Astrophysical BackgroundStirling A. Colgate and Hui Li, T-6, Los Alamos National Lab1. Evidence for Immense Magnetic Field Energy and FluxRadio astronomy aperture synthesis pictures of quasars or active galactic nuclei (AGN) within the special environment of clusters of galaxies reveal massive regions of coherent magnetic flux: Fig. 1, 3C465, 3C75, (Eilek and Owen, 1999) and Fig. 2, Hydra, (Taylor and Perley, 1993). This magnetic field is inferred using the technique of measuring the Faraday rotation by a foreground screen of the vector of polarized emission of an underlying, highly polarized source. In order for this effect to be observed, this screen must have: (1) a highly polarized source, (2) with small internal Rm, (3) lying behind, (4) a region of highly ordered Rm, (5) without significant emission, and (6) further requiring both a highly ordered field and (7) a uniform electron density. A modification of any one of these seven conditions rapidly averages a potentially large Rm to a negligible value. The fact that the field can be measured and is so strong, highly oriented, and uniform and furthermore that the electron density is equally as uniform over dimensions of 100 kpc or 10 times the size of our galaxy is truly extraordinary. The energy derived from the size and therefore volume of coherent field is ~ 1059 to 1060 erg. This energy is so large, a million times that of the field energy of our own galaxy and orders of magnitude larger than the binding energy of typical galaxies, that an extraordinary source of this energy is necessary (Colgate and Li, 1999). The underlying highly polarized source is an AGN. This implies that the AGN is the source of this immense magnetic energy. A massive black hole, ~ 108 Mʘ is presumed to power every AGN from the black hole binding energy, ~ 1062 erg. A significant fraction of this energy is therefore required to create the fields inferred from Faraday measurements. Furthermore an even larger magnetic energy, ~ 1060 to 1061 erg is required to explain the underlying polarized synchrotron source, which is also the giant radio lobes of typical, average field AGN, (not in clusters). One calculates this still larger energy by assuming that the radio emission from the giant radio lobes is synchrotron radiation. This emission requires both a magnetic field and highly energetic electrons. The minimum energy is determined from the minimum sum of the accelerated electrons (plus ions) and magnetic field strength necessary to emit the observed synchrotron radiation. This minimum occurs when the two energies are equal. If the energy in energetic electrons is just a small fraction, ~ 1%, of the ion energy as expected from our galactic cosmic ray spectrum, then the total energy is an order of magnitude larger. The Faraday rotation measures indicate that strong magnetic fields of immense energy are associated with AGN. This substantiates the synchrotron radiation interpretation of still larger energies implied for most sources. Finally, comparable field energies are seen in the cluster as a whole (Clarke,Krönberg, & Böhringer 2000).
Figure 1:
Shows the emission measure maps and the Faraday rotation
measure maps of two galaxies 3C465 and 3C75 in two different Abell
clusters. The scale is of the order of 80 kpc, far larger than normal
galaxy size. The Faraday rotation proves the immense regions of
coherent magnetic fields. The minimum energy of the emission maps
interpreted as synchrotron emission implies a minimum energy ~ 1061 erg for the sum of magnetic and relativistic electrons plus ions,
more than an order of magnitude greater than the field energy implied by
Faraday rotation measures. This energy is so large that we believe
that only a massive black hole is a feasible energy source (courtesy
Eilek and Owen 2000).
Figure 2: Shows the emission measure maps and the Faraday rotation measure maps of a galaxy in the Hydra cluster of galaxies. The scale is of the order of 100 kpc, far larger than normal galaxy size. The Faraday rotation proves the immense regions of coherent magnetic fields. The minimum energy of the emission maps interpreted as synchrotron emission implies a minimum energy ~ 1061 erg for the sum of magnetic and relativistic electrons plus ions, more than an order of magnitude greater than the field energy implied by Faraday rotation measures(courtesy Taylor and Perley 1993).
