arXiv subjects

Stefan Reissl

Publications and source records attributed to Stefan Reissl.

At least 19 recordsLinked to original sources

arXiv2026

IRIS: Deciphering Spectral-Line Imagery of the Galactic Center by Machine-Learning on Simulations

In understanding the 3D structure of the Milky Way's Central Molecular Zone (CMZ), we are limited by our edge-on perspective. Towards addressing this problem, we introduce Imagery Reversion Informed by Simulation (IRIS). IRIS is a novel machine-learning code base featuring a deep convolutional neural network (CNN), which we have designed to translate edge-on observations of our Milky Way Galaxy into top-down images by training on data generated from AREPO galaxy simulations and synthetic observations of those simulations. We develop a large custom dataset on which we train our bespoke model, and then test the trained model on synthetic data to probe the potential of this machine-learning method, which we call supervised reversion. We then apply our trained model to real observations from the SEDIGISM 13CO(2-1) survey, yielding new top-down views of our CMZ. Though our SEDIGISM reversions are not fully consistent across model training runs, we posit that this lack of convergence can be alleviated by expansion of the training dataset. We argue that these results represent a strong proof-of-concept for the use of supervised reversion to decipher our CMZ's 3D structure. Crucial in generating our training dataset's 100k synthetic observations, we introduce IRIS Synthetic Observation (IRIS-SO), a new GPU-accelerated and fully differentiable code implemented in PyTorch for the non-LTE synthetic observation of spectral lines and dust. We find that IRIS-SO provides up to 10,000x speedups in comparison to the synthetic-observation code RADMC-3D. We release all the IRIS code open-source at https://github.com/bldubois/IRIS.

astro-ph.GA↗

arXiv2026

SIMPLIFI -- Study of Interstellar Magnetic Polarization: a Legacy Investigation of Filaments. I. Magnetically-Guided Accretion onto the DR21 Ridge

We present first results from SIMPLIFI (Study of Interstellar Magnetic Polarization: a Legacy Investigation of Filaments), a SOFIA/HAWC+ $214~μ\rm{}m$ polarimetric survey of Galactic molecular cloud filaments. We trace magnetic field morphology from the DR21 Main Ridge into surrounding sub-filaments at $\sim{}0.1~\rm{}pc$ resolution, extending polarimetric detections for the first time beyond high-column-density regions probed by prior submillimeter observations. We compare the plane-of-sky orientations of the magnetic field $\hat{B}_{\rm{}pos}$, the projected gravitational acceleration $\vec{g}_{\rm{}pos}$, and the intensity gradient rotated by $90^{\circ}$. The relative orientation of $\hat{B}_{\rm{}pos}$ and the rotated gradient transitions from preferentially parallel in sub-filaments to perpendicular in the Main Ridge at $N({\rm{}H_2})\sim{}2\times{}10^{22}~\rm{}cm^{-2}$, consistent with thresholds seen with Planck. This is expected in clouds formed from strongly magnetized, sub-Alfvenic, magnetically sub-critical gas. We find region-to-region and pixel-to-pixel variations at fixed column density, indicating that column density alone is not sufficient to encode changes in magnetic field structure. Our central finding is that $\vec{g}_{\rm{}pos}$ and $\hat{B}_{\rm{}pos}$ remain aligned throughout the cloud regardless of column density or environment, unlike the environment-dependent behavior of either quantity vs. the intensity gradient. This persistent alignment is consistent with magnetically-guided accretion: sub-filaments channel material along field lines at several $10^{-3}\,M_{\odot}\,\rm{}yr^{-1}$, sufficient to assemble the Ridge within $\sim{}1~\rm{}Myr$ and sustain high-mass star formation. The framework also explains why observed radial velocities $\sim{}2~\rm{}km\,s^{-1}$ fall well below free-fall expectations $\sim{}8~\rm{}km\,s^{-1}$ due to projection effects.

astro-ph.GA↗

arXiv2025

Polarization at millimeter wavelengths caused by drifting grains in protoplanetary disks

