arXiv subjects

A. Lazarian

Publications and source records attributed to A. Lazarian.

At least 19 recordsLinked to original sources

arXiv2026

The topology of the magnetic field in Abell 2255 out to its virial radius. Results from the LOFAR Galaxy Cluster Ultra-Deep Field

We present the LOFAR Galaxy Cluster Ultra-Deep Field, in which 336 h of LOFAR observations at 120$-$168 MHz have been collected on the nearby ($z=0.080$) cluster Abell 2255. This massive and merging system is known to host spectacular radio emission from both cluster galaxies and the intracluster medium. Previous LOFAR observations revealed pervasive diffuse synchrotron emission extending from the cluster center to its dynamically active outskirts, tracing relativistic electrons propagating in large-scale magnetic fields. In this work, we present a set of new ultra-deep images at the central frequency of 144 MHz based on the 224 h of data with the best quality, which reach a sensitivity of 24 $\mu$Jy beam$^{-1}$ at 7.1" $\times$ 4.3" resolution. These images represent the deepest radio observations of a galaxy cluster obtained to date and provide a glimpse of what should be routinely observed in clusters with SKA-Low in the near future. Using these data, we investigate the topology of the cluster magnetic field out to its virial radius by applying the synchrotron intensity gradient technique. We find that the inferred magnetic field exhibits preferential orientations in distinct regions of the cluster, such as in the radio halo extensions (bridges) and in the relics, suggesting that the dynamics of the cluster formation process is shaping the large-scale magnetic field. This interpretation is supported by the comparison with the magnetic field orientation obtained from cosmological magnetohydrodynamic simulations. This work provides the first indication of a coherent, large-scale magnetic field topology across an entire galaxy cluster, from core to outskirts, and demonstrates the unique power of ultra-deep, low-frequency observations to trace the structure of cluster magnetic fields on megaparsec scales, thereby probing the magnetization of the large-scale structure of the Universe.

astro-ph.CO↗

arXiv2026

Probing Anomalous Microwave Emission with the Square Kilometre Array

Anomalous microwave emission (AME) represents an excess of radiation in the 10-60 GHz range, distinct from synchrotron, free-free, or thermal dust emission. Although most commonly attributed to electric dipole radiation from rapidly rotating small dust grains (spinning dust), alternative mechanisms such as magnetic dipole emission (MDE) remain plausible. The detection of AME across diverse environments, from diffuse interstellar clouds to protoplanetary disks and external galaxies, suggests that multiple physical processes or carriers may contribute to its origin. Understanding AME is essential for both Galactic astrophysics and cosmology, as it constitutes a significant foreground for cosmic microwave background (CMB) studies, potentially biasing measurements. This chapter reviews current theoretical frameworks and observational evidence for AME, highlighting the key outstanding questions concerning its emission mechanisms, carriers, and polarization properties. We discuss how the Square Kilometre Array Observatory (SKAO), through its unprecedented sensitivity, angular resolution, and frequency coverage, will transform AME studies. SKA observations will enable detailed mapping of AME morphology, precise characterisation of its spectral energy distribution, and the identification of its carriers in Galactic and extragalactic environments. By combining SKA-mid data with higher-frequency observations from ALMA and other facilities such as SPHEREx, it will be possible to disentangle competing models and exploit AME as a diagnostic probe of interstellar grain physics and the small-scale structure of the interstellar medium.

astro-ph.GA↗

arXiv2026

Do plasmoids induce fast magnetic reconnection in well-resolved current sheets in 2D MHD simulations?

