Magnetic spiral arms and galactic outflows

Chamandy, Luke; Shukurov, Anvar; Subramanian, Kandaswamy

doi:10.1093/mnrasl/slu156

Cited by 39 publications

(40 citation statements)

References 36 publications

(37 reference statements)

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“…This leaves a certain range of outflow velocities for optimal dynamo action . As outflows are strongest above spiral arms, the α − Ω dynamo is expected to be more efficient between the spiral arms (Chamandy et al 2015).…”

Section: ∂B/∂t = ∇ × (V × B) + ∇ × αBmentioning

confidence: 99%

Magnetic fields in spiral galaxies

Beck¹

2015

Astron Astrophys Rev

337

260

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Radio synchrotron emission, its polarization and Faraday rotation of the polarization angle are powerful tools to study the strength and structure of magnetic fields in galaxies. Unpolarized synchrotron emission traces isotropic turbulent fields which are strongest in spiral arms and bars (20-30 μG) and in central starburst regions (50-100 μG). Such fields are dynamically important; they affect gas flows and drive gas inflows in central regions. Polarized emission traces ordered fields, which can be regular or anisotropic turbulent, where the latter originates from isotropic turbulent fields by the action of compression or shear. The strongest ordered fields (10-15 μG) are generally found in interarm regions. In galaxies with strong density waves, ordered fields are also observed at the inner edges of spiral arms. Ordered fields with spiral patterns exist in grand-design, barred and flocculent galaxies and in central regions. Ordered fields in interacting galaxies have asymmetric distributions and are a tracer of past interactions between galaxies or with the intergalactic medium.-Faraday rotation measures of the diffuse polarized radio emission from galaxy disks reveal large-scale spiral patterns that can be described by the superposition of azimuthal modes; these are signatures of regular fields generated by mean-field dynamos. "Magnetic arms" between gaseous spiral arms may also be products of dynamo action, but need a stable spiral pattern to develop. Helically twisted field loops winding around spiral arms were found in two galaxies so far. Large-scale field reversals, like the one found in the Milky Way, could not yet be detected in external galaxies. In radio halos around edgeon galaxies, ordered magnetic fields with X-shaped patterns are observed. The origin and evolution of cosmic magnetic fields, in particular their first occurrence in young

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Section: ∂B/∂t = ∇ × (V × B) + ∇ × αBmentioning

confidence: 99%

Magnetic fields in spiral galaxies

Beck¹

2015

Astron Astrophys Rev

337

260

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“…A different approach to the problem of displaced magnetic and gaseous spiral patterns emerges if the material pattern is not a solidly rotating structure, as suggested by the density wave theory, but rather a transient and evolving system of spiral arm segments produced by a bar, galactic encounters, local instabilities, etc., and wound up by the galactic differential rotation. As shown by Chamandy et al (2013Chamandy et al ( , 2014Chamandy et al ( , 2015, the mechanisms mentioned above are relieved of their problems in this case, producing diverse interlaced or intersecting magnetic and gaseous spiral patterns depending on the relative contribution of each mechanism. 5.…”

Section: Magnetic Armsmentioning

confidence: 94%

“…The mean-field dynamo action can be suppressed within the gaseous arms by either a presumably enhanced fluctuation dynamo driven by stronger star formation (Moss et al 2013) or a stronger galactic outflow driven by stronger star formation (Sur et al 2007;Chamandy et al 2015; see also Shukurov 1998). A generic problem of such mechanisms is that they produce the desired displacement only within a few kiloparsecs of the corotation radius of the spiral pattern (Shukurov 1998;Chamandy et al 2013Chamandy et al , 2014, because the residence time of a volume element within a gaseous spiral arm is shorter than the dynamo time scale of order 5 × 10 8 yr at large distances from the corotation.…”