During the evolution of protoplanetary disks, dust grains start to grow, form larger particles, settle to the midplane, and rearrange the disk, mainly by the inward radial drift. Because of this, dust pebbles with an irregular shape usually align mechanically and thus cause polarization signatures in their thermal radiation due to dichroic emission or absorption. The goal of this paper is to evaluate the potential to trace the impact of mechanical grain alignment in protoplanetary disks on the observed degree and orientation of linear polarization at millimeter wavelengths. We combined 3D radiation hydrodynamical simulations to determine the density distribution and the velocity field of gas and dust particles, Monte Carlo dust-gas interaction simulations to calculate the mechanical alignment of dust in a gas flow, and, finally, 3D Monte Carlo polarized radiative transfer simulations to obtain synthetic polarimetric observations. We find that large grains, which contribute the most to the net polarization, are potentially mechanically aligned in the protoplanetary disk under the effect of the vertical shear instability (VSI). Thereby, the drift velocity is parallel to the rotational disk axis. Assuming oblate dust grains that are aligned with their short axis parallel to the direction of the drift velocity, the resulting polarization is usually along the major axis of the disk. This is in contrast to typical drift models that propose either a radial or azimuthal drift velocity component. If hydrodynamical instabilities, such as the VSI, dominate the kinematics in protoplanetary disks, the mechanical alignment of dust is a promising mechanism for grain alignment in these systems. In that case, the resulting millimeter polarization allows us to trace the orientation of aligned millimeter-sized grains.

astro-ph.EP↗

arXiv2025

Modeling Local Bubble analogs II: Synthetic Faraday rotation maps

Faraday rotation describes the change of the linear polarization angle of radiation passing through a magnetized plasma and it is quantified by the rotation measure (RM), which is related to the line-of-sight (LOS) magnetic field component and the thermal electron density traversed by light along its path toward the observer. However, it is challenging to disentangle the signal from different LOS portions and separate the contribution from the local ISM. This is particularly relevant since the Sun is located within the Local Bubble (LB), a low-density and hot cavity formed by past SN events, making it essential to investigate how this environment may influence the observed RM values. The present study investigates the imprint of the local environment on the synthetic RM signal, as measured by an observer within a LB-like cavity. The RM derived from diffuse polarized synchrotron radiation produced by CR electrons at decimeter wavelengths is also analyzed. We produce synthetic RM maps for an observer placed inside a LB candidate, selected from a MHD simulation that resembles the properties of the ISM in the Solar vicinity. Using the capabilities of the radiative transfer code POLARIS, we study the imprint of the cavity walls on the RM signal. As the MHD simulation does not account for CR diffusion, we develop a CR toy-model to study the Faraday rotation of the diffuse polarized synchrotron radiation. We find that (i) the imprint of local structures, such as the walls of the LB candidate and the edges of other supernovae blown cavities, is of fundamental importance for interpreting the global Faraday sky; (ii) the LB has a non negligible contribution to the sinusoidal patterns of RM as a function of Galactic longitude seen in observations; and (iii) the RM signal from diffuse synchrotron emission shows a strong correspondence with the RM signal generated by the LB candidate walls.

astro-ph.GA↗

arXiv2024

The radiative torque spin-up efficiency of ballistic dust grain aggregates

Aims. It is quintessential for the analysis of the observed dust polarization signal to understand the rotational dynamics of interstellar dust grains. Additionally, high rotation velocities may rotationally disrupt the grains, which impacts the grain-size distribution. We aim to constrain the set of parameters for an accurate description of the rotational spin-up process of ballistic dust grain aggregates driven by radiative torques (RATs). Methods. We modeled the dust grains as complex fractal aggregates grown by the ballistic aggregation of uniform spherical particles (monomers) of different sizes. A broad variation of dust materials, shapes, and sizes were studied in the presence of different radiation sources. Results. We find that the canonical parameterization for the torque efficiency overestimates the maximum angular velocity $ω_{\mathrm{RAT}}$ caused by RATs acting on ballistic grain aggregates. To resolve this problem, we propose a new parameterization that predicts $ω_{\mathrm{RAT}}$ more accurately. We find that RATs are most efficient for larger grains with a lower monomer density. Following the constant part, the parameterization has two power laws with different slopes that retain universality for all grain sizes. The maximum grain rotation does not scale linearly with radiation strength because different drag mechanisms dominate, depending on the grain material and environment. The angular velocity $ω_{\mathrm{RAT}}$ of individual single dust grains has a wide distribution and may even differ from the mean by up to two orders of magnitude. Even though ballistic aggregates have a lower RAT efficiency, strong sources of radiation (stronger than $\approx 100$ times the typical interstellar radiation field) may still produce rotation velocities high enough to cause the rotational disruption of dust grains.

