Large-scale Galactic Magnetic Field Research Articles

The H I distribution in the environment of the WR star HD 50896

The neutral matter distribution from the interstellar medium (ISM) located in the vicinity of the galactic Wolf-Rayet (WR) star HD 50896 has been examined by means of H I21-cm line observations obtained with both low [half-power beamwidth (HPBW)≃34 arcmin] and intermediate (HPBW≃9 arcmin) angular resolution. The most interesting discovery is a huge ovoidal H I minimum spanning the velocity range +1.5 to +10.0 km s−1, created, very likely, by the joint action of the progenitor of HD 50896 and the WR itself. Inside this cavity, two minima are clearly discernible. The WR star is offset with respect to either the geometrical centre of the main H I void or the inner H I minima. A physical link between S308, the ring nebula associated with HD 50896, and one of the H I minima is suggested by our data. A kinematical distance of ∼ 1 kpc for HD 50896 is derived based on the radial velocity of the H I cavity. The dual H I minimum geometry observed inside the main H I cavity, a feature also seen in the H I distribution of the ISM located close to other galactic WR stars, may be a consequence of the interaction process itself. The ovoidal shape of the main cavity cannot be explained within the framework of the standard interstellar bubble theory. Elongated bubbles may result when the large-scale galactic magnetic field is taken into account.

The Influence of Spiral Arms and Bar on the Large-Scale Galactic Magnetic Field Evolution

Recent observations of radio polarization from nearby galaxies show that the large-scale galactic magnetic field is aligned with spiral arms and bars and the magnetic field vectors in the interarm regions possess a spiral structure which has the same pitch angle as that in spiral arms. Our present project is going to address the following questions: What is the structure and evolution of the large-scale galactic magnetic field under the influence of spiral and bar structure in a galactic disk? To which extent could the resulting magnetic field account for the observed spiral pattern of magnetic field in nearby galaxies? The model is based on the particle-particle numerical scheme (SPH) involving two components: stars and molecular gas. The magnetic field is connected with the latter one. The magnetic field computations were performed first in two dimensions for 100 velocity fields: from 107 to 109 yrs. The resultant magnetic field is strongly affected by spiral arms, however at the given evolutionary stage its structure is different from the velocity field at the same time. The magnetic pitch angle distribution shows that the magnetic field “remembers” all the past velocity steps. The magnetic pitch angle distribution resulting after beam smoothing could quite well fit observations. The present model with fully 3D velocity field of interstellar gas should clear the problem if the magnetic field under the realistic velocity evolution of gas could explain the observed structure of large-scale magnetic field with constant pitch angle in the whole disk.

Magnetic Field Generation by Galactic Winds

A new mechanism is presented for the local amplification and possible global dynamo maintenance of non-axisymmetric large-scale magnetic fields in disk galaxies. Shear in a galactic wind or large-scale flow of ionised gas with components axial and radial to the disk plane may regenerate large-scale magnetic fields.Numerical results are presented from kinematic mathematical models based on a local (thin disk) approximation and an exact three-dimensional formulation. The one-dimensional thin-disk model illustrates the possibility of exponential amplification and the resulting local axial spatial structure of large-scale galactic magnetic fields. Three-dimensional results support the possibility of global wind dynamo action.

Global Magnetic Structure In Spiral Galaxies

AbstractGlobal magnetic structure has been inferred in spiral galaxies from radio and optical observations. The author has developed a three dimensional, quasi-global kinematic dynamo model which demonstrates the possibility of maintaining large-scale magnetic fields by the αω mechanism. Analytical expressions for large-scale galactic magnetic fields near temporal steady states have been derived. Illustrations of the spatial distribution of vector and absolute large-scale magnetic field strength are presented. When coupled with observations this model may help in understanding the maintenance and morphologies inferred for disk and halo global magnetic fields in spiral galaxies.