We investigate the development of tearing-mode instability using the highest-resolution two-dimensional magnetohydrodynamic simulations of reconnecting current sheets performed on a uniform grid, for Lundquist numbers of $10^3 \le S \le 5 \times 10^5$ , reaching up to $65,536^2$ grid cells. We demonstrate a Sweet--Parker scaling of the reconnection rate $V_{\text{rec}} \sim S^{-1/2}$ up to Lundquist numbers $S \sim 10^4$. For larger values of Lundquist number, between $2\times 10^4\le S \le 2 \times 10^5$, plasmoid formation sets in, leading to a slight enhancement of the reconnection rate, $V_{\text{rec}} \sim S^{-1/3}$, consistent with the prediction from linear tearing mode induced reconnection, indicating that reconnection remains resistivity-dependent and therefore slow. In this range of $S$-values, the plasmoids do not undergo a merger cascade, as they are rapidly advected out of the reconnection layer. Only for $S > 2 \times 10^5$, we observe the nonlinear development of the tearing-mode instability, with plasmoid coalescence and a saturation of the reconnection rate at $V_\text{rec} / V_A \sim 0.01$. At such high $S$, however, the corresponding Reynolds number is large, reaching $\text{Re} > 2000$ even on scales comparable to the current-sheet thickness. We therefore conclude that, in astrophysical systems, it is essential to account for the dominant influence of turbulence and three-dimensional effects in the reconnection process.

physics.plasm-ph↗

arXiv2026

Magnetic field morphological diagnostics with ALMA in the G327.29 protocluster: VGT versus dust polarization

Magnetic fields and turbulence may play a key role in the evolution of protoclusters, influencing the formation of dense cores and stars. Here, we examine the morphology of the magnetic fields in the G327.29 protocluster using both the velocity gradient technique (VGT) extracted from molecular line emissions and linear polarization in the dust continuum emission. The VGT analysis is performed using four molecular tracers: DCN (3-2), C18O (2-1), HN13C (3-2), and H13CO+ (3-2) - which probe gas across different density regimes, observed with the ALMA 12 m array. Owing to its sensitivity to gas dynamics, a comparison between VGT and dust polarization provides a powerful probe of the evolutionary processes in massive star-forming regions. From our analysis we reveal a complex magnetic-field structure, shaped by the combined influence of turbulence and gravity. In addition, it also appears that there is a large-scale (beyond the core scale) gravitational infall from the surrounding medium on to the filament and the central densest region. Furthermore, we observe that cores are dominated by a mix of turbulence and gravity. Overall, this work presents, likely for the first time, the application of VGT to a massive protocluster, G327.29, using high-resolution ALMA observations.

astro-ph.GA↗

arXiv2025

Obtaining Magnetization of Super-Alfvénic Turbulence with the Structure Functions of Gradient Directions

Super-Alfvénic turbulence is widespread in astrophysical environments, including molecular clouds and the diffuse plasma of galaxy clusters. At large scales, magnetic fields play only a minor dynamical role; however, for sufficiently extended turbulent cascades, the motions transition into the MHD regime at a characteristic scale $l_A$. We introduce a new diagnostic based on the structure functions of the gradient directions, which can be obtained directly from spectroscopic and synchrotron intensity observations. We demonstrate that the new measure robustly recovers the transition scale $l_A$. Building on this result, we propose a generalized expression that replaces the traditional Davis-Chandrasekhar-Fermi (DCF) method for estimating magnetic field strength in the super-Alfvénic regime, where the DCF approach fails. We further show how the magnetization and magnetic field strength of diffuse media, such as the intracluster medium, can be inferred using synchrotron intensity maps. Our theoretical predictions are validated through numerical simulations.