2. Field Galaxies and their Magnetic FieldMost galaxies exist outside of clusters, i.e. field galaxies. Nearly all galaxies go through an early phase of being an AGN (Richstone 1998). These field AGN also always emit synchrotron radiation from their radio lobes, which are larger in dimension than in clusters. The Faraday rotation cannot be observed in these field galaxies or field AGN, because of the low, ~ 10-2 electron density, but within the galaxy clusters a stronger gravity binds a thousand times larger electron (and mass) density and so we can observe the Faraday rotation. Thus we are led to conclude that all galaxies produced an immense energy in magnetic flux during their brief, early AGN phase. This magnetic energy may be so large as to explain the missing energy, ~ 90%, lost or not seen from the black hole formation, (Richstone, 1998) aside from the background x-ray sources. In order to explain the large fields requires a dynamo. In order to convert the kinetic energy of the black hole accretion disk into magnetic energy requires an efficient dynamo. In order to explain the missing luminosity from AGN, the dynamo that produces this field would have to be ~ 90% efficient. But this high efficiency is what we expect of an α ω dynamo. If this magnetic energy were the "missing" energy, it would then exceed that of star light emitted during the lifetime of the galaxy. 3. The Efficient DynamoThe only feasible source of this magnetic energy is the gravitational binding energy of forming the central galactic black hole. How can one convert a significant fraction of this energy, 108 Mʘc2 = 2 × 1062 erg, into magnetic energy, B2/8π? Such a conversion mechanism is called a dynamo. In order for this magnetic energy to remain "dark" it must not be dissipated into heat at some later point in time as is the case for most dynamos. We believe that this magnetic energy remains as force-free magnetic fields of ~ 1μ G in the intergalactic space between galaxies, i.e. the "walls". 4. Angular Momentum: The Cosmic PollutantMatter is born with too much angular momentum. The initial Lyman-α clouds, so called because of their UV hydrogen absorption spectra, are the first signature of large density fluctuations, galaxy size, in calculations of the emerging structure of the universe (Zurek, et. al, 1994). The initial specific angular momentum of the matter of these massive clouds, although small, ~ 5%, compared to that necessary for Keplerian support, is nevertheless very large, ~ 3 × 106 greater than the specific angular momentum of the same matter when it reaches the "event horizon" of the black hole. Exchanging this angular momentum with another fraction of matter not reaching the black hole is a central problem in astrophysics. Initially matter and angular momentum are exchanged equally by tidal torquing so that a "flat", constant velocity, "rotation curve" is produced, where Minterior ∝ R. Thus one has a small mass at a small radius. This would never lead to a massive central black hole. Instead at a critical radius and critical condition a hydrodynamic instability is initiated so that almost all the interior matter falls to form the black hole. This process generally called an accretion disk, starts with fluid dynamic processes when the disk is thick enough to remain adiabatic during the growth of this instability (Lovelace, Li, and Colgate, 1999, Li, Lovelace, and Colgate 2000, Li, Wendroff, and Colgate, 2000). When a galaxy is being formed from the collapse of giant Lyman-α clouds emerging from the growth of structure of the universe, we predict that this instability, Fig. 3, is initiated at a critical thickness, ~ 100 to 1000 g/cm2. At this point ~ 108 Mʘ reside within a disk several parsecs in radius. The matter in this accretion disk then evolves by these instabilities into the black hole.
Figure 3: Shows a calculation of the formation of co-rotating "Rossby vortices" in a Keplerian flow. These vortices are calculated as if they are just 2 dimensional. It is these vortices that transport angular momentum. The interaction with the third dimension sets the condition for minimum thickness and contributes to the damping of the vortices (Li et al. 2000).