Section: Magnetic Armsmentioning

confidence: 99%

Magnetic and gaseous spiral arms in M83

Frick

Stepanov

Beck

et al. 2015

A&A

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Context. The magnetic field configurations in several nearby spiral galaxies contain magnetic arms that are sometimes located between the material arms. The nearby barred galaxy M83 provides an outstanding example of a spiral pattern seen in tracers of gas and magnetic field. Aims. We analyse the spatial distribution of magnetic fields in M83 and their relation to the material spiral arms. Methods. Isotropic and anisotropic wavelet transforms are used to decompose the images of M83 in various tracers to quantify structures in a range of scales from 0.2 to 10 kpc. We used radio polarization observations at λ6.2 cm and λ13 cm obtained with the VLA, Effelsberg and ATCA telescopes and APEX sub-mm observations at 870 μm, which are first published here, together with maps of the emission of warm dust, ionized gas, molecular gas, and atomic gas. Results. The spatial power spectra are similar for the tracers of dust, gas, and total magnetic field, while the spectra of the ordered magnetic field are significantly different. As a consequence, the wavelet cross-correlation between all material tracers and total magnetic field is high, while the structures of the ordered magnetic field are poorly correlated with those of other tracers. The magnetic field configuration in M83 contains pronounced magnetic arms. Some of them are displaced from the corresponding material arms, while others overlap with the material arms. The pitch angles of the magnetic and material spiral structures are generally similar. The magnetic field vectors at λ6.2 cm are aligned with the outer material arms, while significant deviations occur in the inner arms and, in particular, in the bar region, possibly due to non-axisymmetric gas flows. Outside the bar region, the typical pitch angles of the material and magnetic spiral arms are very close to each other at about 10• . The typical pitch angle of the magnetic field vectors is about 20• larger than that of the material spiral arms. Conclusions. One of the main magnetic arms in M83 is displaced from the gaseous arms similarly to the galaxy NGC 6946, while the other main arm overlaps a gaseous arm, similar to what is observed in M51. We propose that a regular spiral magnetic field generated by a mean-field dynamo is compressed in material arms and partly aligned with them. The interaction of galactic dynamo action with a transient spiral pattern is a promising mechanism for producing such complicated spiral patterns as in M83.

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“…However, fast outflows can advect large-scale magnetic fields and hence suppress mean-field dynamo action. As the outflow is expected to be stronger above and below the spiral arms, large-scale fields are weaker than in the inter-arm regions which may explain the observation of magnetic arms between material arms (Chamandy et al 2015).…”

Section: Introductionmentioning

confidence: 99%

Resolved magnetic structures in the disk-halo interface of NGC 628

Mulcahy

Beck

Heald

2017

A&A

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Context. Magnetic fields are essential to fully understand the interstellar medium and its role in the disk-halo interface of galaxies is still poorly understood. Star formation is known to expel hot gas vertically into the halo and these outflows have important consequences for mean-field dynamo theory in that they can be efficient in removing magnetic helicity. Aims. We aim to probe the vertical magnetic field and enhance our understanding of the disk-halo interaction of galaxies. Studying a face-on galaxy is essential so that the magnetic field components can be separated in 3D. Methods. We perform new observations of the nearby face-on spiral galaxy NGC 628 with the Karl G. Jansky Very Large Array (JVLA) at S -band (2.6-3.6 GHz effective bandwidth) and the Effelsberg 100-m telescope at frequencies of 2.6 GHz and 8.35 GHz with a bandwidth of 80 MHz and 1.1 GHz, respectively. Owing to the large bandwidth of the JVLA receiving system, we obtain some of the most sensitive radio continuum images in both total and linearly polarised intensity of any external galaxy observed so far. Results. The application of rotation measures synthesis to the interferometric polarisation data over this large bandwidth provides high-quality images of Faraday depth and polarisation angle from which we obtained evidence for drivers of magnetic turbulence in the disk-halo connection. Such drivers include a superbubble detected via a significant Faraday depth gradient coinciding with a H I hole. We observe an azimuthal periodic pattern in Faraday depth with a pattern wavelength of 3.7 ± 0.1 kpc, indicating Parker instabilities. The lack of a significant anti-correlation between Faraday depth and magnetic pitch angle indicates that these loops are vertical in nature with little helical twisting, unlike in IC 342. We find that the magnetic pitch angle is systematically larger than the morphological pitch angle of the polarisation arms which gives evidence for the action of a large-scale dynamo where the regular magnetic field is not coupled to the gas flow and obtains a significant radial component. We additionally discover a lone region of ordered magnetic field to the north of the galaxy with a high degree of polarisation and a small pitch angle, a feature that has not been observed in any other galaxy so far and is possibly caused by an asymmetric H I hole. Conclusions. Until now NGC 628 has been relatively unexplored in radio continuum but with its extended H I disk and lack of active star formation in its central region has produced a wealth of interesting magnetic phenomena. We observe evidence for two drivers of magnetic turbulence in the disk-halo connection of NGC 628, namely, Parker instabilities and superbubbles.

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Magnetic spiral arms and galactic outflows

Cited by 39 publications

References 36 publications

Magnetic fields in spiral galaxies

Magnetic fields in spiral galaxies

Magnetic and gaseous spiral arms in M83

Resolved magnetic structures in the disk-halo interface of NGC 628

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