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

2020 ◽  
Vol 642 ◽  
pp. A201 ◽  
Author(s):  
S. Reissl ◽  
J. M. Stil ◽  
E. Chen ◽  
R. G. Treß ◽  
M. C. Sormani ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Interstellar Medium ◽  
Faraday Rotation ◽  
Milky Way ◽  
Line Of Sight ◽  
Unit Distance ◽  
Spiral Arms ◽  
Rotation Measure ◽  
The Magnetic Field ◽  
Spiral Arm

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.

scholarly journals Fluctuation dynamos and their Faraday rotation signatures

2012 ◽  
Vol 10 (H16) ◽  
pp. 400-400
Author(s):  
Pallavi Bhat ◽  
Kandaswamy Subramanian
Keyword(s):  
Magnetic Field ◽  
Simple Model ◽  
Faraday Rotation ◽  
High Field ◽  
Line Of Sight ◽  
Simulation Data ◽  
Thermal Electron ◽  
Volume Filling ◽  
Rotation Measure ◽  
The Magnetic Field

We study fluctuation dynamo (FD) action in turbulent systems like galaxy-clusters focusing on the Faraday rotation signature. This is defined as RM = K ∫LneB ⋅ dl where ne is the thermal electron density, B is the magnetic field, the integration is along the line of sight from the source to the observer, and K = 0.81 rad m−2 cm−3 μG−1 pc−1. We directly compute, using the simulation data, ∫ B ⋅ dl, and hence the Faraday rotation measure (RM) over 3N2 lines of sight, along each x, y and z-directions. We normalise the RM by the rms value expected in a simple model, where a field of strength Brms fills each turbulent cell but is randomly oriented from one turbulent cell to another. This normalised RM is expected to have a nearly zero mean but a non-zero dispersion, σRM. We show in Fig. 1a and 1b, that a suite of simulations, on saturation, obtain the value of σRM = 0.4−0.5, and this is independent of PM, RM and the resolution of the run. This is a fairly large value for an intermittent random field; as it is of order 40%–50%, of that expected in a model where Brms strength fields volume fill each turbulent cell, but are randomly oriented from one cell to another. We also find that the regions with a field strength larger than 2Brms contribute only 15–20% to the total RM (see Fig. 1a). This shows that it is the general ‘sea’ of volume filling fluctuating fields that contribute dominantly to the RM produced, rather than the the high field regions.

scholarly journals Magnetic fields in star-forming systems – II: Examining dust polarization, the Zeeman effect, and the Faraday rotation measure as magnetic field tracers

2020 ◽  
Vol 500 (1) ◽  
pp. 153-176
Author(s):  
Stefan Reissl ◽  
Amelia M Stutz ◽  
Ralf S Klessen ◽  
Daniel Seifried ◽  
Stefanie Walch
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Faraday Rotation ◽  
Zeeman Effect ◽  
Dominant Factor ◽  
Line Of Sight ◽  
Physical Parameters ◽  
Magnetic Field Direction ◽  
Rotation Measure ◽  
The Magnetic Field

ABSTRACT 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 analysed at the onset of star formation and it is characterized by a line-mass of about $\mathrm{\left(M/L\right) \sim 63\ \mathrm{M}_{\odot }\ pc^{-1}}$ 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 dependence 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.

scholarly journals The Two Component Magnetic Field of the Galaxy

1990 ◽  
Vol 140 ◽  
pp. 54-54
Author(s):  
R.R. Andreassian ◽  
A.N. Makarov
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Field Configuration ◽  
Radio Sources ◽  
Magnetic Field Configuration ◽  
Spiral Arms ◽  
Two Component ◽  
The Galaxy ◽  
The Magnetic Field ◽  
Spiral Arm

The present paper is devoted to a study of the magnetic field configuration of our Galaxy based on Faraday rotation measures (RM) of 185 pulsars and 802 extragalactic radio sources. RM data of pulsars located near the plane of the Galaxy are used for the study of magnetic fields in neighbouring spiral arms. For the distribution of spiral arms the well-known model of Georgelin and Georgelin (1976) is used. The calculations show (for details see Andreassian and Makarov, 1987, 1989) that in the Perseus spiral arm and the local Orion arm the magnetic fields have approximately the same directions (lo;bo) ≈ (80°;0°), while in the Sagittarius-Carina arm the magnetic field has an opposite direction.

Probing the B-fields of AGN jets on kiloparsec scales - NGC 6251

2018 ◽  
Vol 14 (S342) ◽  
pp. 244-245
Author(s):  
Sebastian Knuettel ◽  
Denise Gabuzda
Keyword(s):  
Magnetic Field ◽  
Faraday Rotation ◽  
Large Scale ◽  
Outward Current ◽  
Electrical Current ◽  
Line Of Sight ◽  
Astrophysical Jets ◽  
Rotation Measure ◽  
Jet Structure ◽  
Helical Magnetic Field

AbstractBy constructing images of the Faraday rotation measure (RM) of large scale astrophysical jets, the line-of-sight magnetic field component and electron density in the region of Farady rotation can be investigated. A significant gradient in the RM transverse to the jet direction may indicate a corresponding gradient in the line-of-sight magnetic field, implying a toroidal or helical magnetic field, which would, in turn, imply the presence of an associated electrical current in the jet. The detection of such large scale gradients can reliably demonstrate that helical or toroidal fields can persist to large distances from the central AGN. We present a kiloparsec-scale Faraday rotation map of NGC 6251 that shows statistically significant transverse RM gradients across its kiloparsec scale jet structure that imply an outward current.

