Strong Excess Faraday Rotation on the Inside of the Sagittarius Spiral Arm

Shanahan, R.; Lemmer, Stephen J; Stil, J. M.; Beuther, H.; Wang, Y.; Soler, J. D.; Anderson, L. D.; Bigiel, Frank; Glover, Simon C. O.; Goldsmith, Paul F.; Klessen, Ralf S.; McClure-Griffiths, N. M.; Reißl, Stefan; Rugel, M.; Smith, Rowan J.

doi:10.3847/2041-8213/ab58d4

Cited by 37 publications

(57 citation statements)

References 69 publications

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“…This can explain an increased Faraday depolarization trailing the spiral arm. A recent paper by Shanahan et al (2019) shows that there are large RM values on the inner side of a spiral arm of the Milky Way, going along with that argument.…”

Section: General Explanations and A Simplified Modelmentioning

confidence: 67%

Chang-Es

Stein

Dettmar

Beck

et al. 2020

A&A

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Context. Radio continuum observations of edge-on spiral galaxies reveal the appearance of radio halos as well as the large-scale structure of their magnetic fields. Furthermore, with multiple frequency observations, it is possible to deduce the transport mechanisms of the cosmic ray electrons (CREs). Aims. In order to gain a better understanding of the influence of cosmic rays (CRs) and magnetic fields in the disk-halo interface of edge-on spiral galaxies, we investigate the radio continuum halo, the magnetic field, and the transport processes of the CRs of the edge-on spiral galaxy NGC 4217 using CHANG-ES radio data at two frequencies, 6 GHz (C-band) and 1.5 GHz (L-band), and supplemental LOFAR data of this galaxy at 150 MHz. With additional X-ray Chandra data, we study the connection of radio features to the diffuse hot gas around NGC 4217. Methods. We investigate the total intensity (Stokes I) data in detail and determine the integrated spectral behavior. The radio scale heights of all three radio frequencies for NGC 4217 were extracted via exponential fits to the intensity profiles. From these, individual absolute flux densities of the disk and the halo were also calculated. Furthermore, we present magnetic field orientations from the polarization data using rotation measure synthesis (RM-synthesis), showing the large-scale ordered magnetic field of NGC 4217. After a separation of thermal and nonthermal emission, we calculated the resolved magnetic field strength via the revised equipartition formula. Additionally, we modeled the transport processes of CREs into the halo with the 1D model SPINNAKER. Results. NGC 4217 shows a large-scale X-shaped magnetic field structure, covering a major part of the galaxy with a mean total magnetic field strength in the disk of 9 μG. From the analysis of the rotation measure map at C-band, we found that the direction of the disk magnetic field is pointing inward. A helical outflow structure is furthermore present in the northwestern part of the galaxy, which is extended nearly 7 kpc into the halo. More polarized emission is observed on the approaching side of the galaxy, indicating that Faraday depolarization has to be considered at C-band. With a simplified galaxy disk model, we are able to explain the finding of higher polarized intensity on the approaching side. We generalize the model to predict that roughly 75% of edge-on spiral galaxies will show higher polarized intensity on the approaching side. Many loop and shell structures are found throughout the galaxy in total intensity at C-band. One structure, a symmetric off-center (to southwest of the disk) superbubble-like structure is prominent in total and polarized intensity, as well as in Hα and optical dust filaments. This is at a location where a second peak of total intensity (to the southwest of the disk) is observed, making this superbubble-like structure a possible result of a concentrated star formation region in the disk. The X-ray diffuse emission shows similarities to the polarized diffuse emission of NGC 4217. The flux density extension of the radio continuum halo increases toward lower frequencies. While the total flux density of the disk and halo are comparable at C-band, the contribution of the disk flux density decreases toward LOFAR to 18% of the total flux density. Dumbbell-shaped structures are present at C-band and at the LOFAR frequency. Total intensity profiles at the two CHANG-ES bands and the LOFAR frequency show a clear two-component behavior and were fit best with a two-component exponential fit. The halo scale heights are 1.10 ± 0.04 kpc, 1.43 ± 0.09 kpc, and 1.55 ± 0.04 kpc in C-band, L-band, and 150 MHz, respectively. The frequency dependence of these scale heights between C-band and L-band suggests advection to be the main transport process. The 1D CRE transport modeling shows that advection appears to be more important than diffusion.

