Context.
The magnetic field strength in interstellar clouds can be estimated indirectly from measurements of dust polarization by assuming that turbulent kinetic energy is comparable to the ...fluctuating magnetic energy, and using the spread of polarization angles to estimate the latter. The method developed by Davis (1951, Phys. Rev., 81, 890) and by Chandrasekhar and Fermi (1953, ApJ, 118, 1137) (DCF) assumes that incompressible magnetohydrodynamic (MHD) fluctuations induce the observed dispersion of polarization angles, deriving
B
∝ 1∕
δθ
(or, equivalently,
δθ
∝
M
A
, in terms of the Alfvénic Mach number). However, observations show that the interstellar medium is highly compressible. Recently, two of us (ST) relaxed the incompressibility assumption and derived instead B ∝ 1/√δθ (equivalently,
δθ
∝
M
A
2
).
Aims.
We explored what the correct scaling is in compressible and magnetized turbulence through theoretical arguments, and tested the assumptions and the accuracy of the two methods with numerical simulations.
Methods.
We used 26 magnetized, isothermal, ideal-MHD numerical simulations without self-gravity and with different types of forcing. The range of
M
A
and sonic Mach numbers
M
s
explored are 0.1 ≤
M
A
≤ 2.0 and 0.5 ≤
M
s
≤ 20. We created synthetic polarization maps and tested the assumptions and accuracy of the two methods.
Results.
The synthetic data have a remarkable consistency with the δθ ∝ M
A
2
scaling, which is inferred by ST, while the DCF scaling failed to follow the data. Similarly, the assumption of ST that the turbulent kinetic energy is comparable to the root-mean-square (rms) of the coupling term of the magnetic energy between the mean and fluctuating magnetic field is valid within a factor of two for all
M
A
(with the exception of solenoidally driven simulations at high
M
A
, where the assumption fails by a factor of 10). In contrast, the assumption of DCF that the turbulent kinetic energy is comparable to the rms of the second-order fluctuating magnetic field term fails by factors of several to hundreds for sub-Alfvénic simulations. The ST method shows an accuracy better than 50% over the entire range of
M
A
explored; DCF performs adequately only in the range of
M
A
for which it has been optimized through the use of a “fudge factor”. For low
M
A
, it is inaccurate by factors of tens, since it omits the magnetic energy coupling term, which is of first order and corresponds to compressible modes. We found no dependence of the accuracy of the two methods on
M
s
.
Conclusions.
The assumptions of the ST method reflect better the physical reality in clouds with compressible and magnetized turbulence, and for this reason the method provides a much better estimate of the magnetic field strength over the DCF method. Even in super-Alfvénic cases where DCF might outperform ST, the ST method still provides an adequate estimate of the magnetic field strength, while the reverse is not true.
In diffuse molecular clouds, possible precursors of star-forming clouds, the effect of the magnetic field is unclear. In this work, we compare the orientations of filamentary structures in the ...Polaris Flare, as seen through dust emission by Herschel, to the plane-of-the-sky magnetic field orientation (B
pos) as revealed by stellar optical polarimetry with RoboPol. Dust structures in this translucent cloud show a strong preference for alignment with B
pos. Of the field orientations, 70 per cent are consistent with those of the filaments (within 30°). We explore the spatial variation of the relative orientations and find it to be uncorrelated with the dust emission intensity and correlated to the dispersion of polarization angles. Concentrating on the area around the highest column density filament, and on the region with the most uniform field, we infer the B
pos strength to be 24–120 μG. Assuming that the magnetic field can be decomposed into a turbulent and an ordered component, we find a turbulent-to-ordered ratio of 0.2–0.8, implying that the magnetic field is dynamically important, at least in these two areas. We discuss implications for three-dimensional field properties, as well as for the distance estimate of the cloud.
