arXiv subjects

Ka Ho Yuen

Publications and source records attributed to Ka Ho Yuen.

At least 37 records · Page 2Linked to original sources

arXiv2022

Magnetic field strength from turbulence theory (I): Using differential measure approach (DMA)

The mean plane-of-sky magnetic field strength is traditionally obtained from the combination of polarization and spectroscopic data using the Davis-Chandrasekhar-Fermi (DCF) technique. However, we identify the major problem of the DCF to be its disregard of the anisotropic character of MHD turbulence. On the basis of the modern MHD turbulence theory we introduce a new way of obtaining magnetic field strength from observations. Unlike the DCF, the new technique uses not the dispersion of the polarization angle and line of sight velocities, but increments of these quantities given by the structure functions. To address the variety of the astrophysical conditions for which our technique can be applied, we consider the turbulence in both media with magnetic pressure larger than the gas pressure corresponding e.g. to molecular and the gas pressure larger than the magnetic pressure corresponding to the warm neutral medium. We provide general expressions for arbitrary admixture of Alfvén, slow and fast modes in these media and consider in detail the particular cases relevant to diffuse media and molecular clouds. We successfully test our results using synthetic observations obtained from MHD turbulence simulations. We demonstrate that our Differential Measure Approach (DMA), unlike the DCF, can be used to measure the distribution of magnetic field strengths, can provide magnetic field measurements with limited data and is much more stable in the presence of large scale variations induces of non-turbulent nature. In parallel, our study uncover the deficiencies of the earlier DCF research.

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arXiv2022

Correlation of velocity and density contributions to spectroscopic channel maps: Reality check on Kalberla et.al (2022)

The existence of magnetized turbulence in the interstellar HI is well accepted. A number of techniques to obtain turbulence spectrum and magnetic field direction and strength have been developed and successfully applied to HI spectroscopic data. To better separate the imprints of density and velocity fluctuations to the channel maps, a new theory-based technique, the Velocity Decomposition Algorithm (VDA,Yuen et.al 2021), has been created. The technique demonstrates that the intensity fluctuations are separated into a component pv that mostly arises from velocity fluctuations and pd that mostly arise from density fluctuations. The VDA helps to clarify the nature of the filamentary structure observed in channel maps. A recent publication (Kalberla et.al 2022,K22) claims that the application of VDA to HI4PI data provides negative correlation of pv and pd,which according to the authors invalidates the technique since it requires that pv and pd have zero correlation. However, the quantities pv and pd given by VDA are naturally orthogonal which can be trivially checked analytically or numerically. That means the correct application of the VDA to any data must provide zero correlation. This is the point that we clarify in this paper and search for the cause of the mistake in the application of the VDA in K22 that resulted in the erroneous conclusion. We prove analytically that by construction pv and pd are not correlated. We identify the likely mistake in the VDA expression that K22 used and reproduce their figures with the incorrect expression. We find that 14 out of 15 figures in K22 are invalid, and thus their criticism of the VDA is ill-founded and arises from their use of incorrect expressions. We conclude that the detrimental mistake that K22 made at their analysis completely invalidate their scientific claim that Y21 isnt compatible to observation.

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arXiv2021

Is the recently discovered large scale filamentary feature Cattail in the cold or unstable phase?

A recent publication (Li et al. 2021) discovered one of the largest filamentary neutral hydrogen features dubbed Cattail from high resolution FAST observations that might be a new galactic arm of our own Milky Way. However in the analysis, it was suggested that this neutral hydrogen feature is cold despite having 12km/s total linewidth. We evaluate the probability whether the Cattail is actually cold neutral media via the newly developed Velocity Decomposition Algorithm (Yuen et al. 2021a) and Force Balancing Model (Ho et al. 2021a). We discovered that even with the inclusion of the galactic shear term, the feature is still at the unstable neutral media regime. Moreover, we also discover that the Cattail is two disjoint features in caustics space, suggesting that the Cattail might have two different turbulent systems. We check the spectra of the individual system separated via VDA to confirm this argument. We do not exclude the existence of smaller scale cold media being embedded within this structure.