A possible origin of galactic magnetic fields

view Abstract Citations (63) References (33) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Possible Origin of Galactic Magnetic Fields Daly, R. A. ; Loeb, Abraham Abstract Large-scale galactic magnetic fields with strengths of several μG can result if galaxies typically undergo a brief but active phase during which magnetized plasma is channeled away from a central compact object through jets. When the jets are initially activated, they must tunnel through the ambient protogalactic medium, and, as they do so, material from the jets flows into the protogalactic medium. It is shown that if these jets possess magnetic fields with equipartition field strengths and large coherence lengths, then galactic fields with large coherence lengths and equipartition field strengths result, which suggests another possible origin of galactic magnetic fields. Observations of jets are consistent with the fields having large coherence lengths and equipartition field strengths, making the proposed model viable. The jet magnetic fields may be generated near the central compact object, and, subsequently, may be stretched and amplified by processes within the jets; this interpretation of the ultimate source of the field is not mandatory in this model, but it is discussed for completeness. Implications of the model are considered. Publication: The Astrophysical Journal Pub Date: December 1990 DOI: 10.1086/169429 Bibcode: 1990ApJ...364..451D Keywords: Interstellar Magnetic Fields; Magnetohydrodynamics; Plasma Jets; Astronomical Models; Red Shift; Astrophysics; GALAXIES: JETS; HYDROMAGNETICS; INTERSTELLAR: MAGNETIC FIELDS full text sources ADS | data products SIMBAD (1)

Large-Scale Galactic and Intergalactic Magnetic Fields

Large-scale axisymmetric and bisymmetric spiral (ASS and BSS) structures are found of magnetic fields in spiral galaxies by measuring the Faraday rotation of polarized radio emission. Dynamo theory is introduced to explain the field structures, and strong magnetogravitational interaction is suggested to occur between the BSS magnetic fields and spiral density waves. Up-to-date data about the rotation measures RM and redshifts z of QSOs and distant radio galaxies are given for discussing large-scale intergalactic magnetic fields.

Galactic Dynamos Without Sharp Boundaries: Non-Axisymmetric Fields in Axisymmetric Disks?

Radio polarization observations of spiral galaxies suggest the existence of large-scale galactic magnetic fields which are of either axisymmetric -spiral (ASS) or bisymmetric-spiral (BSS), i.e. non-axisymmetric, structure (cf. Beck, 1939). Clear evidence for a BSS field was indicated for M31 by M. Krause et al. (1989).

Intermittent behavior of galactic dynamo activities

view Abstract Citations (43) References (38) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Intermittent Behavior of Galactic Dynamo Activities Ko, C. M. ; Parker, E. N. Abstract Recent observations by Beck and Golla of far-infrared and radio continuum emission from nearby spiral galaxies suggest that the galactic magnetic field strength is connected to the current star formation rate. The role of star formation on the generation of large-scale galactic magnetic field is studied in this paper. Using a simple galactic model, it is shown how the galactic dynamo depends strongly on the turbulent velocity of the interstellar medium. When the star formation efficiency is high, the ISM is churned which in turn amplifies the galactic magnetic field. Between active star formation epochs, the magnetic field is in dormant state and decays at a negligible rate. If density waves trigger star formation, then they also turn on the otherwise dormant dynamo. Publication: The Astrophysical Journal Pub Date: June 1989 DOI: 10.1086/167540 Bibcode: 1989ApJ...341..828K Keywords: Astronomical Models; Dynamo Theory; Interstellar Magnetic Fields; Interstellar Matter; Spiral Galaxies; Magnetic Field Configurations; Magnetic Flux; Star Formation Rate; Astrophysics; HYDROMAGNETICS; INTERSTELLAR: MAGNETIC FIELDS; STARS: FORMATION full text sources ADS | data products SIMBAD (4)