astro-ph.GA↗

arXiv2025

Model of super-Alfvénic MHD turbulence and structure functions of polarization

MHD turbulence driven at velocities higher than the Alfvén velocity, i.e., super-Alfvénic turbulence, is widely spread in astrophysical environments, including galaxy clusters and molecular clouds. For statistical studies of such turbulence, we explore the utility of the polarization angle structure functions $D^ϕ(R)= \left\langle\sin^2(ϕ_1-ϕ_2) \right\rangle$, where $ϕ$ denotes the polarization angle measured at points separated by a projected distance $\mathbf{R}$ on the plane of the sky. Lazarian, Yuen and Pogosyan, 2022, showed that in the case of super-Alfvénic turbulence, the spectral slope of $D^ϕ(\mathbf{R})$ differs from that of the underlying magnetic fluctuations, limiting its applicability for field strength estimation with known techniques. In this work, we provide an analytical framework that explains the modification of the $D^ϕ(R)$ spectral slope in super-Alfvénic turbulence and validate our predictions with numerical simulations. We demonstrate that for super-Alfvénic turbulence, the structure function $D^ϕ(R)$ gets shallower with the increase of $M_A$. Our study makes $D^ϕ(R)$ a valuable diagnostic of super-Alfvénic turbulence and opens a way to obtain $M_A$ from observations. We also explore numerically the structure function of the polarization degree and the spectrum of the polarization directions, the latter being the Fourier transform of $D^ϕ$. We discuss the implications of our findings for turbulence and magnetic field studies in the intracluster and interstellar media.

astro-ph.GA↗

arXiv2025

Magnetic flux transport via reconnection diffusion in different sonic regimes of interstellar MHD turbulence

Turbulence and magnetic fields are components of the interstellar medium and are interconnected through plasma processes. In particular, the magnetic flux transport in the presence of magneto-hydrodynamic (MHD) turbulence is an essential factor for understanding star formation. The theory of Reconnection Diffusion (RD), based on statistics of Alfvénic turbulence, predicts a dependence of the diffusion coefficient of the magnetic field on the Alfvénic Mach number $M_A$. However, this theory does not consider the effects of compressibility which are important in the regime of supersonic MHD turbulence. In this work, we measure the diffusion coefficient of magnetic fields in sub-Alfvénic MHD turbulence, with different sonic Mach numbers $M_S$. We perform numerical simulations of forced turbulence in periodic domains from the incompressible limit to the supersonic regime. We introduce two methods to extract the diffusion coefficient, based on the analysis of tracer particles. Our results confirm the RD assumption regarding the correspondence between the diffusion of magnetic field and that of fluid Lagrangian particles. The measured diffusion rate provided by incompressible turbulence agrees with the suppression predicted by the RD theory in the presence of strong magnetic fields: $D \propto M_A^3$. Our simulations also indicate an increase in RD efficiency when the turbulence is compressible. The dependency on $M_A$ and $M_S$ from the simulations can be described by the relation $D \propto M_A^α$, where $α(M_S) \approx 3/(1 + M_S)$. This quantitative characterization of $D$ is critical for modeling star formation in turbulent molecular clouds and evaluating the efficiency of this transport compared to other mechanisms.

astro-ph.HE↗

arXiv2025

Machine Learning Approach for Estimating Magnetic Field Strength in Galaxy Clusters from Synchrotron Emission

Magnetic fields play a crucial role in various astrophysical processes within the intracluster medium, including heat conduction, cosmic ray acceleration, and the generation of synchrotron radiation. However, measuring magnetic field strength is typically challenging due to the limited availability of Faraday Rotation Measure sources. To address the challenge, we propose a novel method that employs Convolutional Neural Networks (CNNs) alongside synchrotron emission observations to estimate magnetic field strengths in galaxy clusters. Our CNN model is trained on either magnetohydrodynamic (MHD) turbulence simulations or MHD galaxy cluster simulations, which incorporate complex dynamics such as cluster mergers and sloshing motions. The results demonstrate that CNNs can effectively estimate magnetic field strengths with mean squared error of approximately \SI{0.135}{\micro G}$^2$, \SI{0.044}{\micro G}$^2$, and \SI{0.02}{\micro G}$^2$ for $β= 100$, 200, and 500 conditions, respectively. Additionally, we have confirmed that our CNN model remains robust against noise and variations in viewing angles with sufficient training, ensuring reliable performance under a wide range of observational conditions. We compare the CNN approach with the traditional magnetic field strength estimates method that assumes equipartition between cosmic ray electron energy and magnetic field energy. Different from the equipartition method, this CNN approach relies on the morphological feature of synchrotron images, offering a new perspective for complementing traditional estimates and enhancing our understanding of cosmic ray acceleration mechanisms.

