String Cosmology 2009

DOI: 10.1002/9783527628063.ch8

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Heterotic M‐Theory and Cosmology

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Magnetic Field Structure2

Summary and Discussion1

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Cited by 34 publications

(64 citation statements)

References 110 publications

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“…It has a prominent X-shaped distribution in polarized intensity and a magnetic field orientation centered on the nucleus. Such magnetic field structures have been observed in sev- eral nearby edge-on galaxies (Dahlem et al 1997;Tüllman et al 2000;Krause et al 2006;Krause 2007Krause , 2009). But this is the first example for a galaxy which is only mildly edgeon (i = 78.5 • ) and is dominated by its disk magnetic field.…”

Section: Magnetic Field Structurementioning

confidence: 92%

Transport of cosmic rays in the nearby starburst galaxy NGC 253

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et al. 2009

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Radio halos require the coexistence of extra-planar cosmic rays and magnetic fields. Because cosmic rays are injected and accelerated by processes related to star formation in the disk, they have to be transported from the disk into the halo. A vertical large-scale magnetic field can significantly enhance this transport. We observed NGC 253 using radio continuum polarimetry with the Effelsberg and VLA telescopes. The radio halo of NGC 253 has a dumbbell shape with the smallest vertical extension near the center. With an estimate for the electron lifetime, we measured the cosmic-ray bulk speed as 300 ± 30 km s −1 which is constant over the extent of the disk. This shows the presence of a "disk wind" in NGC 253. We propose that the large-scale magnetic field is the superposition of a disk (r, φ) and halo (r, z) component. The disk field is an inward-pointing spiral with even parity. The conical (even) halo field appears in projection as an X-shaped structure, as observed in other edge-on galaxies. Interaction by compression in the walls of the superbubbles may explain the observed alignment between the halo field and the lobes of hot Hα and soft X-ray emitting gas. The disk wind is a good candidate for the transport of small-scale helical fields, required for efficient dynamo action, and as a source for the neutral hydrogen observed in the halo.

“…It has a prominent X-shaped distribution in polarized intensity and a magnetic field orientation centered on the nucleus. Such magnetic field structures have been observed in sev- eral nearby edge-on galaxies (Dahlem et al 1997;Tüllman et al 2000;Krause et al 2006;Krause 2007Krause , 2009). But this is the first example for a galaxy which is only mildly edgeon (i = 78.5 • ) and is dominated by its disk magnetic field.…”

Section: Magnetic Field Structurementioning

confidence: 92%

Transport of cosmic rays in the nearby starburst galaxy NGC 253

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,

²

,

³

et al. 2009

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Radio halos require the coexistence of extra-planar cosmic rays and magnetic fields. Because cosmic rays are injected and accelerated by processes related to star formation in the disk, they have to be transported from the disk into the halo. A vertical large-scale magnetic field can significantly enhance this transport. We observed NGC 253 using radio continuum polarimetry with the Effelsberg and VLA telescopes. The radio halo of NGC 253 has a dumbbell shape with the smallest vertical extension near the center. With an estimate for the electron lifetime, we measured the cosmic-ray bulk speed as 300 ± 30 km s −1 which is constant over the extent of the disk. This shows the presence of a "disk wind" in NGC 253. We propose that the large-scale magnetic field is the superposition of a disk (r, φ) and halo (r, z) component. The disk field is an inward-pointing spiral with even parity. The conical (even) halo field appears in projection as an X-shaped structure, as observed in other edge-on galaxies. Interaction by compression in the walls of the superbubbles may explain the observed alignment between the halo field and the lobes of hot Hα and soft X-ray emitting gas. The disk wind is a good candidate for the transport of small-scale helical fields, required for efficient dynamo action, and as a source for the neutral hydrogen observed in the halo.

