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1.
Summary Recent years have seen an amazing development in our knowledge of the magnetic fields in the universe. The last ten years were crucial in our realization of the importance of the magnetic fields in galaxies. While a lot of the earlier data on our Galaxy depended on optical observations, the bulk of the recent results depend on radio measurements. The radio Zeeman effect gave us new information on magnetic fields in molecular clouds. The mapping of galaxies at several radio frequencies resulted in new knowledge about the large-scale magnetic fields in these basic building blocks of the universe. These exciting observations have led to new theoretical developments. In particular, the dynamo theory of flat objects received much attention since the observed large-scale structures can best be explained through the action of the dynamo effect. This review will attempt to summarise the observational evidence and to give viable explanations for the magnetic fields in galaxies. 相似文献
2.
T.G. Arshakian R. Stepanov R. Beck M. Krause D. Sokoloff 《Astronomische Nachrichten》2011,332(5):524-536
Future radio observations with the Square Kilometre Array (SKA) and its precursors will be sensitive to trace spiral galaxies and their magnetic field configurations up to redshift z ≈ 3. We suggest an evolutionary model for the magnetic configuration in star‐forming disk galaxies and simulate the magnetic field distribution, the total and polarized synchrotron emission, and the Faraday rotation measures for disk galaxies at z ≲ 3. Since details of dynamo action in young galaxies are quite uncertain, we model the dynamo action heuristically relying only on well‐established ideas of the form and evolution of magnetic fields produced by the mean‐field dynamo in a thin disk. We assume a small‐scale seed field which is then amplified by the small‐scale turbulent dynamo up to energy equipartition with kinetic energy of turbulence. The large‐scale galactic dynamo starts from seed fields of 100 pc and an averaged regular field strength of 0.02 μG, which then evolves to a “spotty” magnetic field configuration in about 0.8 Gyr with scales of about one kpc and an averaged regular field strength of 0.6 μG. The evolution of these magnetic spots is simulated under the influence of star formation, dynamo action, stretching by differential rotation of the disk, and turbulent diffusion. The evolution of the regular magnetic field in a disk of a spiral galaxy, as well as the expected total intensity, linear polarization and Faraday rotation are simulated in the rest frame of a galaxy at 5GHz and 150 MHz and in the rest frame of the observer at 150 MHz. We present the corresponding maps for several epochs after disk formation. Dynamo theory predicts the generation of large‐scale coherent field patterns (“modes”). The timescale of this process is comparable to that of the galaxy age. Many galaxies are expected not to host fully coherent fields at the present epoch, especially those which suffered from major mergers or interactions with other galaxies. A comparison of our predictions with existing observations of spiral galaxies is given and discussed (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
3.
The role of magnetic fields in the dynamical evolution of galaxies and of the interstellar medium (ISM) is not well understood, mainly because such fields are difficult to directly observe. Radio astronomy provides the best tools to measure magnetic fields: synchrotron radiation traces fields illuminated by cosmic-ray electrons, while Faraday rotation and Zeeman splitting allow us to detect fields in all kinds of astronomical plasmas, from lowest to highest densities. Here, we describe how fundamental new advances in studying magnetic fields, both in our own Milky Way and in other nearby galaxies, can be made through observations with the proposed Square Kilometre Array. Underpinning much of what we propose is an all-sky survey of Faraday rotation, in which we will accumulate tens of millions of rotation measure measurements toward background radio sources. This will provide a unique database for studying magnetic fields in individual Galactic supernova remnants and Hii regions, for characterizing the overall magnetic geometry of our Galaxy’s disk and halo, and for understanding the structure and evolution of magnetic fields in galaxies. Also of considerable interest will be the mapping of diffuse polarized emission from the Milky Way in many narrow bands over a wide frequency range. This will allow us to carry out Faraday tomography of the Galaxy, yielding a high-resolution three-dimensional picture of the magnetic field within a few kpc of the Sun, and allowing us to understand its coupling to the other components of the ISM. Finally, direct synchrotron imaging of a large number of nearby galaxies, combined with Faraday rotation data, will allow us to determine the magnetic field structure in these sources, and to test both the dynamo and primordial field theories for field origin and amplification. 相似文献
4.