If a single line of sight (LOS) intercepts multiple dust clouds with different spectral energy distributions and magnetic field orientations, then the frequency scaling of each of the Stokes
Q
and
U
...parameters of the thermal dust emission may be different, a phenomenon we refer to as LOS frequency decorrelation. We present first evidence for LOS frequency decorrelation in
Planck
data using independent measurements of neutral-hydrogen (H
I
) emission to probe the 3D structure of the magnetized interstellar medium (ISM). We use H
I
-based measurements of the number of clouds per LOS and the magnetic field orientation in each cloud to select two sets of sightlines: (i) a target sample of pixels that are likely to exhibit LOS frequency decorrelation and (ii) a control sample of pixels that lack complex LOS structure. We test the null hypothesis that LOS frequency decorrelation is not detectable in
Planck
353 and 217 GHz polarization data at high Galactic latitudes. We reject the null hypothesis at high significance based on data that show that the combined effect of polarization angle variation with frequency and depolarization are detected in the target sample. This detection is robust against the choice of cosmic microwave background (CMB) map and map-making pipeline. The observed change in polarization angle due to LOS frequency decorrelation is detectable above the
Planck
noise level. The probability that the detected effect is due to noise alone ranges from 5 × 10
−2
to 4 × 10
−7
, depending on the CMB subtraction algorithm and treatment of residual systematic errors; correcting for residual systematic errors consistently increases the significance of the effect. Within the target sample, the LOS decorrelation effect is stronger for sightlines with more misaligned magnetic fields, as expected. With our sample, we estimate that an intrinsic variation of ~15% in the ratio of 353 to 217 GHz polarized emission between clouds is sufficient to reproduce the measured effect. Our finding underlines the importance of ongoing studies to map the three-dimensional structure of the magnetized and dusty ISM that could ultimately help component separation methods to account for frequency decorrelation effects in CMB polarization studies.
In the context of cosmic microwave background polarization studies and the characterization of the Galactic foregrounds, the power spectrum analysis of the thermal dust polarization sky has led to ...intriguing evidence of an
E
∕
B
asymmetry and a positive
TE
correlation. In this work, we produce synthesized dust polarization maps from a set of global magneto-hydrodynamic (MHD) simulations of Milky-Way-sized galaxies, and analyze their power spectra at intermediate angular scales (intermediate angular multipoles
ℓ
∈60, 140). We study the role of the initial configuration of the large-scale magnetic field, its strength, and the feedback on the power spectrum characteristics. Using full-galaxy MHD simulations, we were able to estimate the variance induced by the peculiar location of the observer in the galaxy. We find that the polarization power spectra sensitively depend on the observer’s location, impeding a distinction between different simulation setups. In particular, there is a clear statistical difference between the power spectra measured from within the spiral arms and those measured from the inter-arm regions. Also, power spectra from within supernova-driven bubbles share common characteristics, regardless of the underlying model. However, no correlation was found between the statistical properties of the polarization power spectra and the local (with respect to the observer) mean values of physical quantities such as the density and the strength of the magnetic field. Finally, we find some indications that the global strength of the magnetic field may play a role in shaping the power spectrum characteristics; as the global magnetic field strength increases, the
E
∕
B
asymmetry and the
TE
correlation increase, whereas the viewpoint-induced variance decreases. However, we find no direct correlation with the strength of the local magnetic field that permeates the mapped region of the interstellar medium.
Abstract
Filaments in Herschel molecular cloud images are found to exhibit a ‘characteristic width’. This finding is in tension with spatial power spectra of the data, which show no indication of ...this characteristic scale. We demonstrate that this discrepancy is a result of the methodology adopted for measuring filament widths. First, we perform the previously used analysis technique on artificial scale-free data, and obtain a peaked width distribution of filament-like structures. Next, we repeat the analysis on three Herschel maps and reproduce the narrow distribution of widths found in previous studies – when considering the average width of each filament. However, the distribution of widths measured at all points along a filament spine is broader than the distribution of mean filament widths, indicating that the narrow spread (interpreted as a ‘characteristic’ width) results from averaging. Furthermore, the width is found to vary significantly from one end of a filament to the other. Therefore, the previously identified peak at 0.1 pc cannot be understood as representing the typical width of filaments. We find an alternative explanation by modelling the observed width distribution as a truncated power-law distribution, sampled with uncertainties. The position of the peak is connected to the lower truncation scale and is likely set by the choice of parameters used in measuring filament widths. We conclude that a ‘characteristic’ width of filaments is not supported by the available data.
Context . One of the indicators most frequently used to characterize the magnetic field’s influence on star formation is the relation between the magnetic field strength and the gas density (the B − ...p relation), usually expressed as a power law of the form B ∝ ρ κ . The value of κ is an indication of the dynamical importance of the magnetic field during gas compression. Aims . In this work, we investigate the role of the global magnetic field morphology on a galaxy’s B − ρ relation, as well as the evolution of the relation over time. Methods . We developed magnetohydrodynamic simulations of Milky Way-like galaxies that include gravity, star formation, and supernova feedback. The models take into account nonequilibrium chemistry up to H 2 formation, which is used to fuel star formation. We considered two different initial magnetic field morphologies: one completely ordered (toroidal) and the other completely random. Using these models, we studied the dynamical importance of the magnetic field through the plasma ß and the B − ρ relation. Results . For both magnetic morphologies, low-density regions are thermally supported, while high-density regions are magnetically dominated. Equipartition is reached earlier and at lower densities in the toroidal model. However, the B − ρ relation varies, even within the same galaxy, as it consistently includes two different branches for a given density, with κ ranging from about 0.2 to 0.8. The mean value of κ for each model also varies significantly over time, which supersedes the differences between the two models. Conclusions . While our findings suggest that the magnetic field morphology does influence the galactic B − ρ relation, its impact is transient in nature since time-averaged differences between the models fall within the large temporal scatter. The context and time-dependent nature of the B − ρ relation underscore the need for comprehensive research and observations to understand the intricate role of magnetic fields in star formation processes across diverse galactic environments.