astro-ph.GA↗

arXiv2025

Anisotropic Velocity Fluctuations in Galaxy Mergers: A Probe of the Magnetic Field

Magnetic fields and turbulence are fundamental to the evolution of galaxies, yet their precise measurement and analysis present significant challenges. The recently developed Velocity Gradient Technique (VGT), which capitalizes on the anisotropy inherent in magnetohydrodynamic (MHD) turbulence, represents a new method for mapping magnetic fields in galaxies using spectroscopic observations. Most validations of VGT thus far, however, have relied upon idealized MHD turbulence simulations, which lack the more complex dynamics found in galaxies and galaxy mergers. In this study, we scrutinize VGT using an AREPO-based cosmological galaxy merger simulation, testing its effectiveness across pre-merger, merging, and post-merger stages. We examine the underlying assumptions of VGT and probe the statistics of gas density, velocity, and magnetic fields over time. We find that velocity fluctuations are indeed anisotropic at each stage, being larger in the direction perpendicular to the local magnetic field, as required by VGT. We find, additionally, that galaxy mergers substantially intensify velocity and density fluctuations and amplify magnetic fields at all scales. The observed scaling of the velocity fluctuations shows a steeper trend than $r^{1/2}$ between 0.6 and 3~kpc and a shallower trend at larger scales. The scaling of the magnetic field and density fluctuations at scales $\lesssim$ 1.0 kpc also predominantly aligns with $r^{1/2}$. Finally, we compare results from VGT to those derived from polarization-like mock magnetic field measurements, finding consistent and statistically significant global agreement in all cases.

astro-ph.GA↗

arXiv2024

Magnetic field morphology and evolution in the Central Molecular Zone and its effect on gas dynamics

The interstellar medium in the Milky Way's Central Molecular Zone (CMZ) is known to be strongly magnetised, but its large-scale morphology and impact on the gas dynamics are not well understood. We explore the impact and properties of magnetic fields in the CMZ using three-dimensional non-self gravitating magnetohydrodynamical simulations of gas flow in an external Milky Way barred potential. We find that: (1) The magnetic field is conveniently decomposed into a regular time-averaged component and an irregular turbulent component. The regular component aligns well with the velocity vectors of the gas everywhere, including within the bar lanes. (2) The field geometry transitions from parallel to the Galactic plane near $z=0$ to poloidal away from the plane. (3) The magneto-rotational instability (MRI) causes an in-plane inflow of matter from the CMZ gas ring towards the central few parsecs of $0.01-0.1$ M$_\odot$ yr$^{-1}$ that is absent in the unmagnetised simulations. However, the magnetic fields have no significant effect on the larger-scale bar-driven inflow that brings the gas from the Galactic disc into the CMZ. (4) A combination of bar inflow and MRI-driven turbulence can sustain a turbulent vertical velocity dispersion of $σ_z \simeq 5$ km s$^{-1}$ on scales of $20$ pc in the CMZ ring. The MRI alone sustains a velocity dispersion of $σ_z \simeq 3$ km s$^{-1}$. Both these numbers are lower than the observed velocity dispersion of gas in the CMZ, suggesting that other processes such as stellar feedback are necessary to explain the observations. (5) Dynamo action driven by differential rotation and the MRI amplifies the magnetic fields in the CMZ ring until they saturate at a value that scales with the average local density as $B \simeq 102 (n/10^3 cm^{-3})^{0.33}$ $μ$G. Finally, we discuss the implications of our results within the observational context in the CMZ.