astro-ph.GA↗

arXiv2024

A deep-learning approach to the 3D reconstruction of dust density and temperature in star-forming regions

Aims: We introduce a new deep-learning approach for the reconstruction of 3D dust density and temperature distributions from multi-wavelength dust emission observations on the scale of individual star-forming cloud cores (<0.2pc). Methods: We construct a training data set by processing cloud cores from the Cloud Factory simulations with the POLARIS radiative transfer code to produce synthetic dust emission observations at 23 wavelengths between 12 and 1300 $μ$m. We simplify the task by reconstructing the cloud structure along individual lines of sight and train a conditional invertible neural network (cINN) for this purpose. The cINN belongs to the group of normalising flow methods and is able to predict full posterior distributions for the target dust properties. We test different cINN setups, ranging from a scenario that includes all 23 wavelengths down to a more realistically limited case with observations at only seven wavelengths. We evaluate the predictive performance of these models on synthetic test data. Results: We report an excellent reconstruction performance for the 23-wavelengths cINN model, achieving median absolute relative errors of about 1.8% in $\log(n/m^{-3})$ and 1% in $\log(T_{dust}/K)$, respectively. We identify trends towards overestimation at the low end of the density range and towards underestimation at the high end of both the density and temperature values, which may be related to a bias in the training data. Limiting our coverage to a combination of only seven wavelengths, we still find a satisfactory performance with average absolute relative errors of about 3.3% and 2.5% in $\log(n/m^{-3})$ and $\log(T_{dust}/K)$. Conclusions: This proof-of-concept study shows that the cINN-based approach for 3D reconstruction of dust density and temperature is very promising and even compatible with a more realistically constrained wavelength coverage.

astro-ph.GA↗

arXiv2023

Dust attenuation in galaxies at cosmic dawn from the FirstLight simulations

We study the behavior of dust in galaxies at cosmic dawn, z=6-8, by coupling the FirstLight simulations with the radiative transfer code POLARIS. The starburst nature of these galaxies and their complex distribution of dust lead to a large diversity of attenuation curves. These follow the Calzetti model only for relatively massive galaxies, Mstars=10^9Msun. Galaxies with lower masses have steeper curves, consistent with the model for the Small Magellanic Cloud (SMC). The ultraviolet and optical slopes of the attenuation curves are closer to the modified Calzetti model, with a slight preference for the power-law model for galaxies with the highest values of attenuation. We have also examined the relation between the slope in the far-ultraviolet, beta_UV , and the infrared excess, IRX. At z=6, it follows the Calzetti model with a shift to slightly lower beta_UV values due to lower metallicities at lower attenuation. The same relation at z=8 shows a shift to higher IRX values due to a stronger CMB radiation at high-z.

astro-ph.GA↗

arXiv2023

The complex multi-scale structure in simulated and observed emission maps of the proto-cluster cloud G0.253+0.016 (`the Brick')

The Central Molecular Zone (CMZ; the central ~ 500 pc of the Milky Way) hosts molecular clouds in an extreme environment of strong shear, high gas pressure and density, and complex chemistry. G0.253+0.016, also known as `the Brick', is the densest, most compact and quiescent of these clouds. High-resolution observations with the Atacama Large Millimeter/submillimeter Array (ALMA) have revealed its complex, hierarchical structure. In this paper we compare the properties of recent hydrodynamical simulations of the Brick to those of the ALMA observations. To facilitate the comparison, we post-process the simulations and create synthetic ALMA maps of molecular line emission from eight molecules. We correlate the line emission maps to each other and to the mass column density, and find that HNCO is the best mass tracer of the eight emission lines within the simulations. Additionally, we characterise the spatial structure of the observed and simulated cloud using the density probability distribution function (PDF), spatial power spectrum, fractal dimension, and moments of inertia. While we find good agreement between the observed and simulated data in terms of power spectra and fractal dimensions, there are key differences in the density PDFs and moments of inertia, which we attribute to the omission of magnetic fields in the simulations. This demonstrates that the presence of the Galactic potential can reproduce many cloud properties, but additional physical processes are needed to fully explain the gas structure.

