Magnetic fields and star formation in spiral galaxies

TL;DR

This paper summarizes radio observations of magnetic fields in spiral galaxies, revealing how magnetic strength and structure relate to star formation rates and galaxy orientation, with implications for galactic winds and magnetic field configurations.

Contribution

It provides new insights into the nonlinear relationship between magnetic field strength and star formation rate, and shows that large-scale magnetic field patterns are consistent across different galaxy types and star formation activities.

Findings

Nonthermal radio emission and total magnetic field grow nonlinearly with SFR.

Vertical scale heights are about equal in different galaxies, indicating galactic winds.

Large-scale magnetic field patterns are similar regardless of SFR or galaxy type.

Abstract

The main observational results from radio continuum and polarization observations about the magnetic field strength and large-scale pattern for face-on and edge-on spiral galaxies are summarized and compared within our sample of galaxies of different morphological types, inclinations, and star formation rates (SFR). We found that galaxies with low SFR have higher thermal fractions/smaller synchrotron fractions than those with normal or high SFR. Adopting an equipartition model, we conclude that the nonthermal radio emission and the \emph{total magnetic field} strength grow nonlinearly with SFR, while the regular magnetic field strength does not seem to depend on SFR. We also studied the magnetic field structure and disk thicknesses in highly inclined (edge-on) galaxies. We found in four galaxies that - despite their different radio appearance - the vertical scale heights for both, the thin and thick disk/halo, are about equal (0.3/1.8 kpc at 4.75 GHz), independently of their different SFR. This implies that all these galaxies host a galactic wind, in which the bulk velocity of the cosmic rays (CR) is determined by the total field strength within the galactic disk. The galaxies in our sample also show a similar large-scale magnetic field configuration, parallel to the midplane and X-shaped further away from the disk plane, independent of Hubble type and SFR in the disk. Hence we conclude that also the large-scale magnetic field pattern does not depend on the amount of SFR.