms with magnetic field reversals.

Findings

CMR can be triggered by cloud collisions at magnetic field reversals.

Stronger initial magnetic fields promote dense gas formation via CMR.

High spatial resolution is necessary to resolve the collision midplane in simulations.

Abstract

Recently, a collision-induced magnetic reconnection (CMR) mechanism was proposed to explain a dense filament formation in the Orion A giant molecular cloud. A natural question is that whether CMR works elsewhere in the Galaxy. As an initial attempt to answer the question, this paper investigates the triggering of CMR and the production of dense gas in a flat-rotating disk with a modified BiSymmetric Spiral (BSS) magnetic field. Cloud-cloud collisions at field reversals in the disk are modeled with the Athena++ code. Under the condition that is representative of the warm neutral medium, the cloud-cloud collision successfully triggers CMR at different disk radii. However, dense gas formation is hindered by the dominating thermal pressure, unless a moderately stronger initial field $\gtrsim5\mu$G is present. The strong-field model, having a larger Lundquist number $S_L$ and lower plasma $\beta$, activates the plasmoid instability in the collision midplane, which is otherwise suppressed by the disk rotation. We speculate that CMR can be common if more clouds collide along field reversals. However, to witness the CMR process in numerical simulations, we need to significantly resolve the collision midplane with a spatial dynamic range $\gtrsim10^6$. If Milky Way spiral arms indeed coincide with field reversals in BSS, it is possible that CMR creates or maintains dense gas in the arms. High-resolution, high-sensitivity Zeeman/Faraday-Rotation observations are crucial for finding CMR candidates that have helical fields.