ABSTRACT
Non-ideal magnetohydrodynamic (MHD) effects are thought to be gravity’s closest ally in overcoming the support of magnetic fields and in forming stars. Here, we modify the publicly available ...version of the adaptive mesh refinement code flash (Fryxell et al. 2000; Dubey et al. 2008) to include a detailed treatment of non-ideal MHD and study such effects in collapsing pre-stellar cores. We implement two very extended non-equilibrium chemical networks, the largest of which is comprised of ∼ 300 species and includes a detailed description of deuterium chemistry. The ambipolar diffusion, Ohmic and Hall resistivities are then self-consistently calculated from the abundances of charged species. We present a series of 2D axisymmetric simulations where we vary the chemical model, cosmic ray ionization rate, and grain distribution. We benchmark our implementation against ideal MHD simulations and previously published results. We show that, at high densities ($n_{\rm {H_2}}\gt ~10^6~\rm {cm^{-3}}$), the ion that carries most of the perpendicular and parallel conductivities is not $\rm {H_3^+}$ as was previously thought, but is instead $\rm {D_3^+}$.
Magnetohydrodynamic (MHD) turbulence is a cross-field process relevant to many systems. A prerequisite for understanding these systems is to constrain the role of MHD turbulence, and in particular, ...the energy exchange between kinetic and magnetic forms. The energetics of strongly magnetized and compressible turbulence has so far resisted attempts to understand them. Numerical simulations reveal that kinetic energy can be orders of magnitude higher than fluctuating magnetic energy. We solved this lack-of-balance puzzle by calculating the energetics of compressible and sub-Alfvénic turbulence based on the dynamics of coherent cylindrical fluid parcels. Using the MHD Lagrangian, we proved analytically that the bulk of the magnetic energy transferred to kinetic energy is the energy that is stored in the coupling between the ordered and fluctuating magnetic field. The analytical relations are in strikingly good agreement with numerical data, up to second-order terms.
Abstract
We use results of our 3 yr polarimetric monitoring programme to investigate the previously suggested connection between rotations of the polarization plane in the optical emission of blazars ...and their gamma-ray flares in the GeV band. The homogeneous set of 40 rotation events in 24 sources detected by RoboPol is analysed together with the gamma-ray data provided by Fermi-LAT. We confirm that polarization plane rotations are indeed related to the closest gamma-ray flares in blazars and the time lags between these events are consistent with zero. Amplitudes of the rotations are anticorrelated with amplitudes of the gamma-ray flares. This is presumably caused by higher relativistic boosting (higher Doppler factors) in blazars that exhibit smaller amplitude polarization plane rotations. Moreover, the time-scales of rotations and flares are marginally correlated.
Abstract
Compact symmetric objects (CSOs) are jetted active galactic nuclei (AGN) with overall projected size <1 kpc. The classification was introduced to distinguish these objects from the majority ...of compact jetted AGN in centimeter-wavelength very long baseline interferometry observations, where the observed emission is relativistically boosted toward the observer. The original classification criteria for CSOs were (i) evidence of emission on both sides of the center of activity and (ii) overall size <1 kpc. However, some relativistically boosted objects with jet axes close to the line of sight appear symmetric and have been misclassified as CSOs, thereby undermining the CSO classification. This is because two essential CSO properties, pointed out in the original papers, have been neglected: (iii) low variability and (iv) low apparent speeds along the jets. As a first step toward creating a comprehensive catalog of “bona fide” CSOs, we identify 79 bona fide CSOs, including 15 objects claimed as confirmed CSOs here for the first time, that match the CSO selection criteria. This sample of bona fide CSOs can be used for astrophysical studies of CSOs without contamination by misclassified CSOs. We show that the fraction of CSOs in complete flux density limited AGN samples with
S
5GHz
> 700 mJy is between (6.8 ± 1.6)% and (8.5 ± 1.8)%.
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