Galactic Dynamos

Brandenburg, Axel; Ntormousi, Evangelia

doi:10.48550/arxiv.2211.03476

Cited by 3 publications

(3 citation statements)

References 178 publications

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“…The magnetic fields in spiral galaxies are observed to generally follow spiral patterns resembling the morphological spiral arms (Beck et al 2019;Lopez-Rodriguez et al 2020). This spatial correlation is in agreement with the current paradigm in which weak seed magnetic fields (Rees 1987;Subramanian 2016) are amplified by dynamo mechanisms such as small-scale (<50-100 pc) turbulence (Beck 1996;Haverkorn et al 2008;Bhat et al 2016;Brandenburg & Ntormousi 2022;Martin-Alvarez et al 2022), and then are reorganized through differential galactic disk rotation and helical turbulence (large-scale dynamos; Gressel et al 2008aGressel et al , 2008bGent 2012;Bendre et al 2015;Ntormousi et al 2020).…”

Section: Discussionsupporting

confidence: 83%

Extragalactic Magnetism with SOFIA (SALSA Legacy Program). V. First Results on the Magnetic Field Orientation of Galaxies

Borlaff

López-Rodríguez

Beck

et al. 2023

ApJ

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We present the analysis of the magnetic field (B-field) structure of galaxies measured with far-infrared (FIR) and radio (3 and 6 cm) polarimetric observations. We use the first data release of the Survey of extragALactic magnetiSm with SOFIA of 14 nearby ( < 20 Mpc) galaxies with resolved ( 5 ″ – 18 ″ ; 90 pc–1 kpc) imaging polarimetric observations using SOFIA/HAWC+ from 53 to 214 μm. We compute the magnetic pitch-angle ( Ψ B ) profiles as a function of the galactocentric radius. We introduce a new magnetic alignment parameter (ζ) to estimate the disordered-to-ordered ratio of spiral B-fields. We find FIR and radio wavelengths to not generally trace the same B-field morphology in galaxies. The Ψ B profiles tend to be more ordered across all galactocentric radii in radio ( ζ 6 cm = 0.93 ± 0.03 ) than in FIR ( ζ 154 μ m = 0.84 ± 0.14 ). For spiral galaxies, FIR B-fields are 2%–75% more turbulent than the radio B-fields. For starburst galaxies, we find that FIR polarization is a better tracer of the B-fields along the galactic outflows than radio polarization. Our results suggest that the B-fields associated with dense, dusty, turbulent star-forming regions (those traced at FIR) are less ordered than warmer, less dense regions (those traced at radio) of the interstellar medium. The FIR B-fields seem to be more sensitive to the activity of the star-forming regions and molecular clouds within a vertical height of a few hundred parsecs in the disk of spiral galaxies than the radio B-fields.

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Section: Discussionsupporting

confidence: 83%

Extragalactic Magnetism with SOFIA (SALSA Legacy Program). V. First Results on the Magnetic Field Orientation of Galaxies

Borlaff

López-Rodríguez

Beck

et al. 2023

ApJ

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“…It is difficult to pinpoint what physical effect terminates the growth of B reg , but radial spreading of the ring, nonlinear quenching due to small-scale helicity, and turbulent diffusion may all play a role (e.g. Brandenburg & Ntormousi 2022).…”

Section: Discussionmentioning

confidence: 99%

Effects of Magnetic Fields on Gas Dynamics and Star Formation in Nuclear Rings

Moon¹,

Kim²,

Kim³

et al. 2023

Preprint

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Nuclear rings at the centers of barred galaxies are known to be strongly magnetized. To explore the effects of magnetic fields on star formation in these rings and nuclear gas flows, we run magnetohydrodynamic simulations in which there is a temporally-constant magnetized inflow to the ring, representing a bar-driven inflow. The mass inflow rate is 1 M yr −1 , and we explore models with a range of field strength in the inflow. We adopt the TIGRESS framework developed by Kim & Ostriker to handle radiative heating and cooling, star formation, and resulting supernova (SN) feedback. We find that magnetic fields are efficiently amplified in the ring due to rotational shear and SN feedback. Within a few 100 Myr, the turbulent component B trb in the ring saturates at ∼ 35 µG (in rough equipartition with the turbulent kinetic energy density), while the regular component B reg exceeds 50 µG. Expanding superbubbles created by clustered SN explosions vertically drag predominantly-toroidal fields from near the midplane to produce poloidal fields in high-altitude regions. The growth of magnetic fields greatly suppresses star formation at late times. Simultaneously, strong magnetic tension in the ring drives radially inward accretion flows from the ring to form a circumnuclear disk in the central region; this feature is absent in the unmagnetized model.

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“…Therefore, they were either seeded by a primordial field in the early Universe and then amplified through a dynamo, or directly generated by an astrophysical process in galaxies (e.g. Brandenburg & Ntormousi 2022).…”

Section: Introductionmentioning

confidence: 99%

A closer look at supernovae as seeds for galactic magnetization

Ntormousi

Sordo²,

Cantiello

et al. 2022

A&A

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Context. Explaining the currently observed magnetic fields in galaxies requires relatively strong seeding in the early Universe. One of the current theories proposes that magnetic seeds of the order of µG were expelled by supernova (SN) explosions after primordial, nG or weaker fields were amplified in stellar interiors. Aims. In this work, we take a closer look at this theory and calculate the maximum magnetic energy that can be injected in the interstellar medium by a stellar cluster of mass M cl based on what is currently known about stellar magnetism. Methods. We consider early-type stars and adopt either a Salpeter or a top-heavy IMF. For their magnetic fields, we adopt either a Gaussian or a bimodal distribution. The Gaussian model assumes that all massive stars are magnetized with 10 3 < B * < 10 4 G, while the bimodal, consistent with observations of Milky Way stars, assumes only 5 − 10 per cent of OB stars have 10 3 < B * < 10 4 G, while the rest have 10 < B * < 10 2 G. We ignore the effect of magnetic diffusion and assume no losses of magnetic energy. Results. We find that the maximum magnetic energy that can be injected by a stellar population is between 10 −10 −10 −7 times the total SN energy. The highest end of these estimates is about five orders of magnitude lower than what is usually employed in cosmological simulations, where about 10 −2 of the SN energy is injected as magnetic. Conclusions. Pure advection of the stellar magnetic field by SN explosions is a good candidate for seeding a dynamo, but not enough to magnetize galaxies. Assuming SNe as main mechanism for galactic magnetization, the magnetic field cannot exceed an intensity of 10 −7 G in the best-case scenario for a population of 10 5 solar masses in a superbubble of 300 pc radius, while more typical values are between 10 −10 − 10 −9 G. Therefore, other scenarios for galactic magnetization at high redshift need to be explored.

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Galactic Dynamos

Cited by 3 publications

References 178 publications

Extragalactic Magnetism with SOFIA (SALSA Legacy Program). V. First Results on the Magnetic Field Orientation of Galaxies

Extragalactic Magnetism with SOFIA (SALSA Legacy Program). V. First Results on the Magnetic Field Orientation of Galaxies

Effects of Magnetic Fields on Gas Dynamics and Star Formation in Nuclear Rings

A closer look at supernovae as seeds for galactic magnetization

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