“…If the halo emission is as large as that of the disk, the observed spectrum may suffer substantial wavelength-independent depolarization, since the perpendicular components of the halo and disk fields would in general not be aligned with each other. However, observations showed that the distribution of radio emission from halos of edge-on spiral galaxies can be described by an exponential function, for instance, with the scale heights of about 1.8 kpc (Krause 2009). This suggests that the halo emission is small compared to that of the disk.…”

Section: Summary and Discussionmentioning

confidence: 99%

“…The strength of the vertical magnetic field is among many yet to be constrained in spiral galaxies (see, e.g., Beck 2016 for a summary). It has been observed in several edge-on galaxies showing the X-shaped pattern (see, e.g., Krause 2009;Beck 2015), but such observations have so far told us mostly only the existence and orientation of the field.…”

Section: Introductionmentioning

confidence: 99%

“…Gaensler et al (2008), for instance, suggested L SH ∼ 430-1830 pc for the thermal electron density in the thick disk of our Galaxy. Krause (2009) reported that the L SH of radio emission, which reflects the L SH of CR electrons and magnetic field, is ∼300 pc for thin disks and ∼1.8 kpc for halos (or thick disks) from observations of various edge-on spiral galaxies. So, we set L SH = 1 kpc as the fiducial value and also consider 0.5 and 2 kpc for comparison.…”

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confidence: 99%

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Study of the Vertical Magnetic Field in Face-on Galaxies Using Faraday Tomography

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Faraday tomography allows astronomers to probe the distribution of the magnetic field along the line of sight (LOS), but that can be achieved only after the Faraday spectrum is interpreted. However, the interpretation is not straightforward, mainly because the Faraday spectrum is complicated due to a turbulent magnetic field; it ruins the one-to-one relation between the Faraday depth and the physical depth, and appears as many small-scale features in the Faraday spectrum. In this paper, by employing "simple toy models" for the magnetic field, we describe numerically as well as analytically the characteristic properties of the Faraday spectrum. We show that the Faraday spectrum along "multiple LOSs" can be used to extract the global properties of the magnetic field. Specifically, considering face-on spiral galaxies and modeling turbulent magnetic field as a random field with a single coherence length, we numerically calculate the Faraday spectrum along a number of LOSs and its shape-characterizing parameters, that is, the moments. When multiple LOSs cover a region of (10 coherence length) 2 , the shape of the Faraday spectrum becomes smooth and the shape-characterizing parameters are well specified. With the Faraday spectrum constructed as a sum of Gaussian functions with different means and variances, we analytically show that the parameters are expressed in terms of the regular and turbulent components of the LOS magnetic field and the coherence length. We also consider the turbulent magnetic field modeled with a power-law spectrum, and study how the magnetic field is revealed in the Faraday spectrum. Our work suggests a way to obtain information on the magnetic field from a Faraday tomography study.

“…Highsensitivity observations of NGC 253 [38], NGC 891 (Fig. 5), NGC 4631 [45] and other galaxies revealed "X-shaped" fields in the halo. The field is probably transported from the disk into the halo by an outflow emerging from the disk.…”

Section: Magnetic Fields In Spiral Galaxiesmentioning

confidence: 99%

Towards a New Era of Observing Cosmic Magnetic Fields

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2010

Proceedings of ISKAF2010 Science Meeting — PoS(ISKAF2010)

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The origin of magnetic fields in the Universe is an open problem in astrophysics and fundamental physics. Polarization observations with the forthcoming large radio telescopes will open a new era in the observation of magnetic fields and should help to understand their origin. At low frequencies, LOFAR (30-240 MHz) will allow us to map the structure of weak magnetic fields in the outer regions and halos of galaxies, in galaxy clusters and in the Milky Way. Small Faraday rotation measures can also be best measured at low frequencies. Polarization at higher frequencies (1-10 GHz) as observed with the EVLA, MeerKAT and the SKA will trace magnetic fields in the disks and central regions of nearby galaxies in unprecedented detail. The SKA pulsar survey will find many new pulsars; their RMs will map the Milky Way's magnetic field with high precision. All-sky surveys of Faraday rotation measures towards a dense grid of polarized background sources with the SKA and its precursor telescope ASKAP (project POSSUM) are dedicated to measure magnetic fields in distant intervening galaxies, clusters and intergalactic filaments, and will be used to model the overall structure and strength of the magnetic field in the Milky Way. With the SKA, large-scale patterns of regular fields in galaxies can be recognized to about 100 Mpc distance, ordered fields in unresolved galaxies or cluster relics to redshifts of z 0.5, turbulent fields in starburst galaxies or cluster halos to z 3 and regular fields in intervening galaxies towards QSOs to z 5.

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