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}.
Highlights
We focus our attention on the study of the magnetic field strength in the outskirts of four spiral galaxies, following a different approach to those commonly based on Faraday rotation, dust polarization, synchrotron emission, etc
We found that a H2 molecular density (n H2) density of about 103 cm3 is suitable to fit the CO(2-1) line intensity for the four galaxies in our sample
We have considered the upper limits of M and n as fixed values in order to calculate the magnetic field strength
Summary
We focus our attention on the study of the magnetic field strength in the outskirts of four spiral galaxies, following a different approach to those commonly based on Faraday rotation, dust polarization, synchrotron emission, etc. To constrain the magnetic field strength of spiral galaxies, we will follow the Dotson method, described in Section 4.4 of [1]; i.e., approaching the magneto-hydrodynamic force equation to derive a simple expression to estimate the upper limit of the magnetic field. Where R is the radius of the magnetic field lines, n is the molecular hydrogen gas density, M and r are the total mass and radius of the source, respectively. To estimate the density n and mass M of the source, we use the carbon monoxide emission as a tracer of the molecular gas H2 [2] CO it is a good tracer for molecular hydrogen
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