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Section: General Explanations and A Simplified Modelmentioning

confidence: 67%

Chang-Es

Stein

Dettmar

Beck

et al. 2020

A&A

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“…However, if that was the case, we should also see a significant variation around l ≈ 50 • , toward the tangent of the Sagittarius arm. Such an effect was seen in the strong excess of Faraday rotation toward that position (Shanahan et al 2019), but this is not seen in the orientation of the HI filament, as illustrated in Fig. 7.…”

Section: Regions Of Interest (Roismentioning

confidence: 92%

The history of dynamics and stellar feedback revealed by the H I filamentary structure in the disk of the Milky Way

Soler

Beuther

Syed

et al. 2020

A&A

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We present a study of the filamentary structure in the emission from the neutral atomic hydrogen (HI) at 21 cm across velocity channels in the 40′′ and 1.5-km s−1 resolution position-position-velocity cube, resulting from the combination of the single-dish and interferometric observations in The HI/OH/recombination-line survey of the inner Milky Way. Using the Hessian matrix method in combination with tools from circular statistics, we find that the majority of the filamentary structures in the HI emission are aligned with the Galactic plane. Part of this trend can be assigned to long filamentary structures that are coherent across several velocity channels. However, we also find ranges of Galactic longitude and radial velocity where the HI filamentary structures are preferentially oriented perpendicular to the Galactic plane. These are located (i) around the tangent point of the Scutum spiral arm and the terminal velocities of the Molecular Ring, around l ≈ 28° and vLSR ≈ 100 km s−1, (ii) toward l ≈ 45° and vLSR ≈ 50 km s−1, (iii) around the Riegel-Crutcher cloud, and (iv) toward the positive and negative terminal velocities. A comparison with numerical simulations indicates that the prevalence of horizontal filamentary structures is most likely the result of large-scale Galactic dynamics and that vertical structures identified in (i) and (ii) may arise from the combined effect of supernova (SN) feedback and strong magnetic fields. The vertical filamentary structures in (iv) can be related to the presence of clouds from extra-planar HI gas falling back into the Galactic plane after being expelled by SNe. Our results indicate that a systematic characterization of the emission morphology toward the Galactic plane provides an unexplored link between the observations and the dynamical behavior of the interstellar medium, from the effect of large-scale Galactic dynamics to the Galactic fountains driven by SNe.

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“…However, observations of synchrotron radiation and far-infrared dust emission (Beuermann et al 1985;Beuther et al 2012), very long baseline interferometry (VLBI) parallaxes of maser sources associated with high-mass star-forming regions (Reid et al 2019), A&A proofs: manuscript no. paper and Faraday rotation measurements (Shanahan et al 2019) open a window to infer the spiral structure of our home galaxy. In detail, the observations of diffuse synchrotron emission in wellresolved face-on galaxies show evidence for amplified ordered magnetic fields associated with tracers of spiral arm shocks (e.g., Jansson & Farrar 2012).…”

Section: Introductionmentioning

confidence: 99%

“…Recently, Shanahan et al (2019) presented a pattern of Faraday rotation along the Galactic plane based on data provided by the THOR survey (see Beuther et al 2016;Wang et al 2020, for details). Here Shanahan et al (2019) suggests that the Sagittarius arm is a much stronger Faraday screen within the Milky Way than previously thought.…”

Section: Introductionmentioning

confidence: 99%

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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Strong Excess Faraday Rotation on the Inside of the Sagittarius Spiral Arm

Cited by 37 publications

References 69 publications

Chang-Es

Chang-Es

The history of dynamics and stellar feedback revealed by the H I filamentary structure in the disk of the Milky Way

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

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Strong Excess Faraday Rotation on the Inside of the Sagittarius Spiral Arm

Cited by 37 publications

References 69 publications

Chang-Es

Chang-Es

The history of dynamics and stellar feedback revealed by the H I filamentary structure in the disk of the Milky Way

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

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Product

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The history of dynamics and stellar feedback revealed by the H I filamentary structure in the disk of the Milky Way