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arXiv2021

Technique for separating velocity and density contributions in spectroscopic data and its application to studying turbulence and magnetic fields

Based on the theoretical description of Position-Position-Velocity(PPV) statistics in Lazarian & Pogosyan(2000), we introduce a new technique called the Velocity Decomposition Algorithm(VDA) in separating the PPV fluctuations arising from velocity and density fluctuations. Using MHD turbulence simulations, we demonstrate its promise in retrieving the velocity fluctuations from PPV cube in various physical conditions and its prospects in accurately tracing the magnetic field. We find that for localized clouds, the velocity fluctuations are most prominent at the wing part of the spectral line, and they dominate the density fluctuations. The same velocity dominance applies to extended HI regions undergoing galactic rotation. Our numerical experiment demonstrates that velocity channels arising from the cold phase of atomic hydrogen (HI) are still affected by velocity fluctuations at small scales. We apply the VDA to HI GALFA-DR2 data corresponding to the high-velocity cloud HVC186+19-114 and high latitude galactic diffuse HI data. Our study confirms the crucial role of velocity fluctuations in explaining why linear structures are observed within PPV cubes. We discuss the implications of VDA for both magnetic field studies and predicting polarized galactic emission that acts as the foreground for the Cosmic Microwave Background studies. Additionally, we address the controversy related to the filamentary nature of the HI channel maps and explain the importance of velocity fluctuations in the formation of structures in PPV data cubes. VDA will allow astronomers to obtain velocity fluctuations from almost every piece of spectroscopic PPV data and allow direct investigations of the turbulent velocity field in observations.

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arXiv2020

Velocity Gradient in the Presence of Self-Gravity: Identifying Gravity-induced Inflow and Determining Collapsing Stage

Understanding how star formation is regulated requires studying the energy balance between turbulence, magnetic fields, stellar feedback, and gravity within molecular clouds. However, identifying the transition region where the gravity takes over remains elusive. Recent studies of the Velocity Gradient Technique (VGT), which is an advanced tool for magnetic field studies, reveal that the gradients of spectroscopic observables change their directions by 90 degrees with respect to the magnetic fields in the regions of gravitational collapse. In this study, we perform 3D MHD numerical simulations. We observe that star formation successfully proceeds in strongly magnetized and fully ionized media. We confirm that the self-gravity induces the change of gradients' orientation and high gradients' amplitude. We explore two ways of identifying collapsing self-gravitating regions through the double-peak feature in the histogram of gradients' orientation and the curvature of gradients. We show that velocity gradients' morphology and amplitude can be synthetically used to trace the convergent inflows. By comparing with the column density Probability Density Functions (N-PDFs) method, we show that VGT is a powerful new tool for studying the gas dynamics and tracing magnetic field in star-forming regions. By analogy with VGT, we extend the Intensity Gradient Technique (IGT) to locate the gravitational collapsing region and shocks. We demonstrate the synergy of VGT and IGT can determine the collapsing stages in a star-forming region.

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arXiv2020

Advancing the Velocity Gradient Technique: Using Gradient Amplitudes and handling thermal broadening

The recent development of the Velocity Gradient Technique allows observers to map magnetic field orientations and magnetization using the direction of velocity gradients. Aside from the directions, amplitudes of velocity gradients also contain valuable information about the underlying properties of magneto-hydrodynamic (MHD) turbulence. In this paper, we explore what physical information is contained in the amplitudes of velocity gradients and discuss how this information can be used to diagnose properties of turbulence in both diffuse and self-gravitating interstellar media. We identify the relations between amplitudes of both intensity and velocity centroid gradients and the sonic Mach number $M_s$ and they are consistent with the theory's predictions. We test the robustness of the method and discuss how to utilize the amplitudes of gradients into self-gravitating media. To extend the velocity gradient technique we also discuss the usage of amplitude method to Position-Position Velocity (PPV) space as a possible way to retrieve the velocity channel maps before the contamination of thermal broadening. We discuss that the Velocity Gradient Technique with these advancements could potentially give a significantly more accurate statistical insight into the properties magnetized turbulence.