astro-ph.GA↗

arXiv2023

A reproduction of the Milky Way's Faraday rotation measure map in galaxy simulations from global to local scales

Magnetic fields are of critical importance for our understanding of the origin and long-term evolution of the Milky Way. This is due to their decisive role in the dynamical evolution of the interstellar medium (ISM) and their influence on the star-formation process. Faraday rotation measures (RM) along many different sightlines across the Galaxy are a primary means to infer the magnetic field topology and strength from observations. However, the interpretation of the data has been hampered by the failure of previous attempts to explain the observations in theoretical models and to synthesize a realistic multi-scale all-sky RM map. We here utilize a cosmological magnetohydrodynamic (MHD) simulation of the formation of the Milky Way, augment it with a novel star cluster population synthesis model for a more realistic structure of the local interstellar medium, and perform detailed polarized radiative transfer calculations on the resulting model. This yields a faithful first principles prediction of the Faraday sky as observed on Earth. The results reproduce the observations of the Galaxy not only on global scales, but also on local scales of individual star-forming clouds. They also imply that the Local Bubble containing our Sun dominates the RM signal over large regions of the sky. Modern cosmological MHD simulations of the Milky Way's formation, combined with a simple and plausible model for the fraction of free electrons in the ISM, explain the RM observations remarkably well, thus indicating the emergence of a firm theoretical understanding of the genesis of magnetic fields in our Universe across cosmic time.

astro-ph.GA↗

arXiv2023

The rotational disruption of porous dust aggregates from ab-initio kinematic calculations

Context: The sizes of dust in the interstellar medium follows a distribution where most of the dust mass is in smaller grains. However, the re-distribution from larger grains towards smaller sizes especially by means of rotational disruption is poorly understood. Aims: We aim to study the dynamics of porous grain aggregates under accelerated ration. Especially, we determine the deformation of the grains and the maximal angular velocity up to the rotational disruption event by caused by centrifugal forces. Methods: We pre-calculate aggregates my means of ballistic aggregation analogous to the interstellar dust as input for subsequent numerical simulations. In detail, we perform three-dimensional N-body simulations mimicking the radiative torque spin-up process up to the point where the grain aggregates become rotationally disrupted. Results: Our simulations results are in agreement with theoretical models predicting a characteristic angular velocity $ω_{\mathrm{disr}}$ of the order of ${ 10^8 - 10^9\ \mathrm{rad\ s^{-1}} }$, where grains become rotationally disrupted. In contrast to theoretical predictions, we show that for large porous aggregates ($< 300\ \mathrm{nm}$) $ω_{\mathrm{disr}}$ reaches a lower asymptotic value. Hence, such grains can withstand an accelerated ration more efficiently up to a factor of 10 because the displacement of mass by centrifugal forces and the subsequent mechanical deformation supports the buildup of new connections within the aggregate. Furthermore, we report that the rapid rotation of grains deforms an ensemble with initially 50:50 prolate and oblate shapes, respectively, preferentially into oblate shapes. Finally, we present a best fit formula to predict the average rotational disruption of an ensemble of porous dust aggregates dependent on internal grain structure, total number of monomers, and applied material properties.

astro-ph.GA↗

arXiv2023

The Mechanical Alignment of Dust (MAD) I: On the spin-up process of fractal grains by a gas-dust drift

