arXiv subjects

Alex Lazarian

Publications and source records attributed to Alex Lazarian.

At least 55 records · Page 3Linked to original sources

arXiv2022

SOFIA observations of 30 Doradus: II -- Magnetic fields and large scale gas kinematics

The heart of the Large Magellanic Cloud, 30 Doradus, is a complex region with a clear core-halo structure. Feedback from the stellar cluster R$\,$136 has been shown to be the main source of energy creating multiple pc-scale expanding-shells in the outer region, and carving a nebula core in the proximity of the ionization source. We present the morphology and strength of the magnetic fields (B-fields) of 30 Doradus inferred from the far-infrared polarimetric observations by SOFIA/HAWC+ at 89, 154, and 214$\,μ$m. The B-field morphology is complex, showing bending structures around R$\,$136. In addition, we use high spectral and angular resolution [\textsc{CII}] observations from SOFIA/GREAT and CO(2-1) from APEX. The kinematic structure of the region correlates with the B-field morphology and shows evidence of multiple expanding shells. Our B-field strength maps, estimated using the Davis-Chandrasekhar-Fermi method and structure-function, show variations across the cloud within a maximum of 600, 450, and 350$\,μ$G at 89, 154, and 214$\,μ$m, respectively. We estimated that the majority of the 30 Doradus clouds are sub-critical and sub-Alfvénic. The probability distribution function of the gas density shows that the turbulence is mainly compressively driven, while the plasma beta parameter indicates supersonic turbulence. We show that the B-field is sufficient to hold the cloud structure integrity under feedback from R$\,$136. We suggest that supersonic compressive turbulence enables the local gravitational collapse and triggers a new generation of stars to form. The velocity gradient technique (VGT) using [\textsc{CII}] and CO(2-1) is likely to confirm these results.

astro-ph.GA↗

arXiv2022

Turbulent reconnection acceleration

The ubiquitous turbulence in astrophysical plasmas is important for both magnetic reconnection and reconnection acceleration. We study the particle acceleration during fast 3D turbulent reconnection with reconnection-driven turbulence. Particles bounce back and forth between the reconnection-driven inflows due to the mirror reflection and convergence of strong magnetic fields. Via successive head-on collisions, the kinetic energy of the inflows is converted into the accelerated particles. Turbulence not only regulates the inflow speed but also introduces various inflow obliquities with respect to the local turbulent magnetic fields. As both the energy gain and escape probability of particles depend on the inflow speed, the spectral index of particle energy spectrum is not universal. We find it in the range from $\approx 2.5$ to $4$, with the steepest spectrum expected at a strong guide field, i.e. a small angle between the total incoming magnetic field and the guide field. Without scattering diffusion needed for confining particles, the reconnection acceleration can be very efficient at a large inflow speed and a weak guide field.

physics.plasm-ph↗

arXiv2022

Turbulent Magnetic Field Amplification by the Interaction of Shock Wave and Inhomogeneous Medium

Magnetic fields on the order of 100 $μ$G observed in young supernova remnants cannot be amplified by shock compression alone. To investigate the amplification caused by turbulent dynamo, we perform three-dimensional MHD simulations of the interaction between shock wave and inhomogeneous density distribution with a shallow spectrum in the preshock medium. The postshock turbulence is mainly driven by the strongest preshock density contrast and follows the Kolmogorov scaling. The resulting turbulence amplifies the postshock magnetic field. The time evolution of the magnetic fields agrees with the prediction of the nonlinear turbulent dynamo theory in Xu & Lazarian (2016). When the initial weak magnetic field is perpendicular to the shock normal, the maximum amplification of the field's strength achieves a factor of $\approx200$, which is twice larger than that for a parallel shock. We find that the perpendicular shock exhibits a smaller turbulent Alfvén Mach number in the vicinity of the shock front than the parallel shock. However, the strongest magnetic field has a low volume filling factor and is limited by the turbulent energy due to the reconnection diffusion taking place in a turbulent and magnetized fluid. The magnetic field strength averaged along the $z$-axis is reduced by a factor $\gtrsim10$. We decompose the turbulent velocity and magnetic field into solenoidal and compressive modes. The solenoidal mode is dominant and evolves to follow the Kolmogorov scaling, even though the preshock density distribution has a shallow spectrum. When the preshock density distribution has a Kolmogorov spectrum, the turbulent velocity's compressive component increases.

