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

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 154 µm 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 12 CO(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.

Extragalactic magnetism with SOFIA (SALSA Legacy Program) -- III: First data release and on-the-fly polarization mapping characterization

“…Table 1 shows the details of the new observations presented in this manuscript. In addition, we also include observations of Centaurus A (Lopez-Rodriguez 2021) at 89 µm and Circinus at 53, 89, and 214 µm from program ID 07 0032 (PI: Lopez-Rodriguez, E.), M51 (Borlaff et al 2021) at 154 µm under programs 70 0509 (Guaranteed Time Observations by the HAWC+ Team), 76 0003 (Discretionary Director Time), and 08 0260 (PI: Dowell, D.), and NGC 1097 (Lopez-Rodriguez et al 2021) at 89 and 154 µm from program ID 07 0034 (PI: Lopez-Rodriguez, E.). These additional observations comprised a total exposure time of 11.84h (including overheads) taken from 2017 to 2021.…”

“…A total on-source time of 47.54h (51.34h including overheads) is presented, which corresponds to 33% of the total awarded time of 155.70h (including overheads) for this SOFIA Legacy Program. An additional 11.84h (including overheads) of observations for Centaurus A (Lopez-Rodriguez 2021), Circinus, M51 (Borlaff et al 2021), and NGC 1097(Lopez-Rodriguez et al 2021) are included in this data release. Each file contains Stokes IQU , P , P A, P I values and their associated uncertainties with a pixel scale equal to the half-beam size for a given band.…”

Extragalactic magnetism with SOFIA (Legacy Program) -- II: A magnetically-driven flow in the starburst ring of NGC 1097

“…The decrease in the FIR polarization with column density may be due to 1) variations of the dust grain alignment efficiency, 2) variations of the B-field orientation in the plane of the sky due to an increase of gas turbulence, and/or 3) complex kinematics within the beam (Lopez-Rodriguez et al 2018b;Chuss et al 2019;King et al 2019;Lopez-Rodriguez 2021;Borlaff et al 2021). We suggest that the decrease of FIR polarization with increasing column density may be due to an increment of the small-scale turbulent fields arising from the star forming regions at smaller scales than the beam of our observations (< 0.72 kpc).…”