The ordered magnetic field observed via polarised synchrotron emission in nearby disc galaxies can be explained by a mean‐field dynamo operating in the diffuse interstellar medium (ISM). Additionally, vertical‐flux initial conditions are potentially able to influence this dynamo via the occurrence of the magnetorotational instability (MRI). We aim to study the influence of various initial field configurations on the saturated state of the mean‐field dynamo. This is motivated by the observation that different saturation behaviour was previously obtained for different supernova rates. We perform direct numerical simulations (DNS) of three‐dimensional local boxes of the vertically stratified, turbulent interstellar medium, employing shearing‐periodic boundary conditions horizontally. Unlike in our previous work, we also impose a vertical seed magnetic field. We run the simulations until the growth of the magnetic energy becomes negligible. We furthermore perform simulations of equivalent 1D dynamo models, with an algebraic quenching mechanism for the dynamo coefficients. We compare the saturation of the magnetic field in the DNS with the algebraic quenching of a mean‐field dynamo. The final magnetic field strength found in the direct simulation is in excellent agreement with a quenched α) dynamo. For supernova rates representative of the Milky Way, field losses via a Galactic wind are likely responsible for saturation. We conclude that the relative strength of the turbulent and regular magnetic fields in spiral galaxies may depend on the galaxy's star formation rate. We propose that a mean field approach with algebraic quenching may serve as a simple sub‐grid scale model for galaxy evolution simulations including a prescribed feedback from magnetic fields. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
5.
Andrew Fletcher Anvar Shukurov 《Monthly notices of the Royal Astronomical Society》2001,325(1):312-320
Hydrostatic equilibrium of the multiphase interstellar medium in the solar vicinity is reconsidered, with the regular and turbulent magnetic fields treated separately. The regular magnetic field strength required to support the gas is consistent with independent estimates, provided that energy equipartition is maintained between turbulence and random magnetic fields. Our results indicate that a mid-plane value of B 0 =4 μG for the regular magnetic field near the Sun leads to more attractive models than B 0 =2 μG . The vertical profiles of both the regular and random magnetic fields contain disc and halo components, the parameters of which we have determined. The layer at 1≲| z |≲4 kpc can be overpressured and an outflow at a speed of about 50 km s−1 may occur there, presumably associated with a Galactic fountain flow, if B 0 ≃2 μG .
We show that hydrostatic equilibrium in a warped disc must produce asymmetric density distributions in z , in rough agreement with H i observations in the outer Galaxy. This asymmetry may be a useful diagnostic of the details of the warping mechanism in the Milky Way and other galaxies. We find indications that gas and magnetic field pressures are different above and below the warped midplane in the outer Galaxy, and quantify the difference in terms of turbulent velocity and/or magnetic field strength. 相似文献
We show that hydrostatic equilibrium in a warped disc must produce asymmetric density distributions in z , in rough agreement with H i observations in the outer Galaxy. This asymmetry may be a useful diagnostic of the details of the warping mechanism in the Milky Way and other galaxies. We find indications that gas and magnetic field pressures are different above and below the warped midplane in the outer Galaxy, and quantify the difference in terms of turbulent velocity and/or magnetic field strength. 相似文献
6.
脉冲星的偏振信息是理解脉冲星辐射区的重要手段,利用澳大利亚的64m射电望远镜进行大量的脉冲星观测,得到了一批脉冲星的偏振轮廓和偏振参数,编辑了几乎所有发表的脉冲星轮廓资料,系统总结了脉冲星圆偏辐射的规律,为理论上解释脉冲星辐射这一重要难题提供观测依据和物理限制,利用脉冲星作为探针,研究了银河系磁场结构和模型,确定了银河系BS磁场模型,发现了银河系上下反对称的环向磁场,并首次对星系尺度的发电机类型进行判别,证认出A0型发电机运行于银河系,发现了银晕中的垂直磁场和M31及银盘中的非常延展的磁场,探测到NGC2997星系中由内到外的旋涡磁场,并提出可能有两种发电机在这个星系的不同区域运行。 相似文献
7.
韩金林 《中国天文和天体物理学报》1996,(4)
在分析河外射电源和脉冲星的法拉弟旋度(RotationMeasure,RM)时,发现银道面上下有规则的奇对称环向磁场分量,这是任何银盘磁场模型所无法解释的.这种奇对称的环向磁场说明在银冕中运行着AO模式的Dynamo。 相似文献
8.