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arXiv2020

Curvature of magnetic field lines in compressible magnetized turbulence: Statistics, magnetization predictions, gradient curvature, modes and self-gravitating media

Magnetic field lines in interstellar media have a rich morphology, which could be characterized by geometrical parameters such as curvature and torsion. In this paper, we explore the statistical properties of magnetic field line curvature $κ$ in compressible magnetized turbulence. We see that both the mean and standard deviation of magnetic field line curvature obey power-law relations to the magnetization. Moreover, the power-law tail of the curvature probability distribution function is also proportional to the Alfvenic Mach number. We also explore whether the curvature method could be used in the field-tracing Velocity Gradient Technique. In particular, we observe that there is a relation between the mean and standard deviation of the curvature probed by velocity gradients to $M_A$. Finally we discuss how curvature is contributed by different MHD modes in interstellar turbulence, and suggests that the eigenvectors of MHD modes could be possibly represented by the natural Fernet-Serrat frame of the magnetic field lines. We discuss possible theoretical and observational applications of the curvature technique, including the extended understanding on a special length scale that characterize the importance of magnetic field curvature in driving MHD turbulence, and how it could be potentially used to study self-gravitating system.

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arXiv2020

Magnetic Field Morphology in Interstellar Clouds with the Velocity Gradient Technique

Magnetic fields, while ubiquitous in many astrophysical environments, are challenging to measure observationally. Based on the properties of anisotropy of eddies in magnetized turbulence, the Velocity Gradient Technique is a method synergistic to dust polarimetry that is capable of tracing plane-of-the-sky magnetic field, measuring the magnetization of interstellar media and estimating the fraction of gravitational collapsing gas in molecular clouds using spectral line observations. In this paper, we apply this technique to five low-mass star-forming molecular clouds in the Gould Belt and compare the results to the magnetic-field orientation obtained from polarized dust emission. We find the estimates of magnetic field orientations and magnetization for both methods are statistically similar. We estimate the fraction of collapsing gas in the selected clouds. By means of the Velocity Gradient Technique, we also present the plane-of-the-sky magnetic field orientation and magnetization of the Smith cloud, for which dust polarimetry data are unavailable.

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arXiv2020

Obtaining magnetic field strength using differential measure approach and velocity channel maps

We introduce two new ways of obtaining the strength of plane-of-sky (POS) magnetic field by simultaneous use of spectroscopic Doppler-shifted lines and the information on magnetic field direction. The latter can be obtained either through polarization measurements or using the velocity gradient technique. We show the advantages that our techniques have compared to the traditional Davis-Chandrasekhar-Fermi (DCF) technique of estimating magnetic field strength from observations. The first technique that we describe in detail employs structure functions of velocity centroids and structure functions of Stokes parameters. We provide analytical expressions for obtaining magnetic field strength from observational data. We successfully test our results using synthetic observations obtained with results of MHD turbulence simulations. We measure velocity and magnetic field fluctuations at small scales using two, three and four point structure functions and compare the performance of these tools. We show that, unlike the DCF, our technique is capable of providing the detailed distribution of POS magnetic field and it can measure magnetic field strength in the presence of both velocity and magnetic field distortions arising from external shear and self-gravity. The second technique applies the velocity gradient technique to velocity channel maps in order to obtain the Alfven Mach number and uses the amplitudes of the gradients to obtain the sonic Mach number. The ratio of these two Mach numbers provides the intensity of magnetic field in the region contributing to the emission in the channel map. We test the technique and discuss obtaining the 3D distribution of POS galactic Magnetic field with it. We discuss the application of the second technique to synchrotron data.

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arXiv2019

Predictions of Cosmic Microwave Background Foregrounds Dust Polarization Using Velocity Gradients

The observations of fluctuations in the cosmic microwave background provide information about primordial inhomogeneities in the universe. However, the B-mode polarization of the inflationary gravitational wave is contaminated by the Galactic foreground polarized radiation arising from dust aligned by interstellar magnetic fields. To trace magnetic fields we use the Velocity Gradient Technique, which employs a modern understanding of the nature of magneto-hydrodynamics turbulent motions. In this paper, we combine the VGT with the Principal Component Analysis (PCA) to improve the accuracy of magnetic field tracing. We apply the VGT-PCA to the high-resolution neutral hydrogen data from the GALFA-HI survey to predict the polarization of dust. We report that the predicted directions of dust polarization provide good correspondence with those reported by Planck 353GHz, the alignment measure between the two AM=0.79. We show that our results statistically agree with the Planck polarization in terms of magnetic field tracing. We find that the variation of dust emission efficiency across the sky is small. Using our maps of predicted polarization we calculate the ratio of the E and B modes and show that BB/EE=0.53, which is similar to the result from Plank polarization.