Figure 4: The α ω
dynamo in a galactic black hole accretion disk (ANIMATE
5. The α ω Accretion Disk DynamoThis accretion disk we believe is a natural dynamo because of one peculiar circumstance. A small fraction of the accreting matter must be in the form of stars, Pop III stars, because of the metalicity observed in the initial Lyman-α clouds, ~ 10-3 to 10-4. These stars, not in the orbit of the disk, plunge through the disk many times, ~ 109, before falling into the black hole. Each collision with the disk carries a fraction of the disk matter, as well as entrained magnetic flux, to high altitude, like plumes, above and below the disk. Remarkably these plumes rotate, actually un-rotate, relative to the Keplerian rotation of the disk (Beckley et al 2000). The differential rotation is in the same direction regardless of their orbit. The twist is opposite to the rotation of the disk, because the moment of inertia of the expanding plume always increases and therefore the plume slows down relative to the frame. Thus the translation and twist of the plumes is strikingly coherent. It is this coherent motion, the helicity, (α = v • (∇ × v)) that is the basis of the α-deformation of the dynamo. The differential rotation of Keplerian motion of the disk will always wrap up the radial component of an initial poloidal field provided it is immersed in a conducting medium, the Ω-deformation. A field line wrapped up n turns will be larger by n and, hence, for large n the toroidal field can be much larger than an initial poloidal one. Then only a small fraction of the enhanced toroidal field needs to be converted back to poloidal field to result in a dynamo of positive gain. This conversion of toroidal to poloidal is produced by the helicity of the plume rotation. What is remarkable is that such a random phenomena as star-disk collisions can convert a fraction of this wrapped up, now larger toroidal field, back into the original poloidal field and therefore give rise to a coherent amplification. We have simulated this processes with a kinematic dynamo, vector potential code and are proceeding to demonstrate it in the laboratory. As Paul Roberts and Torkel Jensen, (1993) once lamented, "And yet laboratory models of such dynamos, working under controlled conditions, are still lacking! They are, at this time, merely sterile fantasies of the armchair scientists: the theoreticians. This seems to be a too unhealthy state to be allowed to persist. As Fermi once warned, in magnetohydrodynamics one should not believe the product of a long and complicated piece of mathematics if it is unsupported by observation". Now that the "α2" dynamo has been demonstrated in two experiments, Karlsruhe (Rädler, 1988) and Riga (Gailitis et al., 2000) we know that the basic principle of a dynamo is valid. Such an α2 dynamo can not usefully create the magnetic fields of the universe. We need the experiment to substantiate our understanding of the the α - Ω dynamo in the accretion disk, the largest energy machine in the universe. Theory, mathematics, calculations, and some experiments now agree.
6. REFERENCESBeckley, H.F., Colgate, S.A., Romero, V.D., Ferrel, R., 2000, " Rotation of a Pulsed Jet in a Rotating Annulus: A Source of Helicity for an α - Ω Dynamo ", 2000, Submitted to Physics of FluidsBurbidge, G.R., 1958, ApJ, 129, 849 Clarke, T.E., Krönberg, P.P., and Böhringer, H., 2000, ApJ., Letter in press Colgate, S.A. & Hui Li, 1998, "The Galactic Dynamo, the Helical Force Free Field and the Emissions of AGN", Proc of the Conference on "Relativistic Jets in AGNs", ed. Michal Ostrowski, Sikora, Madejski, and Begelman, Cracow, Poland, 27-30 May 1997, pp. 170-179 Colgate, S.A. and Li, H., 1999, Astrophys. Space Sci. 264 357 Colgate, S.A. and Li, H., 2000, IAU Symposium, 195, 255 Colgate, S.A. and Li, H., & Pariev,V., 2001,"The Origin of the Magnetic Fields of the Universe:The Plasma Astrophysics of the Free Energy of the Universe" Physics of Plasmas, accepted Eilek, J.A., 1999, Magnetic Fields in Clusters: Theory vs. Observation, Ringberg workshop, Germany, MPE-Report Eilek, J.A. & Owen, F., 2000, Astrophys. J., submitted Li, H., Finn, J.M., Lovelace, R.V.E., and Colgate, S.A., 2000 Astrophys. J. 533, 1023 Richstone, D., 1998, Nature, 395, 14 Roberts, P.H., and Jensen, T.H., 1993, Phys. Fluids B 5, 2657 Taylor, G.B., & Perley, R.A., 1993, Astrophys. J., 416, 554 Zurek, W.H., Quinn, P.J., Salmon, J.K., and Warren, M.S., 1994, Astrophys. J., 431, 559-568 About this document ...This document was generated using the LaTeX2HTML translator Version 97.1 (release) (July 13th, 1997) Copyright © 1993, 1994, 1995, 1996, 1997, Nikos Drakos, Computer Based Learning Unit, University of Leeds.
This document was last updated by
Kate Weatherall and Howard Beckley on 24 April 2001
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