arXiv subjects

Ka Ho Yuen

Publications and source records attributed to Ka Ho Yuen.

43 records · Page 3Linked to original sources

arXiv2018

Gradients of Synchrotron Polarization: Tracing 3D distribution of magnetic fields

We describe a new technique for probing galactic and extragalactic 2D and 3D magnetic field distribution using gradients of polarized synchrotron emission. The fluctuations of magnetic field are elongated along the ambient magnetic field. Therefore, the field variations are maximal perpendicular to the B-field. This allows tracing B-field with synchrotron polarization gradients (SPGs). We demonstrate that the Faraday depolarization allows to map 3D B-field structure by. The depolarization ensures that the polarization gradients sample the regions close to the observer with the sampling depth controlled by the frequency of radiation. We also analyze the B-field properties along the line-of-sight by applying the gradient technique to the wavelength derivative of synchrotron polarization. This Synchrotron Derivative Polarization Gradients (SDPGs) technique can recover the 3D vectors of the underlying B-fields. The new techniques are different from the Faraday tomography as they provide a way to map the 3D distribution of B-fields components perpendicular to the line of sight. In addition, we find that the alignment of gradients of polarization with the synchrotron polarization can be used to separate the contribution of the foreground from the polarization of cosmological origin. We notice that the same alignment is also present for the dust polarization.

astro-ph.GA↗

arXiv2018

Improving the Accuracy of Magnetic Field Tracing by Velocity Gradients: Principal Component Analysis

Tracing of the magnetic field with Velocity Gradient Technique (VGT) allows observers to probe magnetic field directions with spectroscopic data. In this paper, we employ the method of Principal Component Analysis (PCA) to extract the spectroscopic information most valuable for VGT. By using synthetic observation data from numerical simulations, we show that PCA acts in a way similar to spatial filtering along the velocity axis. We study both subsonic and supersonic simulations and show that with the PCA filtering the tracing of magnetic fields by the VGT is significantly improved. Using 21 cm GALFA data, we demonstrate that the PCA filtering improves the alignment of the velocity gradients and the Planck dust polarization.

astro-ph.GA↗

arXiv2018

Synchrotron intensity gradients as tracers of magnetic field

On the basis of the modern understanding of MHD turbulence, we propose a new way of using synchrotron radiation, namely using synchrotron intensity gradients for tracing astrophysical magnetic fields. We successfully test the new technique using synthetic data obtained with the 3D MHD simulations and provide the demonstration of the practical utility of the technique by comparing the directions of magnetic field that are obtained with PLANCK synchrotron intensity dats to the directions obtained with PLANCK synchrotron polarization data. We demonstrate that the synchrotron intensity gradients (SIGs) can reliably trace magnetic field in the presence of noise and can provide detailed maps of magnetic-field directions. We also show that the SIGs are relatively robust for tracing magnetic fields while the low spacial frequencies of the synchrotron image are removed. This makes the SIGs applicable to tracing of magnetic fields using interferometric data with single dish measurement absent. We discuss the synergy of using the SIGs together with synchrotron polarization in order to find the actual direction of the magnetic field and quantify the effects of Faraday rotation as well as with other ways of studying astrophysical magnetic fields. We test our method in the presence of noise and the resolution effects. We stress the complementary nature of the studies using the SIG technique and those employing the recently-introduced velocity gradient techniques that traces the magnetic fields using spectroscopic data.

astro-ph.GA↗

arXiv2017

Tracing magnetic fields with spectroscopic channel maps

We identify velocity channel map intensities as a new way to trace magnetic fields in turbulent media. This work makes use of both of the modern theory of MHD turbulence predicting that the magnetic eddies are aligned with the local direction of magnetic field, and also the theory of the spectral line Position-Position-Velocity (PPV) statistics describing how the velocity and density fluctuations are mapped into the PPV space. In particular, we use the fact that, the fluctuations of intensity of thin channel maps are mostly affected by the turbulent velocity, while the thick maps are dominated by density variations. We study how contributions of the fundamental MHD modes affect the Velocity Channel Gradients (VChGs) and demonstrate that the VChGs arising from Alfven and slow modes are aligned perpendicular to the local direction of magnetic field, while the VChGs produced by the fast mode are aligned parallel to the magnetic field. The dominance of the Alfven and slow modes in the interstellar media will therefore allow a reliable magnetic field tracing using the VChGs. We explore ways of identifying self-gravitating regions that do not require polarimetric information. In addition, we also introduce a new measure termed "Reduced Velocity Centroids" (RVCGs) and compare its abilities with the VChGs. We employed both measures to the GALFA 21cm data and successfully compared thus magnetic field directions with the {\it PLANCK} polarization. The applications of the suggested techniques include both tracing magnetic field in diffuse interstellar media and star forming regions, as well as removing the galactic foreground in the framework of cosmological polarization studies.

