Radiative Transfer with POLARIS. II. Modeling of Synthetic Galactic Synchrotron Observations

Reißl, Stefan; Brauer, Robert; Klessen, Ralf S.; Pellegrini, E.

doi:10.3847/1538-4357/ab3664

Cited by 37 publications

(62 citation statements)

References 124 publications

(199 reference statements)

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“…Observational tracers such as dust and synchrotron emission are highly sensitive to the local conditions (Reissl et al 2019). For the purpose of the follow-up radiative transfer postprocessing we manually added three additional bubbles driven by clustered supernovae chosen to be roughly at solar distances from the Galactic center, each associated with a spiral arm.…”

Section: The Simulation Setupmentioning

confidence: 99%

“…The RT simulations are performed with the RT code Polaris 2 (Reissl et al 2016). The advantage of Polaris is that it unites the physics of dust emission (Reissl et al 2016(Reissl et al , 2018, line transfer including the Zeeman effect (Brauer et al 2017a,b), and synchrotron emission (Reissl et al 2019) under a common framework. Furthermore, the code allows one to perform RT simulations on a Voronoi grid.…”

Section: Radiative Transfer Post-processingmentioning

confidence: 99%

“…However, the scale height and extension of the disk of the MHD simulations is not the same as that of the Milky Way. Hence, we follow the approach discussed in Reissl et al (2019) and assume equipartition between the CR energy and the magnetic field energy, so that n CR B 2 /(16πγ min m e c 2 ). Here B is the magnetic field in each cell of the model galaxy, m e is the electron mass, and c is the speed of light.…”

Section: Synchrotron Radiation and Faraday Rmmentioning

confidence: 99%

“…where P ν (γ) is the synchrotron power per frequency ν (see, e.g., Pandya et al 2016;Reissl et al 2019, for further details). We note that the synchrotron power has an angular dependency P ν (γ) ∝ sin ϑ, where the angle ϑ is defined to be between magnetic field direction and the line of sight (LOS).…”

Section: Synchrotron Radiation and Faraday Rmmentioning

confidence: 99%

“…For each of the LOSs we perform ray-tracing by solving the RT problem as outlined above for each individual tracer. Here Polaris utilizes a Runge-Kutta-Fehlberg solver with built-in error correction working within an error limit of < 10 −6 for each Voronoi cell (see, e.g., Ober et al 2015;Reissl et al 2019, for details).…”

Section: The Ray-tracing Schemementioning

confidence: 99%

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

Reißl

Stil

Chen

et al. 2020

A&A

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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. Recent observations indicate an RM magnitude in the spiral arms that exceeds the commonly assumed range. This raises the question of how and under what conditions spiral arms create such strong Faraday rotation. Aims. We investigate the effect of spiral arms on Galactic Faraday rotation through shock compression of the interstellar medium. It has recently been suggested that the Sagittarius spiral arm creates a strong peak in Faraday rotation where the line of sight is tangent to the arm, and that enhanced Faraday rotation follows along side lines which intersect the arm. Here our aim is to understand the physical conditions that may give rise to this effect and the role of viewing geometry. Methods. We apply a magnetohydrodynamic simulation of the multi-phase interstellar medium in a Milky Way-type spiral galaxy disk in combination with radiative transfer in order 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 which spiral arm tangents are observable. Faraday rotation measures are calculated through the disk and evaluated in the context of different observer positions. The observer’s perspectives are related to the parameters of the local bubbles surrounding the observer and their contribution to the total Faraday rotation measure along the line of sight. Results. We reproduce a scattering of tangent points for the different tracers of about 6° per spiral arm similar to the Milky Way. For the RM, the model shows that compression of the interstellar medium and associated amplification of the magnetic field in spiral arms enhances Faraday rotation by a few hundred rad m−2 in addition to the mean contribution of the disk. The arm–interarm contrast in Faraday rotation per unit distance along the line of sight is approximately ~10 in the inner Galaxy, fading to ~2 in the outer Galaxy in tandem with the waning contrast of other tracers of spiral arms. We identify a shark fin pattern in the RM Milky Way observations and in the synthetic data that is characteristic for a galaxy with spiral arms.

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Section: The Simulation Setupmentioning

confidence: 99%

Section: Radiative Transfer Post-processingmentioning

confidence: 99%

Section: Synchrotron Radiation and Faraday Rmmentioning

confidence: 99%

Section: Synchrotron Radiation and Faraday Rmmentioning

confidence: 99%

Section: The Ray-tracing Schemementioning

confidence: 99%

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

Reißl

Stil

Chen

et al. 2020

A&A

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

Reissl,

Klessen,

Pellegrini

et al. 2023

Nat Astron

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

Reißl

Guillet

Brauer

et al. 2020

A&A

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Context. Analyses of Planck data have demonstrated that the grain alignment efficiency is almost constant in the diffuse and translucent interstellar medium (ISM). Aims. We aim to test whether the radiative torque (RAT) theory is compatible with these new observational constraints on grain alignment. Methods. We combine a numerical magnetohydrodynamical simulation with a state-of-the-art radiative transfer post-processing code POLARIS which incorporates a physical dust model and the detailed physics of grain alignment by RATs. A dust model based on two distinct power-law-sized distributions of spherical graphite grains and oblate silicate grains was designed to reproduce the mean spectral dependence of extinction and polarization observed in the diffuse ISM. From a simulation of interstellar turbulence obtained with the adaptive-mesh-refinement code RAMSES, we extracted a data cube with physical conditions representative of the diffuse ISM. We post-process the RAMSES cube with POLARIS to compute the grain temperature and alignment efficiency in each cell of the cube. Finally, we simulate synthetic dust emission and polarization observations. Results. In our simulation, the grain alignment efficiency is well-correlated with the gas pressure, but not with the radiative torque intensity. Because of the low dust extinction in our simulation, the magnitude of the radiative torque varies little, decreasing only for column densities larger than 1022 cm−2. In comparing our synthetic maps with those obtained assuming a uniform alignment efficiency, we find no systematic difference and very small random differences. The dependencies of the polarization fraction p with the column density NH or with the dispersion in polarization angle S are also similar in both cases. The drop of grain alignment produced by the RAT model in the denser cells of the data cube does not significantly affect the patterns of the synthetic polarization maps, the polarization signal being dominated by the line-of-sight and beam integration of the geometry of the magnetic field. If a star is artificially inserted at the center of the simulation, the polarization fraction is increased everywhere, with no specific pattern around the star. The angle-dependence of the RAT efficiency is not observed in simulated maps and where the magnetic field is artificially set to a uniform configuration in the plane of the sky, it is only seen to be very weak in the optimal configuration. Conclusions. The RAT alignment theory is found to be compatible with the Planck polarization data for the diffuse and translucent ISM in the sense that both uniform alignment and RAT alignment lead to very similar simulated maps. To further test the predictions of the RAT theory in an environment where an important drop of grain alignment is expected, high-resolution polarization observations of dense regions must be confronted with numerical simulations sampling high-column densities (NH > 1022 cm−2) through dense clouds, given a sufficient statistical basis.

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Radiative Transfer with POLARIS. II. Modeling of Synthetic Galactic Synchrotron Observations

Cited by 37 publications

References 124 publications

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

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

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

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

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