Anisotropic Velocity Fluctuations in Galaxy Mergers: A Probe of the Magnetic Field

TL;DR

This paper evaluates the Velocity Gradient Technique (VGT) for mapping magnetic fields in galaxy mergers using cosmological simulations, confirming its effectiveness across different merger stages and scales.

Contribution

It demonstrates VGT's reliability in complex galaxy merger environments and compares its results with mock polarization measurements, validating its application.

Findings

Velocity fluctuations are anisotropic at all merger stages.

Galaxy mergers amplify magnetic fields and turbulence.

VGT results agree with polarization-based magnetic field measurements.

Abstract

Magnetic fields and turbulence are fundamental to the evolution of galaxies, yet their precise measurement and analysis present significant challenges. The recently developed Velocity Gradient Technique (VGT), which capitalizes on the anisotropy inherent in magnetohydrodynamic (MHD) turbulence, represents a new method for mapping magnetic fields in galaxies using spectroscopic observations. Most validations of VGT thus far, however, have relied upon idealized MHD turbulence simulations, which lack the more complex dynamics found in galaxies and galaxy mergers. In this study, we scrutinize VGT using an AREPO-based cosmological galaxy merger simulation, testing its effectiveness across pre-merger, merging, and post-merger stages. We examine the underlying assumptions of VGT and probe the statistics of gas density, velocity, and magnetic fields over time. We find that velocity fluctuations are indeed anisotropic at each stage, being larger in the direction perpendicular to the local magnetic field, as required by VGT. We find, additionally, that galaxy mergers substantially intensify velocity and density fluctuations and amplify magnetic fields at all scales. The observed scaling of the velocity fluctuations shows a steeper trend than $r^{1/2}$ between 0.6 and 3~kpc and a shallower trend at larger scales. The scaling of the magnetic field and density fluctuations at scales $\lesssim$ 1.0 kpc also predominantly aligns with $r^{1/2}$. Finally, we compare results from VGT to those derived from polarization-like mock magnetic field measurements, finding consistent and statistically significant global agreement in all cases.