Subhashis Roy A. Pramesh Rao Ravi Subrahmanyan 《Monthly notices of the Royal Astronomical Society》2005,360(4):1305-1324
We have observed a sample of 64 small-diameter sources towards the central −6° < l < 6°, −2° < b < 2° of the Galaxy with the aim of studying the Faraday rotation measure near the Galactic Centre region. All the sources were observed at 6- and 3.6-cm wavelengths using the ATCA and the VLA. 59 of these sources are inferred to be extragalactic. The observations presented here constitute the first systematic study of the radio polarization properties of the background sources towards this direction and increase the number of known extragalactic radio sources in this part of the sky by almost an order of magnitude. Based on the morphology, spectral indices and lack of polarized emission, we identify four Galactic H ii regions in the sample. 相似文献
9.
The distribution of relative position angles between the integrated intrinsic polarization (perpendicular to the direction of the intrinsic magnetic field) and the major axis of an extragalactic radio source were studied for different types of radio sources. Data for 280 extragalactic radio sources were used and it was found that there are large differences in the relative orientation of different types of radio sources. The directions of the intrinsic integrated magnetic fields correlate with the major radio axes of more elongated radio sources (K > 2.5, where K is the ratio of lengths of the major and minor axes of the radio images) and for radio sources of type FR II, whereas for less elongated objects (K < 2.5) and for radio sources of type FR I the magnetic fields do not correlate at all with the radio axes. An alternative mechanism for the formation of a radio galaxy from relativistic plasma ejected from the central part of an optical galaxy and moving in its large-scale, dipole magnetic field may be a theoretical basis for classification with respect to the elongation parameter K of the radio image. 相似文献
10.
A. Noutsos S. Johnston M. Kramer A. Karastergiou 《Monthly notices of the Royal Astronomical Society》2008,386(4):1881-1896
We measured a sample of 150 pulsar rotation measures (RMs) using the 20-cm receiver of the Parkes 64-m radio telescope. 46 of the pulsars in our sample have not had their RM values previously published, whereas 104 pulsar RMs have been revised. We used a novel quadratic fitting algorithm to obtain an accurate RM from the calibrated polarization profiles recorded across 256 MHz of receiver bandwidth. The new data are used in conjunction with previously known dispersion measures and the NE2001 electron-density model to study models of the direction and magnitude of the Galactic magnetic field. 相似文献
11.
The possibility of magnetic flux expulsion from the Galaxy in superbubble (SB) explosions, important for the α –Ω dynamo, is considered. Special emphasis is put on investigation of the downsliding of the matter from the top of the shell formed by the SB explosion, which is able to influence the kinematics of the shell. It is shown that either Galactic gravity or the development of the Rayleigh–Taylor instabilities in the shell limit the SB expansion, thus making magnetic flux expulsion impossible. The effect of cosmic rays in the shell on the sliding is considered, and it is shown that it is negligible compared with Galactic gravity. Thus the question of the possible mechanism of flux expulsion in the α –Ω dynamo remains open. 相似文献
12.
Hardcastle Birkinshaw & Worrall 《Monthly notices of the Royal Astronomical Society》1998,294(4):615-621
We report on ROSAT HRI observations of the nearby powerful radio galaxies 3C 33 and 111, which both have detected optical hotspots. We find nuclear X-ray sources in both objects, but no X-ray emission from the hotspots. This confirms the presence of a high-energy cut-off in the spectrum of synchrotron-emitting electrons. Since these electrons necessarily scatter the synchrotron photons by the inverse Compton process, our upper limits on the X-ray fluxes of the hotspots allow us to set lower limits of a few nanotesla on their magnetic flux density, close to or greater than the fields implied by equipartition of energy between radiating particles and magnetic field. 相似文献
13.
