Magnetic fields in nearby normal galaxies: Energy equipartition

TL;DR

This study maps magnetic fields in five nearby galaxies, revealing their strength, distribution, and energy balance with interstellar gas, highlighting the magnetic field's role in galactic pressure equilibrium.

Contribution

First detailed sub-kpc scale magnetic field maps in multiple galaxies using equipartition assumptions, analyzing their distribution and energy balance.

Findings

Magnetic fields are strongest near galaxy centers (~20-25 μG).

Magnetic energy density is comparable to gas energy density within the arms.

Magnetic fields dominate over gas kinetic energy in interarm regions.

Abstract

We present maps of total magnetic field using 'equipartition' assumptions for five nearby normal galaxies at sub-kpc spatial resolution. The mean magnetic field is found to be ~11 \mu G. The field is strongest near the central regions where mean values are ~20--25 \mu G and falls to ~15 \mu G in disk and ~10 \mu G in the outer parts. There is little variation in the field strength between arm and interarm regions, such that, in the interarms, the field is < 20 percent weaker than in the arms. There is no indication of variation in magnetic field as one moves along arm or interarm after correcting for the radial variation of magnetic field. We also studied the energy densities in gaseous and ionized phases of the interstellar medium and compared to the energy density in the magnetic field. The energy density in the magnetic field was found to be similar to that of the gas within a factor of <2 at sub-kpc scales in the arms, and thus magnetic field plays an important role in pressure balance of the interstellar medium. Magnetic field energy density is seen to dominate over the kinetic energy density of gas in the interarm regions and outer parts of the galaxies and thereby helps in maintaining the large scale ordered fields seen in those regions.