Magnetic Fields, Star Formation Rates, and Gas Densities at Sub-kiloparsec Scales in a Pilot Sample of Nearby Galaxies

Abstract

We have estimated the magnetic field strengths of a sample of seven galaxies using their nonthermal synchrotron radio emission at meter wavelengths, and assuming energy equipartition between magnetic fields and cosmic-ray particles. We tested for deviation of magnetic fields from energy equipartition with cosmic-ray particles, and found that deviations of ∼25% are typical for the sample galaxies. Spatially resolved star formation rates (SFRs) were estimated for the seven galaxies along with five galaxies studied previously. For the combined sample of 12 galaxies, the equipartition magnetic fields ( B _eq ) are correlated with the SFR surface densities (Σ _SFR ) at sub-kiloparsec scales with B _eq ∝ ${{\rm{\Sigma }}}_{\mathrm{SFR}}^{0.31\pm 0.06}$ , consistent with model predictions. We estimated gas densities ( ρ _gas ) for a subsample of seven galaxies using archival observations of the CO rotational transitions and the atomic hydrogen (H i ) 21 cm line and studied the spatially resolved correlation between the magnetic fields and ρ _gas . Magnetic fields and gas densities are found to be correlated at sub-kiloparsec scale as B _eq ∝ ${\rho }_{\mathrm{gas}}^{0.40\pm 0.09}$ . This is broadly consistent with models, which typically predict B ∝ ${\rho }_{\mathrm{gas}}^{0.5}$ .

Keywords

• Magnetic fields
• Star formation
• Interstellar medium
• Radio continuum emission