[Paper Review] Extragalactic magnetism with SOFIA (SALSA Legacy Program) -- V: First results on the magnetic field orientation of galaxies

Alejandro S. Borlaff, Enrique López-Rodríguez+14 more|arXiv (Cornell University)|Mar 23, 2023

Astrophysics and Star Formation Studies4 citations

TL;DR

This study presents the first resolved far-infrared (FIR) and radio (3–6 cm) polarimetric observations of 14 nearby galaxies using SOFIA/HAWC+, measuring magnetic field orientation via magnetic pitch angle (Ψ_B) and introducing a new magnetic alignment parameter (ζ). It finds that FIR-traced magnetic fields are significantly more turbulent than radio-traced fields, especially in spiral galaxies, indicating that dense, dusty star-forming regions trace less ordered fields than warmer, less dense ISM phases.

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 on extragALactic magnetiSm with SOFIA (SALSA) of 14 nearby ($<20$ Mpc) galaxies with resolved (5 arcsec-18 arcsec; $90$ pc--$1$ kpc) imaging polarimetric observations using HAWC+/SOFIA from $53$ to $214$ \um. We compute the magnetic pitch angle ($Ψ_{B}$) profiles as a function of the galactrocentric radius. We introduce a new magnetic alignment parameter ($ζ$) to estimate the disordered-to-ordered $B$-field ratio in 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 with galactocentric radius in radio ($ζ_{ m{6cm}} = 0.93\pm0.03$) than in FIR ($ζ_{ m{154μm}} = 0.84\pm0.14$). 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 the morphology of the molecular clouds within a vertical height of few hundred pc in the disk of spiral galaxies than the radio $B$-fields.

Motivation & Objective

• To investigate the magnetic field structure in nearby galaxies using multi-wavelength polarimetry.
• To compare magnetic field morphology and order between far-infrared (FIR) and radio (3–6 cm) wavelengths.
• To quantify the degree of order in magnetic fields across galactocentric radii using a new magnetic alignment parameter (ζ).
• To assess how magnetic fields in different interstellar medium (ISM) phases—dense, cold dust-emitting regions vs. warm, diffuse regions—differ in organization.
• To determine whether FIR polarization better traces magnetic fields in galactic outflows than radio polarization.

Proposed method

• Acquired resolved (5–18 arcsec) imaging polarimetric data at 53–214 μm using SOFIA/HAWC+ for 14 nearby galaxies (≤20 Mpc).
• Computed magnetic pitch angle (Ψ_B) profiles as a function of galactocentric radius using pixel-level polarization position angles.
• Introduced a new magnetic alignment parameter (ζ) to estimate the disordered-to-ordered ratio of spiral magnetic fields.
• Applied Monte Carlo simulations (10,000 iterations) to propagate uncertainties in tilt angle, inclination, and Stokes parameters into robust median Ψ_B profiles and their confidence intervals (68%, 95%).
• Averaged Ψ_B over radial bins using the atan2 method: ȳΨ_B(R) = atan2(⟨cos Ψ_B⟩, ⟨sin Ψ_B⟩), with robust median estimation.
• Excluded central 2–3 beams (4 pixels diameter) to avoid statistical bias from low-number-of-measurements regions.

Experimental results

Research questions

• RQ1 How do the magnetic field orientations traced in the far-infrared (FIR) compare to those traced at radio (3–6 cm) wavelengths in nearby galaxies?
• RQ2 To what extent are FIR magnetic fields more turbulent than radio magnetic fields in spiral and starburst galaxies?
• RQ3 Does the magnetic alignment parameter (ζ) reveal a systematic difference in field order between FIR and radio bands?
• RQ4 Are FIR polarimetric observations more sensitive to magnetic fields in dense, dusty, star-forming regions than radio observations?
• RQ5 Do FIR magnetic fields better trace magnetic fields along galactic outflows in starburst galaxies than radio fields?

Key findings

• FIR and radio polarimetry do not generally trace the same magnetic field morphology, with FIR showing more complex, turbulent structures.
• The magnetic alignment parameter ζ indicates that radio 6 cm fields are more ordered (ζ_6cm = 0.93 ± 0.03) than FIR 154 μm fields (ζ_154μm = 0.84 ± 0.14).
• In spiral galaxies, FIR magnetic fields are 2–75% more turbulent than radio magnetic fields, indicating stronger disordered components in dense, cold ISM regions.
• For starburst galaxies, FIR polarization better traces magnetic fields along galactic outflows than radio polarization, suggesting FIR is more sensitive to magnetic structures in outflowing, dusty gas.
• The FIR magnetic fields are more sensitive to local activity in star-forming regions and molecular clouds within ~300 pc of the disk plane than radio fields.
• Magnetic fields in dense, cold, dusty regions (traced by FIR) are less ordered than in warmer, less dense regions (traced by radio), indicating phase-dependent magnetic field organization.