astro-ph.CO↗

arXiv2019

Intensity Gradients Technique: Synergy with Velocity Gradients and Polarization Studies

Magnetic fields are ubiquitous in the interstellar medium but are notoriously difficult to study through observation. Making use of the advances in our understanding of MHD turbulence and turbulent reconnection, the Velocity Gradient Technique (VGT) was suggested and successfully applied to study magnetic fields utilizing spectroscopic data. Applying the tools developed for VGT to intensity statistics, we introduce the Intensity Gradients Technique (IGT) as a complementary tool that can be used synergistically with VGT. In this paper, we apply IGT to a diffuse HI region selected from the GALFA-HI survey and compare the intensity gradient maps with those obtained using velocity gradients as well as Planck polarization measurements. We demonstrate the possibility of using IGT and VGT for both studying the magnetic field and identifying shocks in the diffuse interstellar medium. We also explore the ability of IGT in locating self-gravitating regions and calculating Alfvenic Mach number, both alone and in combination with VGT and polarimetry. We compare IGT with the Histogram of Relative Orientation (HRO), which utilizes intensity gradients to characterize the relative orientation of column density structures and local magnetic fields.

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arXiv2019

Tracing Multi-Scale Magnetic Field Structure Using Multiple Chemical Tracers in Giant Molecular Clouds

Probing magnetic fields in Giant Molecular Clouds is often challenging. Fortunately, recently simulations show that analysis of velocity gradients (the Velocity Gradient Technique) can be used to map out the magnetic field morphology of different physical layers within molecular clouds when applied CO isotopologues with different optical depths. Here, we test the effectiveness of the Velocity Gradient Technique in reconstructing the magnetic field structure of the molecular cloud Vela C, employing seven chemical tracers that have different optical depths, i.e. 12CO, 13CO, C18O, CS, HNC, HCO+, and HCN. Our results show good correspondence between the magnetic field morphology inferred from velocity gradients using these different molecular tracers and the magnetic field morphology inferred from BLASTPol polarization observations. We also explore the possibility of using a combination of velocity gradients for multiple chemical tracers to explain the structure of the magnetic field in molecular clouds. We search for signatures of gravitational collapse in the alignment of the velocity gradients and magnetic field and conclude that collapsing regions constitute a small fraction of the cloud.

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arXiv2019

Tracing Magnetic Field with Synchrotron Polarization Gradients: Parameter Study

We employ synthetic observations obtained with MHD simulations to study how to trace the distribution of turbulent magnetic fields using the synchrotron polarization gradient techniques suggested by Lazarian \& Yuen (2018). Both synchrotron polarization gradients and its derivative gradients with regard to the squared wavelength $λ^2$ are used to explore the probing ability of the technique in magnetic fields from sub- to super-Alfv{é}nic turbulence. We focus on studies that involve multi-frequency measurements in the presence of strong Faraday rotation and show the ways of how to recover the projected mean magnetic fields in the plane of the sky and the local magnetic fields within a tomographic slice. We conclude that the new techniques can successfully reconstruct the 3D magnetic field within our Milky Way and other galaxies. This paper opens an avenue for applying our new techniques to a large number of data cubes such as those from the Low Frequency Array for Radio astronomy and the Square Kilometer Array.

astro-ph.HE↗

arXiv2019

Comment on Clark et al. (2019) "The Physical Nature of Neutral Hydrogen Intensity Structure"

A recent publication by Clark et.al (2019, CX19) uses both GALFA-HI observational data and numerical simulations to address the nature of intensity fluctuations in Position-Position-Velocity (PPV) space. The study questions the validity and applicability of the statistical theory of PPV space fluctuations formulated in Lazarian & Pogosyan (2000, LP00) to HI gas and concludes that $"${\it a significant reassessment of many observational and theoretical studies of turbulence in HI}$"$. This implies that dozens of papers that used LP00 theory to explore interstellar turbulence as well as the ongoing research based on LP00 theory are in error. This situation motivates the urgency of our public response. In our Comment we explain why we believe the criticism in CX19 is based on the incorrect understanding of the LP00 theory. In particular, we illustrate that the correlation between PPV slices and dust emissions in CX19 does not properly reveal the relative importance of velocity and density fluctuations in velocity channel maps. While CX19 provides an explanation of the change of the spectral index with respect to the thickness of PPV slice based on the two-phase nature of H1 gas, we failed to see any observational support for this idea. On the contrary, we show that the observations both in two-phase HI and one phase CO show similar results. Moreover, the observed change is in good agreement with LP00 predictions and spectral indexes of velocity and density spectra that are obtained following LP00 procedures are in good agreement with the numerically confirmed expectations of compressible MHD turbulence theory. In short, we could not find any justification of the criticism of LP00 theory that is provided in CX19. On the contrary, our analysis testifies that both available observational and numerical data agree well with the predictions of LP00 theory.