astro-ph.GA↗

arXiv2024

Effects of Radiative Transfer on the Observed Anisotropy in MHD Turbulent Molecular Simulations

We study the anisotropy of centroid and integrated intensity maps with synthetic observations. We perform post-process radiative transfer including the optically thick regime that was not covered in Hernández-Padilla et al. (2020). We consider the emission in various CO molecular lines, that range from optically thin to optically thick ($\mathrm{^{12}CO}$, $\mathrm{^{13}CO}$, $\mathrm{C^{18}O}$, and $\mathrm{C^{17}O}$). The results for the velocity centroids are similar to those in the optically thin case. For instance, the anisotropy observed can be attributed to the Alfvén mode, which dominates over the slow and fast modes when the line of sight is at a high inclination with respect to the mean magnetic field. A few differences arise in the models with higher opacity, where some dependence on the sonic Mach number becomes evident. In contrast to the optically thin case, maps of integrated intensity become more anisotropic in optically thick lines. In this situation the scales probed are restricted, due to absorption, to smaller scales which are known to be more anisotropic. We discuss how the sonic Mach number can affect the latter results, with highly supersonic cases exhibiting a lower degree of anisotropy.

astro-ph.GA↗

arXiv2024

Magnetic Field of Molecular Gas Measured with the Velocity Gradient Technique II: Curved Magnetic Field in kpc-Scale Bubble of NGC\,628

We report the detection of the ordered alignment between the magnetic field and kpc-scale bubbles in the nearby spiral galaxy, NGC\,628. Applying the Velocity Gradient Technique (VGT) on CO spectroscopic data from the ALMA-PHANGS, the magnetic field of NGC\,628 is measured at the scale of 191\,pc ($\sim$ 4\,$''$). The large-scale magnetic field is oriented parallel to the spiral arms and curves around the galactic bubble structures in the mid-infrared emission observed by the James Webb Space Telescope (JWST). Twenty-one bubble structures have been identified at the edges of spiral arms with scales over 300\,pc, which includes two kpc-scale structures. These bubbles are caused by supernova remnants and prolonged star formation and are similar to the outflow chimneys found in neutral hydrogen in galactic disks. At the edge of the bubbles, the shocks traced by the OIII emission present a curved magnetic field that parallels the bubble's shell. The magnetic field follows the bubble expansion and binds the gas in the shell to trigger further star formation. By analyzing the larger sample of 1694 bubbles, we found a distinct radial-size distribution of bubbles in NGC\,628 indicating the star formation history in the galaxy.

astro-ph.GA↗

arXiv2024

Synchrotron Intensity Gradient Revealing Magnetic Fields in Galaxy Clusters

Magnetic fields and their dynamical interplay with matter in galaxy clusters contribute to the physical properties and evolution of the intracluster medium. However, the current understanding of the origin and properties of cluster magnetic fields is still limited by observational challenges. In this article, we map the magnetic fields at hundreds-kpc scales of five clusters RXC J1314.4 -2515, Abell 2345, Abell 3376, MCXC J0352.4 -7401, and El Gordo using the innovative synchrotron intensity gradient technique in conjunction with high-resolution radio observations from JVLA and MeerKAT. We demonstrate that magnetic field orientation of radio relics derived from synchrotron intensity gradients is in very good agreement with that obtained with synchrotron polarization. Most important, synchrotron intensity gradients is not limited by Faraday depolarization in the cluster central regions and allows us to map magnetic fields in the radio halos of RXC J1314.4 -2515 and El Gordo. We find that magnetic fields in radio halos exihibit a preferential direction along the major merger axis and show turbulent structures at higher angular resolution. Results are consistent with expectations from numerical simulations which predict turbulent magnetic fields in cluster mergers that are stirred and amplified by matter motions.

astro-ph.CO↗

arXiv2024

Velocity Gradient and Stellar Polarization: Magnetic Field Tomography towards the L1688 Cloud