Magnetic braking in galactic flows

view Abstract Citations (15) References (32) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Magnetic braking in galactic flows Sparke, L. S. Abstract The nuclear fireworks of active galaxies are believed to derive their power from the kinetic energy of gas falling onto a massive central object; mass shed from evolving galactic stars is an obvious source of fuel for this process. But this ejected material shares the galactic rotation, and a centrifugal barrier will prevent it from reaching the nucleus, if its angular momentum is not removed. This paper shows that, if the large-scale galactic magnetic field has a strong enough radial component, magnetic torques can act to spin down the infalling matter. Rotation of the interstellar gas induces a toroidal magnetic field, amd Maxwell stresses remove angular momentum from the flow; gas can then fall inward to the galactic center. In this way, the monster in the nucleus can be fed on gas from a galaxy's own stars. The magnetic fields in M87 and NGC 1275, giant elliptical galaxies which are accreting from an intracluster medium, appear to be strong enough to allow magnetic braking. Publication: The Astrophysical Journal Pub Date: September 1982 DOI: 10.1086/160237 Bibcode: 1982ApJ...260..104S Keywords: Galactic Nuclei; Galactic Rotation; Interstellar Magnetic Fields; Magnetohydrodynamic Flow; Momentum Transfer; Stellar Mass Ejection; Active Galaxies; Angular Momentum; Elliptical Galaxies; Interstellar Gas; Magnetic Effects; Radial Velocity; Astrophysics full text sources ADS | data products NED (2)

Massive magnetic monopoles: Indirect and direct limits on their number density and flux

The existence of very massive magnetic monopoles is a necessary consequence of most unified theories of the strong, electromagnetic, and weak interactions. Estimates of their masses range from ${10}^{15}$ to ${10}^{17}$ that of a proton. Monopoles should have been produced in the "big bang" and survived to the present, either trapped in matter or in flight. By virtue of their huge mass, these monopoles might have eluded previous searches designed to detect monopoles with much smaller mass. I review the indirect limits which can be placed on monopoles because of the existence of large-scale galactic magnetic fields. I also review the experimental searches and reinterpret them to obtain limits for very massive monopoles. If the monopole masses are \ensuremath{\lesssim}5\ifmmode\times\else\texttimes\fi{}${10}^{14}$ GeV/${\mathit{c}}^{2}$, their concentration must be \ensuremath{\lesssim}${10}^{\ensuremath{-}27}$ monopoles/nucleon from searches for monopoles trapped in meteorites. This limit is several orders of magnitude lower than the astrophysical limits.

The scale and strength of the galactic magnetic field according to the pulsar data

In accordance with the data on the Faraday rotation, angular coordinates, and dispersion measurements and distances of 38 pulsars, the strengthB=2.1±1.1 μG and directionl=99°±24°,b≅0° of the large-scale galactic magnetic field and the mean electron density in the galactic discNe=0.03±0.01 cm−3 are determined. A comparison with the results of a study of the measures of rotation of extragalactic radio sources enabled us to estimate the characteristic half-width of the distribution of the electron density on the Z-coordinate (h≅400 ps). The characteristic size of galactic magnetic field flucturations is shown to be ℒ=100–150 ps.

Hot interstellar tunnels. I - Simulation of interacting supernova remnants

The theory required to build a numerical simulation of interacting supernova remnants is developed. The hot cavities within a population of remnants will become connected, with varying ease and speed, for a variety of assumed conditions in the outer shells of old remnants. Apparently neither radiative cooling nor thermal conduction in a large-scale galactic magnetic field can destroy hot cavity regions, if they grow, faster than they are reheated by supernova shock waves, but interstellar mass motions disrupt the contiguity of extensive cavities necessary for the dispersal of these shocks over a wide volume. Monte Carlo simulations show that a quasi-equilibrium is reached in the test space within 10 million yrs of the first supernova and is characterized by an average cavity filling fraction of the interstellar volume. Aspects of this equilibrium are discussed for a range of supernova rates. Two predictions are not confirmed within this range: critical growth of hot regions to encompass the entire medium, and the efficient quenching of a remnant's expansion by interaction with other cavities.