astro-ph.GA↗

arXiv2017

Tracing interstellar magnetic field using the velocity gradient technique in shock and self-gravitating media

This study proceeds with the development of the technique employing velocity gradients that were identified in (\cite{GL17}, henceforth GL17) as a means of probing magnetic field in interstellar media. We demonstrate a number of practical ways on improving the accuracy of tracing magnetic fields in diffuse interstellar media using velocity centroid gradients (VCGs). Addressing the magnetic field tracing in super-Alfvenic turbulence we introduce the procedure of filtering low spatial frequencies, that enables magnetic field tracing in the situations when the kinetic energy of turbulent plasmas dominate its magnetic energy. We propose the synergic way of of using VCGs together with intensity gradients (IGs), synchrotron intensity gradients (SIGs) as well as dust polarimetry. We show that while the IGs trace magnetic field worse than the VCGs, the deviations of the angle between the IGs and VCGs trace the shocks in diffuse media. Similarly the perpendicular orientation of the VCGs and the SIGs or to the dust polarimetry data trace the regions of gravitational collapse. We demonstrate the utility of combining the VCGs, IGs and polarimetry using GALFA HI and Planck polarimetry data. We also provide an example of synergy of the VCGs and the SIGs using the HI4PI full-sky HI survey together with the Planck synchrotron intensity data.

astro-ph.GA↗

arXiv2017

Tracing interstellar magnetic field using velocity gradient technique: Application to Atomic Hydrogen data

The advancement of our understanding of MHD turbulence opens ways to develop new techniques to probe magnetic fields. In MHD turbulence, the velocity gradients are expected to be perpendicular to magnetic fields and this fact was used by Gonsalvez-Casanova & Lazarian to introduce a new technique to trace magnetic fields using velocity centroid gradients. The latter can be obtained from spectroscopic observations. We apply the technique to GALFA HI survey data and compare the directions of magnetic fields obtained with our technique with the direction of magnetic fields obtained using PLANCK polarization. We find excellent correspondence between the two ways of magnetic field tracing, which is obvious via visual comparison and through measuring of the statistics of magnetic field fluctuations obtained with the polarization data and our technique. This suggests that the velocity centroid gradients can provide a reliable way of measuring of the foreground magnetic field fluctuations and thus provide a new way of separating foreground and CMB polarization signals.

astro-ph.GA↗

arXiv2015

Self-similar Fragmentation Regulated by Magnetic Fields in a Massive Star Forming Filament

Most molecular clouds are filamentary or elongated. Among those forming low-mass stars, their long axes tend to be either parallel or perpendicular to the large-scale (10-100 pc) magnetic field (B-field) in the surrounding inter cloud medium. This arises because, along the dynamically dominant B-fields, the competition between self-gravity and turbulent pressure will shape the cloud to be elongated either perpendicular or parallel to the fields. Recent study also suggested that, on the scales of 0.1-0.01 pc, fields are dynamically important within cloud cores forming massive stars. But whether the core field morphologies are inherited from the inter cloud medium or governed by cloud turbulence is under vigorous debate, so is the role played by B-fields in cloud fragmentation at 10 - 0.1 pc scales. Here we report B-field maps covering 100-0.01 pc scales inferred from polarimetric observations of a massive-star forming region, NGC 6334. First, the main filament also lies perpendicular to the ambient field. NGC 6334 hosts young star-forming sites where fields are not severely affected by stellar feedback, and their directions do not change significantly over the entire scale range. This means that the fields are dynamically important. At various scales, we find that the hourglass-shaped field lines are pinched where the gas column density peaks and the field strength is proportional to the 0.4-power of the density. We conclude that B-fields play a crucial role in the fragmentation of NGC 6334.

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