Magnetic fields are a defining yet enigmatic aspect of the interstellar medium (ISM), with their three-dimensional mapping posing a substantial challenge. In this study, we harness the innovative Velocity Gradient Technique (VGT), underpinned by magnetohydrodynamic (MHD) turbulence theories, to elucidate the magnetic field structure by applying it to the atomic neutral hydrogen (HI) emission line and the molecular tracer $^{12}$CO. We construct the tomography of the magnetic field in the low-mass star-forming region L1688, utilizing two approaches: (1) VGT-HI combined with the Galactic rotational curve, and (2) stellar polarization paired with precise star parallax measurements. Our analysis reveals that the magnetic field orientations deduced from stellar polarization undergo a distinct directional change in the vicinity of L1688, providing evidence that the misalignment between VGT-HI and stellar polarization stems from the influence of the molecular cloud's magnetic field on the polarization of starlight. When comparing VGT-$^{12}$CO to stellar polarization and Planck polarization data, we observe that VGT-$^{12}$CO effectively reconciles the misalignment noted with VGT-HI, showing statistical alignment with Planck polarization measurements. This indicates that VGT-$^{12}$CO could be integrated with VGT-HI, offering vital insights into the magnetic fields of molecular clouds, thereby enhancing the accuracy of our 3D magnetic field reconstructions.

astro-ph.GA↗

arXiv2023

Probing Three-Dimensional Magnetic Fields: II -- An Interpretable Convolutional Neural Network

Observing 3D magnetic fields, including orientation and strength, within the interstellar medium is vital but notoriously difficult. However, recent advances in our understanding of anisotropic magnetohydrodynamic (MHD) turbulence demonstrate that MHD turbulence and 3D magnetic fields leave their imprints on the intensity features of spectroscopic observations. Leveraging these theoretical frameworks, we propose a novel Convolutional Neural Network (CNN) model to extract this embedded information, enabling the probe of 3D magnetic fields. This model examines not only the plane-of-the-sky magnetic field orientation ($ϕ$), but also the magnetic field's inclination angle ($γ$) relative to the line-of-sight, and the total magnetization level (M$_A^{-1}$) of the cloud. We train the model using synthetic emission lines of $^{13}$CO (J = 1 - 0) and C$^{18}$O (J = 1 - 0), generated from 3D MHD simulations that span conditions from sub-Alfvénic to super-Alfvénic molecular clouds. Our tests confirm that the CNN model effectively reconstructs the 3D magnetic field topology and magnetization. The median uncertainties are under $5^\circ$ for both $ϕ$ and $γ$, and less than 0.2 for M$_A$ in sub-Alfvénic conditions (M$_A\approx0.5$). In super-Alfvénic scenarios (M$_A\approx2.0$), they are under $15^\circ$ for $ϕ$ and $γ$, and 1.5 for M$_A$. We applied this trained CNN model to the L1478 molecular cloud. Results show a strong agreement between the CNN-predicted magnetic field orientation and that derived from Planck 353 GHz polarization data. The CNN approach enabled us to construct the 3D magnetic field map for L1478, revealing a global inclination angle of $\approx76^\circ$ and a global M$_A$ of $\approx1.07$.