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arXiv2019

A comparison between Faraday Tomography and Synchrotron Polarization Gradients

Observations of synchrotron polarization at multiple frequencies in the presence of Faraday rotation can provide a way to reconstruct the 3D magnetic field distribution. In this paper we compare the well known Faraday Tomography (FT, Burn 1966) technique to the new approach named Synchrotron Polarization Gradients (SPG, Lazarian & Yuen 2018b). We compare the strengths and limitations of the two techniques, and describe their synergy. In particular, we show that in the situations when FT technique fails, e.g. due to insufficient frequency coverage, the SPG can still trace the 3D structure of magnetic field.

astro-ph.GA↗

arXiv2019

Tracing magnetic field morphology using the Velocity Gradient Technique in the presence of CO self-absorption

Probing magnetic fields in self-gravitating molecular clouds are generally difficult even with the use of the polarimetry. Based on the properties of magneto-hydrodynamic (MHD) turbulence and turbulent reconnection, Velocity Gradient Technique (VGT) provides a new way in tracing magnetic field orientation and strength based on the spectroscopic data. Our study tests the applicability of VGT in various molecular tracers, e.g. 12CO, 13CO, and C18O. By inspecting synthetic molecular line maps of CO isotopologue generated through radiative transfer calculations, we show that the VGT method can be successfully applied in probing the magnetic field direction in the diffuse interstellar medium as well as in self-gravitating molecular clouds.

astro-ph.GA↗

arXiv2018

Statistical tracing of magnetic fields: comparing and improving the techniques

Magnetohydrodynamic(MHD) turbulence displays velocity anisotropies which reflect the direction of the magnetic field. This anisotropy has led to the development of a number of statistical techniques for studying magnetic fields in the interstellar medium. In this paper, we review and compare three techniques that use radio position-position-velocity data for determining magnetic field strength and morphology : the correlation function anisotropy (CFA), Principal Component Analysis of Anisotropies (PCAA), and the more recent Velocity Gradient Technique (VGT). We compare these three techniques and suggest improvements to the CFA and PCAA techniques to increase their accuracy and versatility. In particular, we suggest and successfully implement a much faster way of calculating non-periodic correlation functions for the CFA. We discuss possible improvements to the current implementation of the PCAA. We show the advantages of the VGT in terms of magnetic field tracing and stress the complementary nature with the other two techniques.

astro-ph.GA↗

arXiv2018

Distribution of velocity gradient orientations: Mapping magnetization with the Velocity Gradient Technique

Recent developments of the Velocity Gradient Technique (VGT) show that the velocity gradients provide a reliable tracing of magnetic field direction in turbulent plasmas. In this paper, we explore the ability of velocity gradients to measure the magnetization of interstellar medium. We demonstrate that the distribution of velocity gradient orientations provides a reliable estimation of the magnetization of the media. In particular, we determine the relation between Alfvenic Mach number $M_A$ in the range of $M_A \in [0.2,1.7]$ and properties of the velocity gradient distribution, namely, with the dispersion of velocity gradient orientation as well as with the peak to base ratio of the amplitudes. We apply our technique for a selected GALFA-HI region and find the results consistent with the expected behavior of $M_A$. Using 3D MHD simulations we successfully compare the results with our new measure of magnetization that is based on the dispersion of starlight polarization. We demonstrate that, combined with the velocity dispersion along the line of sight direction, our technique is capable to delivering the magnetic field strength. The new technique opens a way to measure magnetization using other gradient measures such as synchrotron intensity gradients (SIGs) and synchrotron polarization gradients (SPGs).

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