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Le Ngoc Tram

Publications and source records attributed to Le Ngoc Tram.

5 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2021

Extragalactic Magnetism with SOFIA (Legacy Program). I. The Magnetic Field in the Multiphase Interstellar Medium of M51*

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 whether any significant structural differences are detectable in the kiloparsec-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 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 H i 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 multiwavelength polarization observations are key to disentangling the interlocked relation between star formation, magnetic fields, and gas kinematics in the multiphase ISM.

Spiral galaxies↗
NASA NTRS2021

Far-infrared Polarization Spectrum of the OMC-1 Star-forming Region

We analyze the wavelength dependence of the far-infrared polarization fraction toward the OMC-1 star-formingregion using observations from HAWC+/SOFIA at 53, 89, 154, and 214μm. Wefind that the shape of the far-infrared polarization spectrum is variable across the cloud and that there is evidence of a correlation between theslope of the polarization spectrum and the average line-of-sight temperature. The slope of the polarization spectrumtends to be negative(falling toward longer wavelengths)in cooler regions and positive orflat in warmer regions.This is very similar to what was discovered inρOph A via SOFIA polarimetry at 89 and 154μm. Like the authorsof this earlier work, we argue that the most natural explanation for our falling spectra is line-of-sight superpositionof differing grain populations, with polarized emission from the warmer regions and less-polarized emission fromthe cooler ones. In contrast with the earlier work onρOph A, we do notfind a clear correlation of polarizationspectrum slope with column density. This suggests that falling spectra are attributable to variations in grainalignment efficiency in a heterogeneous cloud consistent with radiative torques theory. Alternative explanations inwhich variations in grain alignment efficiency are caused by varying gas density rather than by varying radiationintensity are disfavored.

Joseph M. Michail↗