The Galactic distribution of magnetic fields in molecular clouds and HII regions

Han, J. L.; Zhang, J. S.

doi:10.1051/0004-6361:20065801

Cited by 58 publications

(56 citation statements)

References 77 publications

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“…Wright et al 2004) and the ensemble of field measurements across several hundred star forming regions have the potential to delineate the magnetic field pattern in the spiral arms of our Galaxy (e.g. Davies 1974;Reid & Silverstein 1990;Han & Zhang 2007;.…”

Section: Introductionmentioning

confidence: 99%

Excited-state hydroxyl maser polarimetry: who ate all the πs?

Green¹,

Caswell²,

McClure-Griffiths³

2015

Monthly Notices of the Royal Astronomical Society

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We present polarimetric maser observations with the Australia Telescope Compact Array (ATCA) of excited-state hydroxyl (OH) masers. We observed 30 fields of OH masers in full Stokes polarization with the Compact Array Broadband Backend (CABB) at both the 6030 and 6035 MHz excited-state OH transitions, and the 6668-MHz methanol maser transition, detecting 70 sites of maser emission. Amongst the OH we found 112 Zeeman pairs, of which 18 exhibited candidate π components. This is the largest single full polarimetric study of multiple sites of star formation for these frequencies, and the rate of 16% π components clearly indicates the π component exists, and is comparable to the percentage recently found for ground-state transitions. This significant percentage of π components, with consistent proportions at both ground-and excited-state transitions, argues against Faraday rotation suppressing the π component emission. Our simultaneous observations of methanol found the expected low level of polarisation, with no circular detected, and linear only found at the 10% level for the brightest sources.

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Section: Introductionmentioning

confidence: 99%

Excited-state hydroxyl maser polarimetry: who ate all the πs?

Green¹,

Caswell²,

McClure-Griffiths³

2015

Monthly Notices of the Royal Astronomical Society

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“…The Zeeman splitting can be used to measure the fields (both strength and directioon) in clouds or clumps of a scale of pc or even AU. Surprisingly, the distribution of available magnetic field measurements from maser lines around a large number of HII regions is very coherent with the large-scale spiral structure and large-scale magnetic fields (Han & Zhang 2007). This is strong evidence that magnetic fields have preserved their directions from the kpc scale to AU scale during cloud formation and star formation processes.…”

Section: Magnetic Fields In the Galactic Diskmentioning

confidence: 81%

Magnetic fields in our Milky Way Galaxy and nearby galaxies

Han

2012

Proc. IAU

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Abstract. Magnetic fields in our Galaxy and nearby galaxies have been revealed by starlight polarization, polarized emission from dust grains and clouds at millimeter and submillimeter wavelength, the Zeeman effect of spectral lines or maser lines from clouds or clumps, diffuse radio synchrotron emission from relativistic electrons in interstellar magnetic fields, and the Faraday rotation of background radio sources as well as pulsars for our Milky Way. It is easy to get a global structure for magnetic fields in nearby galaxies, while we have observed many details of magnetic fields in our Milky Way, especially by using pulsar rotation measure data. In general, magnetic fields in spiral galaxies probably have a large-scale structure. The fields follow the spiral arms with or without the field direction reversals. In the halo of spiral galaxies magnetic fields exist and probably also have a large-scale structure as toroidal and poloidal fields, but seem to be slightly weaker than those in the disk. In the central region of some galaxies, poloidal fields have been detected as vertical components. Magnetic field directions in galaxies seem to have been preserved during cloud formation and star formation, from large-scale diffuse interstellar medium to molecular clouds and then to the cloud cores in star formation regions or clumps for the maser spots. Magnetic fields in galaxies are passive to dynamics.

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“…In the central region of our galaxy, the field strength of the gas can be as high as ∼ mGauss (Han & Zhang 2007). A weaker magnetic field requires the gas with a lower angular momentum at the Bondi radius for RL AGNs, because the circularization radius decreases with decreasing angular momentum of the gas, which increases the amplification of the external magnetic field in the region between the Bondi radius and the circularization radius.…”

Section: Discussionmentioning

confidence: 99%

On the Radio Dichotomy of Active Galactic Nuclei

Cao

2016

ApJ

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It is still a mystery why only a small fraction of active galactic nuclei (AGNs) contain relativistic jets. A strong magnetic field is a necessary ingredient for jet formation, however, the advection of the external field in a geometrically thin disk is inefficient. Gas with a small angular velocity may fall from the Bondi radius R B nearly freely to the circularization radius R c , and a thin accretion disk is formed within R c . We suggest that the external magnetic field is substantially enhanced in this region, and the magnetic field at R c can be sufficiently strong to drive outflows from the disk if the angular velocity of the gas is low at R B . The magnetic field is efficiently dragged in the disk, because most angular momentum of the disk is removed by the outflows that lead to a significantly high radial velocity. The strong magnetic field formed in this way may accelerate jets in the region near the black hole, either by the Blandford-Payne or/and Blandford-Znajek mechanisms. We suggest that the radio dichotomy of AGNs predominantly originates from the angular velocity of the circumnuclear gas. An AGN will appear as a radio-loud (RL) one if the angular velocity of the circumnuclear gas is lower than a critical value at the Bondi radius, otherwise, it will appear as a radio-quiet (RQ) AGN. This is supported by the observations that RL nuclei are invariably hosted by core galaxies. Our model suggests that the mass growth of the black holes in RL quasars is much faster than that in RQ quasars with the same luminosity, which is consistent with the fact that the massive black holes in RL quasars are systematically a few times heavier than those in their RQ counterparts.

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The Galactic distribution of magnetic fields in molecular clouds and HII regions

Cited by 58 publications

References 77 publications

Excited-state hydroxyl maser polarimetry: who ate all the πs?

Excited-state hydroxyl maser polarimetry: who ate all the πs?

Magnetic fields in our Milky Way Galaxy and nearby galaxies

On the Radio Dichotomy of Active Galactic Nuclei

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