Evolution of magnetic fields in galaxies and future observational tests with the Square Kilometre Array

Arshakian, T. G.; Beck, Rainer; Krause, M.; Sokoloff, Dmitry

doi:10.1051/0004-6361:200810964

Cited by 191 publications

(264 citation statements)

References 103 publications

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“…The small-scale field is probably generated by local injections of kinetic energy (from supernova explosions), which produce currents that generate local amplification of the field on the relevant scales. In dynamo theory, the coexistence of the large-and small-scale fields is a natural consequence of the feeding of the large-scale field from the small-scale field (see, e.g., Arshakian et al 2009). This phenomenon ("inverse cascade") is seen in direct numerical simulations, e.g., by Brandenburg (2001), where a robust and well-defined average field (which was not imposed) coexists with a small-scale field that has a dispersion ∼3 times larger than this average field.…”

Section: Summary and Discussionmentioning

confidence: 99%

The large-scale magnetic field in the fourth Galactic quadrant

Nota¹,

Katgert²

2010

A&A

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Aims. We have re-examined the published rotation measures (RMs) of extragalactic point sources and pulsars with |b| < 3 • to study the magnetic field in the fourth Galactic quadrant. Methods. We reduced the influence of structure in electron density as much as possible by excluding objects for which Hα-data indicate large fluctuations in n e somewhere along the line of sight. We also excluded objects for which the RM may have been significantly "corrupted" by an intervening supernova remnant. We modeled RM(l), the longitude dependence of RM of the unaffected extragalactic sources and pulsars. We assumed several geometries for the large-scale field. All but one of those are based on logarithmic spiral arms (with various pitch angles and widths), while one has circular symmetry. We also made different assumptions about the large-scale n e -distribution. Results. The data suggest the following generic behaviour of the large-scale field in the 4th Galactic quadrant. The field is most likely organized along logarithmic spiral arms and shows two significant reversals: from the Norma arm (CCW field) to the Norma-Crux interarm region (CW field), and from the Norma-Crux interarm region to the Crux arm (CCW field). The present data do not constrain the field in and beyond the Crux-Carina interarm region. Although the models give a good description of the global character of RM(l), individual RM-estimates deviate by typically 15 times their measurement errors. We argue that these large deviations are most likely due to the "small-scale" field that dominates on scales of up to several hundred pc. Conclusions. The picture that emerges is thus of a field that has significant structure on smaller scales, but for which the average values in arms and interarm regions are nevertheless well-defined. In addition, this smaller-amplitude large-scale field appears to reverse at each arm-interarm boundary that we can study with the present data. We briefly discuss the link between these results and theoretical predictions.

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Section: Summary and Discussionmentioning

confidence: 99%

The large-scale magnetic field in the fourth Galactic quadrant

Nota¹,

Katgert²

2010

A&A

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“…This strong dependence can be intuitively understood because the local SFR determines the population of supernova explosions, which constitute the main source of the turbulent energy, which is in turn vital to the dynamo process (Arshakian et al 2009, see also Sect. 5.4).…”

Section: Main Factors Regulating Magnetic Fieldmentioning

confidence: 99%

Magnetic fields in Local Group dwarf irregulars

Chyży

Weżgowiec

Beck

et al. 2011

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Aims. We wish to clarify whether strong magnetic fields can be effectively generated in typically low-mass dwarf galaxies and to assess the role of dwarf galaxies in the magnetization of the Universe. Methods. We performed a search for radio emission and magnetic fields in an unbiased sample of 12 Local Group (LG) irregular and dwarf irregular galaxies with the 100-m Effelsberg telescope at 2.64 GHz. Three galaxies were detected. A higher frequency (4.85 GHz) was used to search for polarized emission in five dwarfs that are the most luminous ones in the infrared domain, of which three were detected. Results. Magnetic fields in LG dwarfs are weak, with a mean value of the total field strength of <4.2 ± 1.8 μG, three times lower than in the normal spirals. The strongest field among all LG dwarfs of 10 μG (at 2.64 GHz) is observed in the starburst dwarf IC 10. The production of total magnetic fields in dwarf systems appears to be regulated mainly by the star-formation surface density (with the power-law exponent of 0.30 ± 0.04) or by the gas surface density (with the exponent 0.47 ± 0.09). In addition, we find systematically stronger fields in objects of higher global star-formation rate. The dwarf galaxies follow a similar far-infrared relationship (with a slope of 0.91 ± 0.08) to that determined for high surface brightness spiral galaxies. The magnetic field strength in dwarf galaxies does not correlate with their maximum rotational velocity, indicating that a small-scale rather than a large-scale dynamo process is responsible for producting magnetic fields in dwarfs. If magnetization of the Universe by galactic outflows is coeval with its metal enrichment, we show that more massive objects (such as Lyman break galaxies) can efficiently magnetize the intergalactic medium with a magnetic field strength of about 0.8 nG out to a distance of 160-530 kpc at redshifts 5-3, respectively. Magnetic fields that are several times weaker and shorter magnetization distances are expected for primordial dwarf galaxies. We also predict that most star-forming local dwarfs might have magnetized their surroundings up to a field strength about 0.1 μG within about a 5 kpc distance. Conclusions. Strong magnetic fields (>6 μG) are observed only in dwarfs of extreme characteristics (e.g. NGC 4449, NGC 1569, and the LG dwarf IC 10). They are all starbursts and more evolved objects of statistically much higher metallicity and global star-formation rate than the majority of the LG dwarf population. Typical LG dwarfs are unsuitable objects for the efficient supply of magnetic fields to the intergalactic medium.

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“…A better comparison with the predictions of dynamo theory (see e.g. Arshakian et al 2009) will thus become possible.…”

Section: High-redshift Rotation Measuresmentioning

confidence: 95%