S.C. Marsden 《Astronomische Nachrichten》2010,331(6):577-580
Today the Sun has a regular magnetic cycle driven by a dynamo action. But how did this regular cycle develop? How do basic parameters such as rotation rate, age, and differential rotation affect the generation of magnetic fields? Zeeman Doppler imaging (ZDI) is a technique that uses high‐resolution observations in circularly polarised light to map the surface magnetic topology on stars. Utilising the spectropolarimetric capabilities of future large solar telescopes it will be possible to study the evolution and morphology of the magnetic fields on a range of Sun‐like stars from solar twins through to rapidly‐rotating active young Suns and thus study the solar magnetic dynamo through time. In this article I discuss recent results from ZDI of Sun‐like stars and how we can use night‐time observations from future solar telescopes to solve unanswered questions about the origin and evolution of the Sun's magnetic dynamo (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
14.
Shan-Jie Qian Xi-Zhen ZhangNational Astronomical Observatories Chinese Academy of Sciences Beijing rqsj@bao.ac.cn 《中国天文和天体物理学报》2003,3(1):75-86
Polarization position angle swings of - 180 ° observed in extragalactic radio sources are a regular behavior of variability in polarization. They should be due to some kind of physically regular process. We consider relativistic shocks which propagate through and 'illuminate' regular configurations of magnetic field, producing polarization angle swing events. Two magnetic field configurations (force-free field and homogeneous helical field) are considered to demonstrate the results. It is shown that the properties of polarization angle swings and the relationship between the swings and variations in total and polarized flux density are critically dependent on the configuration of magnetic field and the dynamical behavior of the shock. In particular, we find that in some cases polarization angle swings can occur when the total and polarized flux densities only vary by a very small amount. These results may be useful for understanding the polarization variability with both long and short timescales obser 相似文献
15.
In order to simulate evolution of a large-scale magnetic field in a barred galaxy possessing a gaseous halo we apply a three-dimensional
(3D) MHD numerical model. We solve a induction equation using a time-dependent velocity field of molecular gas resulting from
self-consistent 3D N-body simulations of a galactic disk. The gaseous halo rotates differentially co-rotating with the disk.
In our model we introduce the dynamo process causing the amplification of the magnetic field as well as the formation of field
structures high above the galactic disk. The simulated magnetic fields are used to construct the models of a high-frequency
(Faraday rotation-free) polarized radio emission that accounts for effects of projection and limited resolution, and is thus
suitable for direct comparison with observations. We found that the resultant magnetic field correctly reproduces the observed
structures of polarization B-vectors, forming coherent patterns well aligned with spiral arms and with the bar. The process
initializing a wave-like behavior of the magnetic field, which efficiently forms magnetic maxima between the spiral arms,
is demonstrated.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
16.
Markus Böttcher 《Astrophysics and Space Science》2007,307(1-3):69-75
Some recent developments in the study of relativistic jets in active galactic nuclei and microquasars are reviewed. While
it has been well established for some time that extragalactic jets found in radio galaxies, quasars, and BL Lac objects are
the site of ultrarelativistic particle acceleration, the recent identification of the Galactic jet source and microquasar
LS~5039 as a source of very-high-energy gamma-ray emission has underlined the striking similarity between the two types of
astrophysical jet sources. In this paper, I will present an overview of the dominant radiation and particle acceleration processes
and observational tests to distinguish between such processes. The wide-ranging analogies between Galactic and extragalactic
jets, but also their distinct differences, in particular those caused by the presence of the companion star in Galactic microquasar
systems, will be exposed. 相似文献
17.
18.
Subhashis Roy Sharanya Sur Kandaswamy Subramanian Arun Mangalam T. R. Seshadri Hum Chand 《Journal of Astrophysics and Astronomy》2016,37(4):42
Origin of magnetic fields, its structure and effects on dynamical processes in stars to galaxies are not well understood. Lack of a direct probe has remained a problem for its study. The first phase of Square Kilometre Array (SKA-I), will have almost an order of magnitude higher sensitivity than the best existing radio telescope at GHz frequencies. In this contribution, we discuss specific science cases that are of interest to the Indian community concerned with astrophysical turbulence and magnetic fields. The SKA-I will allow observations of a large number of background sources with detectable polarization and measure their Faraday depths (FDs) through the Milky Way, other galaxies and their circum-galactic mediums. This will probe line-of-sight magnetic fields in these objects well and provide field configurations. Detailed comparison of observational data (e.g., pitch angles in spirals) with models which consider various processes giving rise to field amplification and maintenance (e.g., various types of dynamo models) will then be possible. Such observations will also provide the coherence scale of the fields and its random component through RM structure function. Measuring the random component is important to characterize turbulence in the medium. Observations of FDs with redshift will provide important information on magnetic field evolution as a function of redshift. The background sources could also be used to probe magnetic fields and its coherent scale in galaxy clusters and in bridges formed between interacting galaxies. Other than FDs, sensitive observations of synchrotron emission from galaxies will provide complimentary information on their magnetic field strengths in the sky plane. The core shift measurements of AGNs can provide more precise measurements of magnetic field in the sub parsec region near the black hole and its evolution. The low band of SKA-I will also be useful to study circularly polarized emission from Sun and comparing various models of field configurations with observations. 相似文献
19.
