Extragalactic Magnetism with SOFIA (SALSA Legacy Program). V. First Results on the Magnetic Field Orientation of Galaxies

Abstract

We present the analysis of the magnetic field ( B -field) structure of galaxies measured with far-infrared (FIR) and radio (3 and 6 cm) polarimetric observations. We use the first data release of the Survey of extragALactic magnetiSm with SOFIA of 14 nearby ( $\lt 20$ Mpc) galaxies with resolved ( $5^{\prime\prime} \mbox{--}18^{\prime\prime} ;$ 90 pc–1 kpc) imaging polarimetric observations using SOFIA/HAWC+ from 53 to 214 μ m. We compute the magnetic pitch-angle ( ${{\rm{\Psi }}}_{B}$ ) profiles as a function of the galactocentric radius. We introduce a new magnetic alignment parameter ( ζ ) to estimate the disordered-to-ordered ratio of spiral B -fields. We find FIR and radio wavelengths to not generally trace the same B -field morphology in galaxies. The ${{\rm{\Psi }}}_{B}$ profiles tend to be more ordered across all galactocentric radii in radio ( ${\zeta }_{6\mathrm{cm}}\,=\,0.93\pm 0.03$ ) than in FIR ( ${\zeta }_{154\mu {\rm{m}}}=0.84\pm 0.14$ ). For spiral galaxies, FIR B -fields are 2%–75% more turbulent than the radio B -fields. For starburst galaxies, we find that FIR polarization is a better tracer of the B -fields along the galactic outflows than radio polarization. Our results suggest that the B -fields associated with dense, dusty, turbulent star-forming regions (those traced at FIR) are less ordered than warmer, less dense regions (those traced at radio) of the interstellar medium. The FIR B -fields seem to be more sensitive to the activity of the star-forming regions and molecular clouds within a vertical height of a few hundred parsecs in the disk of spiral galaxies than the radio B -fields.

Keywords

• Astrophysical magnetism
• Extragalactic magnetic fields
• Polarimetry
• Dust continuum emission
• Radio continuum emission
• Galaxy evolution