astro-ph.GA↗

arXiv2022

Role of magnetic fields in fueling Seyfert nuclei

Molecular gas is believed to be the fuel for star formation and nuclear activity in Seyfert galaxies. To explore the role of magnetic fields in funneling molecular gas into the nuclear region, measurements of the magnetic fields embedded in molecular gas are needed. By applying the new velocity gradient technique (VGT) to ALMA and PAWS's CO isotopolog data, we obtain the first detection of CO-associated magnetic fields in several nearby Seyfert galaxies and their unprecedented high-resolution magnetic field maps. The VGT-measured magnetic fields in molecular gas globally agree with those inferred from existing HAWC+ dust polarization and VLA synchrotron polarization. An overall good alignment between the magnetic fields traced by VGT-CO and by synchrotron polarization may support the correlation between star formation and cosmic ray generation. We find that the magnetic fields traced by VGT-CO have a significant radial component in the central regions of most Seyferts in our sample, where efficient molecular gas inflows or outflows may happen. In particular, we find local misalignment between the magnetic fields traced by CO and dust polarization within the nuclear ring of NGC 1097, and the former aligns with the central bar's orientation. This misalignment reveals different magnetic field configurations in different gas phases and may provide an observational diagnostic for the ongoing multi-phase fueling of Seyfert activity.

astro-ph.GA↗

arXiv2022

Studying the properties of compressible MHD turbulence by synchrotron fluctuation statistics

We study the observable properties of compressible MHD turbulence covering different turbulence regimes, based on synthetic synchrotron observations arising from 3D MHD numerical simulations. Using the synchrotron emissivity and intensity, we first explore how the cosmic ray spectral indices affect the measurements of turbulence properties by employing normalized correlation functions. We then study how the anisotropy of synchrotron total and polarization intensities arising from three fundamental MHD modes vary with the viewing angle, i.e., the angle between the mean magnetic field and the line of sight. We employ the ratio of quadrupole moment to the monopole one (QM) for this purpose. Our numerical results demonstrate that: (1) the two-point correlation function of synchrotron statistics for the arbitrary cosmic ray spectral index is related to the special case of magnetic field index $γ=2$ in agreement with the analytical formulae provided by Lazarian \& Pogosyan (2012); (2) the anisotropy of synchrotron total and polarization intensities arising from Alfvén and slow modes increases with the increase of the viewing angle, while that of fast mode remains almost unchanged with the viewing angle; (3) the analytical formulae of synchrotron intensities for studying turbulence can be applied to describing statistics of polarization intensities, and the QM can be successfully used to recover turbulence anisotropy. This study validates Lazarian \& Pogosyan's analytical approach and opens a way to study turbulence from observations.

astro-ph.HE↗

arXiv2022

Turbulent universal galactic Kolmogorov velocity cascade over 6 decades

We report the evidence for the existence of the universal, continuous turbulent cascade of velocity fluctuations with Kolmogorov -5/3 slope spanning 6 orders of length scales, from $10^4$ pc down to $10^{-2}$ pc. This was achieved by applying our innovative technique of separating density and velocity fluctuations to a set of spectroscopic surveys featuring various galactic spectral lines. This unified velocity cascade involves different interstellar phases from diffuse galactic media to dense self-gravitating clouds and persists despite interstellar phase transitions. However, the turbulent density fluctuations do not show this universality as the value of the spectral slope changes in different interstellar phases. This agrees with the expectation of compressible turbulence theory and demonstrates that the density is only an indirect tracer of interstellar turbulence. We report that the density fluctuations for clouds and filaments that are preferentially parallel to magnetic fields exhibit the spectral slope of -2. The universal grand turbulent velocity cascade that is established in our paper has significant implications for key galactic physical processes, including star formation, cosmic ray transport etc. We anticipate our result to be a starting point for in vitro models of multiphase interstellar turbulence studies with a significant impact for modeling of spiral galaxies.

astro-ph.GA↗

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.

astro-ph.GA↗

arXiv2022

Cosmic ray streaming in the turbulent interstellar medium

We study the streaming instability of GeV$-100~$GeV cosmic rays (CRs) and its damping in the turbulent interstellar medium (ISM). We find that the damping of streaming instability is dominated by ion-neutral collisional damping in weakly ionized molecular clouds, turbulent damping in the highly ionized warm medium, and nonlinear Landau damping in the Galactic halo. Only in the Galactic halo, is the streaming speed of CRs close to the Alfvén speed. Alfvénic turbulence plays an important role in both suppressing the streaming instability and regulating the diffusion of streaming CRs via magnetic field line tangling, with the effective mean free path of streaming CRs in the observer frame determined by the Alfvénic scale in super-Alfvénic turbulence. The resulting diffusion coefficient is sensitive to Alfvén Mach number, which has a large range of values in the multi-phase ISM. Super-Alfvénic turbulence contributes to additional confinement of streaming CRs, irrespective of the dominant damping mechanism.

astro-ph.HE↗

arXiv2022

Damping of Alfvén waves in MHD turbulence and implications for cosmic ray streaming instability and galactic winds