Context: Aligned dust grains are commonly exploited to probe the magnetic field orientation. However, the exact physical processes that result in a coherent large-scale grain alignment are far from being constrained. Aims: In this work, we aim to investigate the impact of a gas-dust drift leading to a mechanical alignment of dust (MAD) and to dust polarization. Methods: We explore fractal dust aggregates to statistically analyze the average alignment behavior of distinct grain ensembles. The spin-up efficiencies for individual aggregates are determined utilizing MC simulations. These efficiencies are analyzed to identify stable points for the grain alignment in direction of the gas-dust drift and along the magnetic field lines. Finally, the net dust polarization is calculated per grain ensemble. Results: The mechanical spin-up within the CNM is sufficient to drive grains to a stable alignment. A likely mechanical grain alignment is parallel to the drift direction. All grains can align at subsonic conditions. Here, we predict a polarization efficiency in the order of unity for the MAD. A supersonic drift may result in a rapid rotation where dust grains may become rotationally disrupted and the polarization becomes drastically reduced. In the presence of a magnetic field, the drift required for the alignment of elongated grains is roughly one order of magnitude higher compared to the pure MAD. Here, the dust polarization efficiency is 0.6-0.9 indicating that a drift can provide the prerequisites to probe the magnetic field. The alignment is inefficient when the direction of the drift and the field lines are perpendicular. Conclusions: We find that MAD has to be taken into consideration as an alternative driving mechanism where the standard RAT alignment theory fails to account for the full spectrum of available dust polarization observations.

astro-ph.GA↗

arXiv2022

Synthetic observations using POLARIS: an application to simulations of massive prestellar cores

Young massive stars are usually found embedded in dense and massive molecular clumps which are known for being highly obscured and distant. During their formation process, the degree of deuteration can be used as a potential indicator of the very early formation stages. This is particularly effective when employing the abundance of H$_2$D$^+$. However, its low abundances and large distances make detections in massive sources hard to achieve. We present an application of the radiative transfer code POLARIS, with the goal to test the observability of the ortho-H$_2$D$^+$ transition $1_{10}$-$1_{11}$ (372.42 GHz) using simulations of high-mass collapsing cores that include deuteration chemistry. We analyzed an early and a late stage of the collapse of a 60 M$_{\odot}$ core, testing different source distances. For all cases, we generated synthetic single-dish and interferometric observations and studied the differences in both techniques. The column densities we derive are comparable to values reported for similar sources. These estimates depend on the extent over which they are averaged, and sources with compact emission they can be highly affected by beam dilution. Combined ALMA-ACA observations improve in signal-to-noise ratio and lead to better column density estimates as compared to ALMA alone. We confirm the feasibility to study ortho-H$_2$D$^+$ emission up to distances of 7 kpc. We provide a proof-of-concept of our framework for synthetic observations and highlight its importance when comparing numerical simulations with real observations. This work also proves how relevant it is to combine single-dish and interferometric measurements to derive appropriate source column densities.

astro-ph.SR↗

arXiv2020

Filament rotation in the California L1482 cloud

We analyze the gas mass distribution, the gas kinematics, and the young stellar object (YSO) content of the California Molecular Cloud (CMC) L1482 filament. We derive a Gaia DR2 YSO distance of 511$^{+17}_{-16}$ pc. We derive scale-free power-laws for the mean gas line-mass (M/L) profiles; we calculate the gravitational potential and field profiles consistent with these. We present IRAM 30 m C$^{18}$O (1-0) (and other tracers) position-velocity (PV) diagrams that exhibit complex velocity twisting and turning structures. We find a rotational profile in C$^{18}$O perpendicular to the southern filament ridgeline. The profile is regular, confined ($r\lesssim0.4$ pc), anti-symmetric, and to first order linear with a break at $r\sim0.25$ pc. The timescales of the inner (outer) gradients are $\sim$0.7 (6.0) Myr. We show that the centripetal force, compared to gravity, increases toward the break; when the ratio of forces approaches unity, the profile turns over, just before filament breakup is achieved. The timescales and relative roles of gravity to rotation indicate that the structure is stable, long lived ($\sim$ a few times 6 Myr), and undergoing outside-in evolution. Moreover, this filament has practically no star formation, a perpendicular Planck plane-of-the-sky (POS) magnetic field morphology, and POS "zig-zag" morphology, which together with the rotation profile lead to the suggestion that the 3D shape is a corkscrew filament with a helical magnetic field. These results, combined with results in Orion and G035.39-00.33, suggest evolution toward higher densities as rotating filaments shed angular momentum. Thus, magnetic fields may be an essential feature of high-mass (M $\sim10^5$ M$_{\odot}$) cloud filament evolution toward cluster formation.