astro-ph.GA↗

arXiv2023

Damping of MHD Turbulence in A Partially Ionized Medium

The coupling state between ions and neutrals in the interstellar medium plays a key role in the dynamics of magnetohydrodynamic (MHD) turbulence, but is challenging to study numerically. In this work, we investigate the damping of MHD turbulence in a partially ionized medium using 3D two-fluid (ions+neutrals) simulations generated with the AthenaK code. Specifically, we examine the velocity, density, and magnetic field statistics of the two-fluid MHD turbulence in different regimes of neutral-ion coupling. Our results demonstrate that when ions and neutrals are strongly coupled, the velocity statistics resemble those of single-fluid MHD turbulence. Both the velocity structures and kinetic energy spectra of ions and neutrals are similar, while their density structures can be significantly different. With an excess of small-scale sharp density fluctuations in ions, the density spectrum in ions is shallower than that of neutrals. When ions and neutrals are weakly coupled, the turbulence in ions is more severely damped due to the ion-neutral collisional friction than that in neutrals, resulting in a steep kinetic energy spectrum and density spectrum in ions compared to the Kolmogorov spectrum. We also find that the magnetic energy spectrum basically follows the shape of the kinetic energy spectrum of ions, irrespective of the coupling regime. In addition, we find large density fluctuations in ions and neutrals and thus spatially inhomogeneous ionization fractions. As a result, the neutral-ion decoupling and damping of MHD turbulence take place over a range of length scales.

astro-ph.GA↗

arXiv2023

Detection of large-scale synchrotron radiation from the molecular envelope of the Sgr B cloud complex at the Galactic center

We present highly sensitive measurements taken with MeerKAT at 1280 MHz as well as archival GBT, MWA and VLA images at 333, 88 and 74 MHz. We report the detection of synchrotron radio emission from the infrared dark cloud (IRDC) associated with the halo of the Sgr B complex on a scale of ~60 pc. A strong spatial correlation between low-frequency radio continuum emission and dense molecular gas, combined with spectral index measurements, indicates enhanced synchrotron emission by cosmic-ray electrons. Correlation of the FeI 6.4 keV Kalpha line and synchrotron emission provides compelling evidence that the low energy cosmic-ray electrons are responsible for producing the Kalpha line emission. The observed synchrotron emission within the halo of the Sgr B cloud complex has mean spectral index alpha -1+/-1 gives the magnetic field strength ~100 muG for cloud densities nH = 10^4-10^5 cm-3 and estimate cosmic-ray ionization rates between 10^-13 and 10^-14 s^-1. Furthermore, the energy spectrum of primary cosmic-ray electrons is constrained to be E^-3 +/-1 for typical energies of few hundred MeV. The extrapolation of this spectrum to higher energies is consistent with X-ray and gamma-ray emission detected from this cloud. These measurements have important implications on the role that high cosmic-ray electron fluxes at the Galactic center play in production of radio synchrotron emission, the FeI Kalpha line emission at 6.4 keV and ~GeV gamma-ray emission throughout the central molecular zone (CMZ).

astro-ph.HE↗

arXiv2023

Nature of Striation in 21 cm Channel Maps: Velocity Caustics

The alignment of striated intensity structures in thin neutral hydrogen (HI) spectroscopic channels with Galactic magnetic fields has been observed. However, the origin and nature of these striations are still debatable. Some studies suggest that the striations result solely from real cold-density filaments without considering the role of turbulent velocity fields, i.e., the velocity caustics effect in shaping the channel's intensity distribution. To determine the relative contribution of density and velocity in forming the striations in channel maps, we analyze synthetic observations of channel maps obtained with simulations that represent realistic magnetized multi-phase HI. We vary the thickness of the channel maps and apply the Velocity Decomposition Algorithm to separate the velocity and density contributions. In parallel, we analyze GALFA HI observations and compare the results. Our analysis shows that the thin channels are dominated by velocity contribution, and velocity caustics mainly generate the HI striations. We show that velocity caustics can cause a correlation between unsharp-masked HI structures and far-infrared emission. We demonstrate that the linear HI fibers revealed by the Rolling Hough Transform (RHT) in thin velocity channels originate from velocity caustics. As the thickness of channel maps increases, the relative contribution of density to fluctuations in channel maps also increases. As a result, more RHT-detected fibers tend to be perpendicular to the magnetic field. Conversely, the alignment with the magnetic field is the most prominent in thin channels. We conclude that similar to the Velocity Channel Gradients (VChGs) approach, RHT traces magnetic fields through the analysis of velocity caustics in thin channel maps.

astro-ph.GA↗