Alfvénic component of MHD turbulence damps Alfvénic waves. The consequences of this effect are important for many processes, from cosmic ray (CR) propagation to launching outflows and winds in galaxies and other magnetized systems. We discuss the differences in the damping of the streaming instability by turbulence and the damping of a plane parallel wave. The former takes place in the system of reference aligned with the local direction of magnetic field along which CRs stream. The latter is in the reference frame of the mean magnetic field and traditionally considered in plasma studies. We also compare the turbulent damping of streaming instability with ion-neutral collisional damping, which becomes the dominant damping effect at a sufficiently low ionization fraction. Numerical testing and astrophysical implications are also discussed.

astro-ph.GA↗

arXiv2021

Velocity Gradients: Magnetic Field Tomography towards the Supernova Remnant W44

As a novel approach for tracing interstellar magnetic fields, the Velocity Gradient Technique (VGT) has been proven to be effective for probing magnetic fields in the diffuse interstellar medium (ISM). In this work, we verify the VGT in a broader context by applying the technique to a molecular cloud interacting with the supernovae remnant (SNR) W44. We probe the magnetic fields with the VGT using CO, $\rm HCO^+$, and H I emission lines and make a comparison with the Planck 353 GHZ dust polarization. We show that the VGT gives an accurate measurement that coheres with the Planck polarization especially in intense molecular gas emission regions. We further study the foreground's contribution to the polarization that results in a misalignment between the VGT and the Planck measurements in low-intensity molecular gas areas. We advance the VGT to achieve magnetic field tomography by decomposing the W44 into various velocity components. We show that W44's velocity component at $v\sim45$ km s$^{-1}$ exhibits the largest coverage and gives the best agreement with Planck polarization in terms of magnetic field orientation.

astro-ph.GA↗

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.

astro-ph.GA↗

arXiv2021

Shock acceleration with oblique and turbulent magnetic fields

We investigate shock acceleration in a realistic astrophysical environment with density inhomogeneities. The turbulence induced by the interaction of the shock precursor with upstream density fluctuations amplifies both upstream and downstream magnetic fields via the turbulent dynamo. The dynamo-amplified turbulent magnetic fields (a) introduce variations of shock obliquities along the shock face, (b) enable energy gain through a combination of shock drift and diffusive processes, (c) give rise to various spectral indices of accelerated particles, (d) regulate the diffusion of particles both parallel and perpendicular to the magnetic field, and (e) increase the shock acceleration efficiency. Our results demonstrate that upstream density inhomogeneities and dynamo amplification of magnetic fields play an important role in shock acceleration, and thus shock acceleration depends on the condition of the ambient interstellar environment. The implications on understanding radio spectra of supernova remnants are also discussed.

astro-ph.HE↗

arXiv2021

Diffusion of cosmic rays in MHD turbulence with magnetic mirrors

As the fundamental physical process with many astrophysical implications, the diffusion of cosmic rays (CRs) is determined by their interaction with magnetohydrodynamic (MHD) turbulence. We consider the magnetic mirroring effect arising from MHD turbulence on the diffusion of CRs. Due to the intrinsic superdiffusion of turbulent magnetic fields, CRs with large pitch angles that undergo mirror reflection, i.e., bouncing CRs, are not trapped between magnetic mirrors, but move diffusively along the turbulent magnetic field, leading to a new type of parallel diffusion, i.e., mirror diffusion. This mirror diffusion is in general slower than the diffusion of non-bouncing CRs with small pitch angles that undergo gyroresonant scattering. The critical pitch angle at the balance between magnetic mirroring and pitch-angle scattering is important for determining the diffusion coefficients of both bouncing and non-bouncing CRs and their scalings with the CR energy. We find non-universal energy scalings of diffusion coefficients, depending on the properties of MHD turbulence.

astro-ph.HE↗

arXiv2021

CRAFT (Cosmic Ray Acceleration From Turbulence) in Molecular Clouds

Low-energy cosmic rays, in particular protons with energies below 1 GeV, are significant drivers of the thermochemistry of molecular clouds. However, these cosmic rays are also greatly impacted by energy losses and magnetic field transport effects in molecular gas. Explaining cosmic ray ionization rates of $10^{-16}$ s$^{-1}$ or greater in dense gas requires either a high external cosmic ray flux, or local sources of MeV-GeV cosmic ray protons. We present a new local source of low-energy cosmic rays in molecular clouds: first order Fermi-acceleration of protons in regions undergoing turbulent reconnection in molecular clouds. We show from energetic-based arguments there is sufficient energy within the magneto-hydrodynamic turbulent cascade to produce ionization rates compatible with inferred ionization rates in molecular clouds. As turbulent reconnection is a volume-filling process, the proposed mechanism can produce a near-homogeneous distribution of low-energy cosmic rays within molecular clouds.

astro-ph.HE↗

arXiv2021

Effect of Dust Rotational Disruption by Radiative Torques and Implications for F-corona decrease revealed by the Parker Solar Probe

