Upper Limits to Magnetic Fields in the Outskirts of Galaxies

TL;DR

This study estimates upper limits of magnetic field strengths in the outskirts of spiral galaxies using CO data and magneto-hydrodynamic modeling, finding values up to approximately 31 microGauss.

Contribution

It introduces a method to constrain magnetic fields in galaxy outskirts based on molecular gas data and simplified MHD equations, providing new upper limit estimates.

Findings

Magnetic field strengths are constrained to be less than about 6-31 microGauss.

Kinetic temperatures in galaxy outskirts are below 38 K.

H$_2$ densities are estimated to be below 10^3 cm$^{-3}$.

Abstract

Based on CO(2-1) public data, we study the monoxide oxygen gas excitation conditions and the magnetic field strength of four spiral galaxies. For the galaxy outskirts, we found kinetic temperatures in the range of $\lesssim$35--38 K, CO column densities $\lesssim 10^{15}$--$10^{16}$ cm$^{-2}$, and H$_2$ masses $\lesssim 4\times 10^6$--$6\times10^8$ M$_\odot$. An H$_2$ density $\lesssim 10^3$ cm$^{-3}$ is suitable to explain the 2$\sigma$ upper limits of the CO(2-1) line intensity. We constrain the magnetic field strength for our sample of spiral galaxies and their outskirts by using their masses and H$_2$ densities to evaluate a simplified magneto-hydrodynamic equation. Our estimations provide values for the magnetic field strength on the order of $\lesssim$6--31 {$\upmu$G}.