Galactic magnetic-field models derived from Faraday-rotation data

1967 ◽  
Vol 31 ◽  
pp. 391-392
Author(s):  
R. D. Davies
Keyword(s):  
Magnetic Field ◽  
Faraday Rotation ◽  
Component Model ◽  
The Sun ◽  
Galactic Magnetic Field ◽  
Two Component ◽  
Net Flux ◽  
The Magnetic Field ◽  
Disk Component ◽  
Spiral Arm

The distribution of rotation measures for 86 sources suggests a two-component model for the magnetic field: a disk component directed towardl= 95°, and a component in the local spiral arm, directed alongl= 70° and 250°, with opposite senses above and below the plane. The latter may be due to a looped field in a cloud surrounding the Sun; its net flux is 3·5 micro-gauss.

scholarly journals AGN dusty plasma polarization features

2014 ◽  
Vol 10 (S313) ◽  
pp. 352-357
Author(s):  
Ericson D. Lopez ◽  
Susana Deustua
Keyword(s):  
Magnetic Field ◽  
Electromagnetic Radiation ◽  
Faraday Rotation ◽  
Significant Contribution ◽  
Theoretical Study ◽  
Outer Region ◽  
Dust Particles ◽  
Rotation Measure ◽  
Electrons And Ions ◽  
The Magnetic Field

AbstractWe present the results of a theoretical study on the influence of dust particles on the polarization properties of the radiation that propagates along the jet in AGNs. First, a model for describing the interaction of dust particles, in addition to the electrons and ions, with electromagnetic radiation in a magneto-active plasma has been developed. From here, the contribution of dust particles to the Faraday rotation of the plane of polarization of the electric vector can be deduced. This model is evaluated for the outer region of the jet where the presence of dust particles are assumed, the magnetic field is weak and the electron density is low. Our results show that the dust particles give a significant contribution to the linear Faraday rotation measure.

scholarly journals A bisymmetric spiral magnetic field in the Milky Way

1985 ◽  
Vol 106 ◽  
pp. 251-252
Author(s):  
Y. Sofue ◽  
M. Fujimoto
Keyword(s):  
Magnetic Field ◽  
Faraday Rotation ◽  
Large Scale ◽  
Milky Way ◽  
Radio Sources ◽  
Extragalactic Radio Sources ◽  
Large Scale Magnetic Field ◽  
Rotation Measure ◽  
The Galaxy ◽  
Field Lines

The distribution of Faraday rotation measure (RM) of extragalactic radio sources shows that a large-scale magnetic field in the Galaxy is oriented along the spiral arms. The field lines change direction from one arm to the next in the inter-arm region.

scholarly journals The magnetic field strength in the Large Magellanic Cloud

1991 ◽  
Vol 148 ◽  
pp. 101-102
Author(s):  
M.E. Costa ◽  
P. M. McCulloch ◽  
P. A. Hamilton
Keyword(s):  
Magnetic Field ◽  
Field Strength ◽  
Magnetic Field Strength ◽  
Large Magellanic Cloud ◽  
Radio Pulsar ◽  
Line Of Sight ◽  
The Sun ◽  
A Value ◽  
Rotation Measure ◽  
The Magnetic Field

We have measured a value of 4±5m--2rad for the rotation measure of the radio pulsar PSR0529-66 in the LMC and, after allowing for the dispersion and rotation measures of our Galaxy on the pulsar's line of sight, we deduce that the magnetic field strength in the LMC is in the range 0 to 5μGauss oriented away from the Sun.

scholarly journals PARSEC-SCALE INVESTIGATION OF THE MAGNETIC FIELD STRUCTURE OF SEVERAL AGN JETS

2008 ◽  
Vol 17 (09) ◽  
pp. 1553-1560 ◽  
Author(s):  
S. P. O'SULLIVAN ◽  
D. C. GABUZDA
Keyword(s):  
Magnetic Field ◽  
Faraday Rotation ◽  
Reliable Determination ◽  
Long Baseline ◽  
Central Engine ◽  
Relativistic Jet ◽  
Rotation Measure ◽  
Transverse Rotation ◽  
Helical Magnetic Field ◽  
The Magnetic Field

Multi-frequency (4.6, 5, 5.5, 8, 8.8, 13, 15, 22 & 43 GHz) polarization observations of six "blazars" were obtained on the American Very Long Baseline Array (VLBA) over a 24-hr period on 2 July 2006. Observing at several frequencies, separated by short and long intervals, enabled reliable determination of the distribution of Faraday rotation on a range of scales. In all cases the magnitude of the RM increases in the higher frequency observations, implying that the electron density and/or magnetic field strength is increasing as we get closer to the central engine. After correcting for Faraday rotation, the polarization orientation in the jet is either parallel or perpendicular to the jet direction. A transverse rotation measure (RM) gradient was detected in the jet of 0954+658, providing evidence for the presence of a helical magnetic field surrounding the jet. For three of the sources (0954+658, 1418+546, 2200+420), the sign of the RM in the core region changes in different frequency-intervals, indicating that the line-of-sight component of the magnetic field is changing with distance from the base of the jet. We suggest an explanation for this in terms of bends in a relativistic jet surrounded by a helical magnetic field; where there is no clear evidence for pc-scale bends, the same effect can be explained by an accelerating/decelerating jet.

Sign in / Sign up

Export Citation Format

Share Document