The first-year results from the Parker Solar Probe (PSP) reveal a gradual decrease of F-coronal dust from distances of $D=0.166-0.336$ AU (or the inner elongations of $\sim 9.22- 18.69~R_{\odot}$) to the Sun (Howard et al. 2019). Such a F-corona decrease cannot be explained by the dust sublimation scenario of the popular silicate composition that implies a dust-free-zone of boundary at heliocentric radius $R\lesssim 4-5R_{\odot}$, but may be explained by appealing to various dust compositions with different sublimation fronts. In this paper, we present an additional explanation for the F-corona decrease using our newly introduced mechanism of dust destruction so-called Radiative Torque Disruption (RATD) mechanism. We demonstrate that RATD rapidly breaks large grains into nanoparticles so that they can be efficiently destroyed by nonthermal sputtering induced by bombardment of energetic protons from slow solar winds, which extends the dust-free-zone established by thermal sublimation to $R_{dfz}\sim 8R_{\odot}$. Beyond this extended dust-free-zone, we find that the dust mass decreases gradually from $R\sim 42R_{\odot}$ toward the Sun due to partial removal of nanodust by nonthermal sputtering. The joint effect of RATD and nonthermal sputtering can successfully reproduce the gradual decrease of the F-corona between $19-9R_{\odot}$ observed by the PSP. Finally, the RATD mechanism can efficiently produce nanoparticles usually observed in the inner solar system.

astro-ph.SR↗

arXiv2021

Small-scale turbulent dynamo in astrophysical environments: nonlinear dynamo and dynamo in a partially ionized plasma

Small-scale turbulent dynamo is responsible for the amplification of magnetic fields on scales smaller than the driving scale of turbulence in diverse astrophysical media. Most earlier dynamo theories concern the kinematic regime and small-scale magnetic field amplification. Here we review our recent progress in developing the theories for the nonlinear dynamo and the dynamo regime in a partially ionized plasma. The importance of reconnection diffusion of magnetic fields is identified for both the nonlinear dynamo and magnetic field amplification during gravitational contraction. For the dynamo in a partially ionized plasma, the coupling state between neutrals and ions and the ion-neutral collisional damping can significantly affect the dynamo behavior and the resulting magnetic field structure. We present both our analytical predictions and numerical tests with a two-fluid dynamo simulation on the dynamo features in this regime. In addition, to illustrate the astrophysical implications, we discuss several examples for the applications of the dynamo theory to studying magnetic field evolution in both preshock and postshock regions of supernova remnants, in weakly magnetized molecular clouds, during the (primordial) star formation, and during the first galaxy formation.

astro-ph.GA↗

arXiv2021

Gradient measurement of synchrotron polarization diagnostic: Application to spatially separated emission and Faraday rotation regions

Considering the spatially separated polarization radiation and Faraday rotation regions to simulate complex interstellar media, we study synchrotron polarization gradient techniques' measurement capabilities. We explore how to trace the direction of projected magnetic field of emitting-source region at the multi-frequency bands, using the gradient technique compared with the traditional polarization vector method. Furthermore, we study how Faraday rotation density in the foreground region, i.e., a product of electron number density and parallel component of magnetic fields along the line of sight, affects the measurement of projected magnetic field. Numerical results show that synchrotron polarization gradient technique could successfully trace projected magnetic field within emitting-source region independent of radio frequency. Accordingly, the gradient technique can measure the magnetic field properties for a complex astrophysical environment.

astro-ph.HE↗

arXiv2021

Intermittency of Fast MHD Modes and Regions of Anomalous Gradient Orientation in Low-beta Plasmas

The strong alignment of small-scale turbulent Alfvénic motions with the direction of the magnetic field that percolates the small-scale eddies and imprints the direction of the magnetic field is a property that follows from the MHD theory and the theory of turbulent reconnection. The Alfvénic eddies mix magnetic fields perpendicular to the direction of the local magnetic field, and this type of motion is used to trace magnetic fields with the velocity gradient technique (VGT). The other type of turbulent motion, fast modes, induces anisotropies orthogonal to Alfvénic eddies and interferes with the tracing of the magnetic field with the VGT. We report a new effect, i.e., in a magnetically dominated low-βsubsonic medium, fast modes are very intermittent, and in a volume, with a small filling factor the fast modes dominate other turbulent motions. We identify these localized regions as the cause of the occasional change of direction of gradients in our synthetic observations. We show that the new technique of measuring the gradients of gradient amplitudes suppresses the contribution from the fast-mode-dominated regions, improving the magnetic field tracing. In addition, we show that the distortion of the gradient measurements by fast modes is also applicable to the synchrotron intensity gradients, but the effect is reduced compared to the VGT.

astro-ph.GA↗