astro-ph.GA↗

arXiv2020

Magnetized filamentary gas flows feeding the young embedded cluster in Serpens South

Observations indicate that molecular clouds are strongly magnetized, and that magnetic fields influence the formation of stars. A key observation supporting the conclusion that molecular clouds are significantly magnetized is that the orientation of their internal structure is closely related to that of the magnetic field. At low column densities the structure aligns parallel with the field, whereas at higher column densities, the gas structure is typically oriented perpendicular to magnetic fields, with a transition at visual extinctions $A_V\gtrsim{}3~\rm{}mag$. Here we use far-infrared polarimetric observations from the HAWC+ polarimeter on SOFIA to report the discovery of a further transition in relative orientation, i.e., a return to parallel alignment at $A_V\gtrsim{}21~\rm{}mag$ in parts of the Serpens South cloud. This transition appears to be caused by gas flow and indicates that magnetic supercriticality sets in near $A_V\gtrsim{}21~\rm{}mag$, allowing gravitational collapse and star cluster formation to occur even in the presence of relatively strong magnetic fields.

astro-ph.GA↗

arXiv2020

Magnetic fields in star-forming systems (II): examining dust polarization, the Zeeman effect, and the Faraday rotation measure as magnetic field tracers

The degree to which the formation and evolution of clouds and filaments in the interstellar medium is regulated by magnetic fields remains an open question. Yet the fundamental properties of the fields (strength and 3D morphology) are not readily observable. We investigate the potential for recovering magnetic field information from dust polarization, the Zeeman effect, and the Faraday rotation measure ($RM$) in a SILCC-Zoom magnetohydrodynamic (MHD) filament simulation. The object is analyzed at the onset of star formation, and it is characterized by a line-mass of about M/L $\sim 63\ M_{\odot}\ pc^{-1}$ out to a radius of $1\,$pc and a kinked 3D magnetic field morphology. We generate synthetic observations via POLARIS radiative transfer (RT) post-processing, and compare with an analytical model of helical or kinked field morphology to help interpreting the inferred observational signatures. We show that the tracer signals originate close to the filament spine. We find regions along the filament where the angular-dependency with the line-of-sight (LOS) is the dominant factor and dust polarization may trace the underlying kinked magnetic field morphology. We also find that reversals in the recovered magnetic field direction are not unambiguously associated to any particular morphology. Other physical parameters, such as density or temperature, are relevant and sometimes dominant compared to the magnetic field structure in modulating the observed signal. We demonstrate that the Zeeman effect and the $RM$ recover the line-of-sight magnetic field strength to within a factor 2.1 - 3.4. We conclude that the magnetic field morphology may not be unambiguously determined in low-mass systems by observations of dust polarization, Zeeman effect, or $RM$, whereas the field strengths can be reliably recovered.

astro-ph.GA↗

arXiv2020

Synthetic observations of deuterated molecules in massive prestellar cores

Young massive stars are usually found embedded in dense massive molecular clumps and are known for being highly obscured and distant. During their formation process, deuteration is regarded as a potentially good indicator of the very early formation stages. In this work, we test the observability of the ground-state transition of ortho-H$_2$D$^+$ $J_{\rm {K_a, K_c}} = 1_{10}$-$1_{11} $ by performing interferometric and single-dish synthetic observations using magneto-hydrodynamic simulations of high-mass collapsing molecular cores, including deuteration chemistry. We studied different evolutionary times and source distances (from 1 to 7 kpc) to estimate the information loss when comparing the column densities inferred from the synthetic observations to the column densities in the model. We mimicked single-dish observations considering an APEX-like beam and interferometric observations using CASA and assuming the most compact configuration for the ALMA antennas. We found that, for centrally concentrated density distributions, the column densities are underestimated by about 51% in the case of high-resolution ALMA observations ($\leqslant$1") and up to 90% for APEX observations (17"). Interferometers retrieve values closer to the real ones, however, their finite spatial sampling results in the loss of contribution from large-scale structures due to the lack of short baselines. We conclude that, the emission of o-H$_2$D$^+$ in distant massive dense cores is faint and would require from $\sim$1 to $\sim$7 hours of observation at distances of 1 and 7 kpc, respectively, to achieve a 14$σ$ detection in the best case scenario. Additionally, the column densities derived from such observations will certainly be affected by beam dilution in the case of single-dishes and spatial filtering in the case of interferometers.

