Upper Limits to Magnetic Fields in the Outskirts of Galaxies
  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}.