Based on our compiled catalogue of fundamental astrophysical parameters for 593 open clusters, we analyze the relations between the chemical composition, spatial positions, Galactic orbital elements, age, and other physical parameters of open star clusters. We show that the population of open clusters is heterogeneous and is divided into two groups differing by their mean parameters, properties, and origin. One group includes the Galactic clusters formed mainly from the interstellar matter of the thin disk with nearly solarmetallicities ([Fe/H] > ?0.2) and having almost circular orbits a short distance away from the Galactic plane, i.e., typical of the field stars of the Galactic thin disk. The second group includes the peculiar clusters formed through the interaction of extragalactic objects (such as high-velocity clouds, globular clusters, or dwarf galaxies) with the interstellar matter of the thin disk, which, as a result, derived abnormally low (for field thin-disk stars) metallicities and/or Galactic orbits typical of objects of the older Galactic subsystems. About 70% of the clusters older than 1Gyr have been found to be peculiar, suggesting a slower disruption of clusters with noncircular high orbits. Analysis of orbital elements has shown that the bulk of the clusters from both groups were formed within a Galactocentric radius of ??10.5 kpc and closer than ??180 pc from the Galactic plane, but owing to their high initial velocities, the peculiar clusters gradually took up the volumes occupied by the objects of the thick disk, the halo, and even the accreted halo of the Galaxy. Analysis of the relative abundances of magnesium (a representative of the ??-elements) in clusters that, according to their kinematical parameters, belong to different Galactic subsystems has shown that all clusters are composed of matter incorporating the interstellar matter of a single protogalactic cloud in different proportions, i.e., reprocessed in genetically related stars of the Galaxy. The [Mg/Fe] ratios for the clusters with thick-disk kinematics are, on average, overestimated, just as for the field stars of the socalled ??metal-rich wing?? of the thick disk. For the clusters with halo kinematics, these ratios exhibit a very large spread, suggesting that they were formed mainly from matter that experienced a history of chemical evolution different from the Galactic one. We point out that a large fraction of the open clusters with thindisk kinematics have also been formed from matter of an extragalactic nature within the last ??30 Myr. 相似文献
20.
Rainer Beck 《Astrophysics and Space Science》2009,320(1-3):77-84
Spiral galaxies host dynamically important magnetic fields which can affect gas flows in the disks and halos. Total magnetic fields in spiral galaxies are strongest (up to 30 μG) in the spiral arms where they are mostly turbulent or tangled. Polarized synchrotron emission shows that the resolved regular fields are generally strongest in the interarm regions (up to 15 μG). Faraday rotation measures of radio polarization vectors in the disks of several spiral galaxies reveal large-scale patterns which are signatures of coherent fields generated by a mean-field dynamo. Magnetic fields are also observed in radio halos around edge-on galaxies at heights of a few kpc above the disk. Cosmic-ray driven galactic winds transport gas and magnetic fields from the disk into the halo. The halo scale height and the electron lifetime allow to estimate the wind speed. The magnetic energy density is larger than the thermal energy density, but smaller than the kinetic energy density of the outflow. There is no observation yet of a halo with a large-scale coherent dynamo pattern. A global wind outflow may prevent the operation of a dynamo in the halo. Halo regions with high degrees of radio polarization at very large distances from the disk are excellent tracers of interaction between galaxies or ram pressure of the intergalactic medium. The observed extent of radio halos is limited by energy losses of the cosmic-ray electrons. Future low-frequency radio telescopes like LOFAR and the SKA will allow to trace halo outflows and their interaction with the intergalactic medium to much larger distances. 相似文献