astro-ph.GA↗

arXiv2020

Synthetic observations of spiral arm tracers of a simulated Milky Way analog

Context: The Faraday rotation measure (RM) is often used to study the magnetic field strength and orientation within the ionized medium of the Milky Way. Observations indicate a RM in the spiral arms that exceeds the commonly assumed range. This raises the question of under what conditions spiral arms create such strong RM. Aims: We investigate the effect of spiral arms on Galactic RMs through shock compression of the interstellar medium (ISM). It has recently been suggested that the Sagittarius spiral arm creates a strong peak in RM where the line of sight (LOS) is tangent to the arm, and that enhanced RM follows along an intersecting LOS. We seek to understand the physical conditions that give rise to this effect and the role of viewing geometry. Methods: We apply a MHD simulation of the multi-phase ISM in a Milky Way type spiral galaxy disk in combination with radiative transfer to evaluate different tracers of spiral arm structures. For observers embedded in the disk, dust intensity, synchrotron emission and the kinematics of molecular gas observations are derived to identify spiral arm tangents. RMs are calculated through the disk and evaluated for different observer positions. The observer's perspective is related to the parameters of the local bubble surrounding the observer. Results: We reproduce a scattering of tangent points for different tracers of about $6^\circ$ per spiral arm similar to the Milky Way. As for the RM, the model shows that compression of the ISM and associated amplification of the magnetic field in spiral arms enhances RM by a few hundred rad $m^{-2}$ on top of the mean contribution of the disk. The arm-inter-arm contrast in RM along the LOS is approximately 10 in the inner Galaxy, fading to ~2 in the outer Galaxy. We identify a shark-fin like pattern in the RM Milky Way observations as well as the synthetic data that is characteristic for spiral arms.

astro-ph.GA↗

arXiv2020

A systematic study of radiative torque grain alignment in the diffuse interstellar medium

Context. Planck observations demonstrated that the grain alignment efficiency is almost constant in the diffuse ISM. Aims. We test if the Radiative Torque (RAT) theory is compatible with observational constraints on grain alignment. Methods. We combine a numerical simulation with the radiative transfer code POLARIS that incorporates a physical dust model and the detailed grain alignment physics of RATs. A dust model is designed to reproduce the spectral dependence of extinction of the ISM. From a RAMSES simulation of interstellar turbulence, we extract a cube representative of the diffuse ISM. We post-process the cube with POLARIS to get the grain temperature and RATs to simulate synthetic dust polarization maps. Results. In our simulation the grain alignment efficiency is correlated with gas pressure, but not with the RAT intensity. Because of the low dust extinction, the magnitude of RATs varies little, decreasing only for high column densities $N_H$. Comparing our maps with a uniform alignment efficiency, we find no systematic difference. The dependence of polarization fraction $p$ with $N_H$ or polarization dispersion $S$ is similar. The drop of RATs in dense regions barely affects the polarization pattern, the signal being dominated by the LOS and magnetic field geometry. If a star is inserted, the polarization increases, with no specific pattern around the star. The angle-dependence of RATs is not observed in the maps, and is weak using a uniform magnetic field. Conclusions. RATs are compatible with Planck data for the diffuse ISM such that both uniform alignment and RAT alignment lead to similar observations. To further test the predictions of RATs where an important drop of grain alignment is expected, polarization observations of dense regions must be confronted to numerical simulations sampling high column densities through dense clouds, with enough statistics.

astro-ph.IM↗