[Paper Review] Extragalactic Magnetism with SOFIA (Legacy Program) -- I: The magnetic field in the multi-phase interstellar medium of M51

Alejandro S. Borlaff, Enrique López-Rodríguez+18 more|arXiv (Cornell University)|May 19, 2021

Astrophysics and Star Formation Studies4 references4 citations

TL;DR

This study uses SOFIA/HAWC+ far-infrared polarization data to investigate the magnetic field structure in the multi-phase interstellar medium of the face-on spiral galaxy M51, revealing that FIR and radio polarization trace distinct magnetic field morphologies. It finds tighter magnetic pitch angles at 154 μm than at 3–6 cm in the spiral arms, especially in the outer disk (R ≥ 7 kpc), and links increased small-scale turbulent fields to higher gas column density and CO velocity dispersion, while radio polarized intensity correlates with star formation rate, suggesting small-scale dynamo amplification.

ABSTRACT

The recent availability of high-resolution far-infrared (FIR) polarization observations of galaxies using HAWC+/SOFIA has facilitated studies of extragalactic magnetic fields in the cold and dense molecular disks.We investigate if any significant structural differences are detectable in the kpc-scale magnetic field of the grand design face-on spiral galaxy M51 when traced within the diffuse (radio) and the dense and cold (FIR) interstellar medium (ISM). Our analysis reveals a complex scenario where radio and FIR polarization observations do not necessarily trace the same magnetic field structure. We find that the magnetic field in the arms is wrapped tighter at 154um than at 3 and 6 cm; statistically significant lower values for the magnetic pitch angle are measured at FIR in the outskirts (R > 7 kpc) of the galaxy. This difference is not detected in the interarm region. We find strong correlations of the polarization fraction and total intensity at FIR and radio with the gas column density and 12CO(1-0) velocity dispersion. We conclude that the arms show a relative increase of small-scale turbulent B-fields at regions with increasing column density and dispersion velocities of the molecular gas. No correlations are found with HI neutral gas. The star formation rate shows a clear correlation with the radio polarized intensity, which is not found in FIR, pointing to a small-scale dynamo-driven B-field amplification scenario. This work shows that multi-wavelength polarization observations are key to disentangling the interlocked relation between star formation, magnetic fields, and gas kinematics in the multi-phase ISM.

Motivation & Objective

• To compare magnetic field structures traced by far-infrared (FIR) and radio polarization in the multi-phase interstellar medium of M51.
• To determine whether the magnetic pitch angle differs between FIR (154 μm) and radio (3–6 cm) wavelengths in spiral arms and interarm regions.
• To investigate correlations between magnetic field properties (polarization fraction, intensity) and gas properties (column density, CO velocity dispersion, H I dispersion).
• To assess the role of star formation in driving magnetic field amplification via small-scale dynamo processes.
• To validate the accuracy of magnetic pitch angle measurements using mock observations with varying signal-to-noise ratios and inclinations.

Proposed method

• Acquired high-resolution far-infrared polarization data at 154 μm using SOFIA’s HAWC+ instrument, sensitive to cold, dense dust in the ISM.
• Compared FIR polarization data with existing 3–6 cm radio polarization data from Fletcher et al. (2011) to assess differences in magnetic field orientation.
• Applied Line Integral Convolution (LIC) technique to visualize magnetic field structures in the plane of the sky, using only measurements with $P/ au_P \> 3$.
• Used mock observations with controlled signal-to-noise ratios (SNR) and inclinations to test the accuracy of magnetic pitch angle estimation, achieving ≤5° uncertainty for $P/ au_P \> 2$.
• Quantified correlations between polarization properties (total intensity, polarized intensity, polarization fraction) and gas properties (H I and 12 CO(1–0) velocity dispersion, column density) via statistical analysis.
• Analyzed the relationship between star formation rate and polarized intensity to infer dynamo-driven field amplification mechanisms.

Experimental results

Research questions

• RQ1 Do the magnetic field structures traced by FIR and radio polarization in M51 exhibit significant structural differences at kpc scales?
• RQ2 Is the magnetic pitch angle in the spiral arms systematically different at 154 μm compared to 3–6 cm, particularly in the outer disk (R ≥ 7 kpc)?
• RQ3 Are there significant correlations between polarization fraction and total intensity in FIR and radio with gas column density and 12 CO(1–0) velocity dispersion?
• RQ4 Does the star formation rate correlate with radio polarized intensity but not FIR polarized intensity, indicating a small-scale dynamo origin?
• RQ5 To what extent do H I neutral gas properties correlate with magnetic field properties in the ISM of M51?

Key findings

• The magnetic pitch angle is statistically significantly lower at 154 μm than at 3–6 cm in the outer disk (R ≥ 7 kpc), indicating tighter field winding in the FIR-traced magnetic field.
• This difference in pitch angle is not detected in the interarm regions, suggesting a localized enhancement of small-scale turbulent magnetic fields in the spiral arms.
• Polarization fraction and total intensity in both FIR and radio show strong positive correlations with gas column density and 12 CO(1–0) velocity dispersion, indicating a link between turbulent gas motions and magnetic field structure.
• No significant correlation is found between magnetic field properties and H I neutral gas velocity dispersion, suggesting that the magnetic field response is more tightly coupled to molecular gas dynamics.
• Radio polarized intensity shows a clear correlation with the star formation rate, while FIR polarized intensity does not, supporting a scenario of small-scale dynamo amplification driven by star formation activity.
• Mock observation tests confirm that magnetic pitch angle can be measured with ≤5° accuracy for $P/ au_P \> 2$, validating the reliability of the analysis in low-SNR regions.