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 ( < 20 Mpc) galaxies with resolved ( 5 ″ – 18 ″ ; 90 pc–1 kpc) imaging polarimetric observations using SOFIA/HAWC+ from 53 to 214 μm. We compute the magnetic pitch-angle ( Ψ 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 Ψ B profiles tend to be more ordered across all galactocentric radii in radio ( ζ 6 cm = 0.93 ± 0.03 ) than in FIR ( ζ 154 μ m = 0.84 ± 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.

The Structure of Magnetic Fields in Spiral Galaxies: A Radio and Far-infrared Polarimetric Analysis

“…The reason for this difference may be caused by the interaction of M51 with M51b (NGC 5195) and/or the injection of kinetic energy driven by the region of strong star formation in the outskirts of the spiral arms of M51. These results have been further confirmed using a sample of seven spiral galaxies with FIR and radio polarimetric observations (Borlaff et al 2023). The difference in the observed B-field structure arises from the different nature of the interstellar medium (ISM) associated with the FIR and radio wavelengths.…”

“…Far-infrared (FIR) polarimetric observations have revealed different components of the large-scale B-fields in the disk of galaxies (e.g., Borlaff et al 2021Borlaff et al , 2023Lopez-Rodriguez et al 2021. The detailed study performed in the spiral galaxy M51 showed that the radio and FIR magnetic pitch angles are similar within the central 6 kpc, but at larger radii Ψ B is wrapped tighter at FIR than at radio wavelengths (Borlaff et al 2021).…”

The Magnetic Fields of Starburst Galaxies. I. Identification and Characterization of the Thermal Polarization in the Galactic Disk and Outflow

“…These studies show that the B-field orientation of M 82 at 89 and 850 μm is predominantly parallel to the galactic outflow with a secondary component parallel to the galaxy disk at galactocentric radius >1 kpc. The B-fields in the outflows have also been reported (Lopez-Rodriguez et al 2021, 2022bBorlaff et al 2023) within the range of 53-214 μm for M 82 and NGC 2146. These results are quite extraordinary because they show that magnetically aligned dust grains are present in the CGMaround starburst galaxies up to scales of several kpc.…”

“…The lines show the E-vectors with the 10% polarization legend shown in the bottom lefthand corner. FIR polarimetry observations reliably trace the B-fields in the galactic outflows (middle panel) (Jones et al 2019;Lopez-Rodriguez et al 2021;Borlaff et al 2023). Radio polarimetric observations have short lifetime cosmic rays along the galactic outflow and only a magnetized bar is measured (right-hand panel) (e.g., Thompson et al 2006;Adebahr et al 2017).…”

“…Another difference may be the assumption of equipartition between cosmic rays and magnetic energy at radio wavelengths, and between turbulent kinetic energy and turbulent magnetic energy at FIR wavelengths. At FIR wavelengths, the B-fields have been measured to be more chaotic than those traced at radio wavelengths (Borlaff et al 2023;Surgent et al 2023). MHD turbulence simulations in a cube have also shown more tangled B-fields in the cold (T <10 3 K) phase than in the warm (T 10 3 K) phase (Seta & Federrath 2022).…”