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Using a closure model for the evolution of magnetic correlations, we uncover an interesting plausible saturated state of the small-scale fluctuation dynamo (SSD) and a novel anology between quantum mechanical tunneling and the generation of large-scale fields. Large scale fields develop via the $\alpha$-effect, but as magnetic helicity can only change on a resistive timescale, the time it takes to organize the field into large scales increases with magnetic Reynolds number. This is very similar to the results which obtain from simulations using full MHD., Comment: 3 pages, 1 figure, to appear in proceedings of the 24th IAU General Assembly, Joint Discussion 14 "The Origins of Galactic Magnetic Fields"

GRAVITATIONAL microlensing of quasars has been invoked to account for their observed optical variability1–3—the timescale being as short as a few years for microlenses the size of Jupiter moving at ~300 km s−1 (ref. 4). But some blazars (conspicuously active quasars) show ultra-rapid variability on timescales as short as 1 hour in the optical5–7 and 1 day at centimetre wavelengths8–11. Blazars are known to contain relativistic jets directed within ~10° of the line of sight12, and bright knots in these jets could appear to move superluminally with respect not only to the blazar nucleus but also to any galaxy near the line of sight. We argue here that any such superluminal motion13, if microlensed by a star in an intervening galaxy, can produce the requisite ultra-rapid variations, even in the absence of intrinsic flux variations in the jet. Moreover, the same mechanism can naturally account for the commonly observed ~ 1-day variability of compact radio sources at centimetre wavelengths14.

During hierarchical clustering, smaller masses generally collapse earlier than larger masses and so are denser on the average. The core of a small mass halo could be dense enough to resist disruption and survive undigested, when it is incorporated into a bigger object. We explore the possibility that a nested sequence of undigested cores in the center of the halo, which have survived the hierarchical, inhomogeneous collapse to form larger and larger objects, determines the halo structure in the inner regions. For a flat universe with $P(k) \propto k^n$, scaling arguments then suggest that the core density profile is, $\rho \propto r^{-\alpha}$ with $\alpha = (9+3n)/(5+n)$. But whether such behaviour obtains depends on detailed dynamics. We first examine the dynamics using a fluid approach to the self-similar collapse solutions for the dark matter phase space density, including the effect of velocity dispersions. We highlight the importance of tangential velocity dispersions to obtain density profiles shallower than $1/r^2$ in the core regions. If tangential velocity dispersions in the core are constrained to be less than the radial dispersion, a cuspy core density profile shallower than 1/r cannot obtain, in self-similar collapse. We then briefly look at the profiles of the outer halos in low density cosmological models where the total halo mass is convergent. Finally, we analyze a suite of dark halo density and velocity dispersion profiles obtained in cosmological N-body simulations of models with n= 0, -1 and -2. We find that the core-density profiles of dark halos, show considerable scatter in their properties, but nevertheless do appear to reflect a memory of the initial power spectrum, with steeper initial spectra producing flatter core profiles. (Abridged), Comment: 31 pages, 7 figures, submitted to ApJ

The evolution and viscous damping of cosmic magnetic fields in the early universe, is analysed. Using the fact that the fluid, electromagnetic, and shear viscous energy-momentum tensors are all conformally invariant, the evolution is transformed from the expanding universe setting into that in flat spacetime. Particular attention is paid to the evolution of nonlinear Alfven modes. For a small enough magnetic field, which satisfies our observational constraints, these wave modes either oscillate negligibly or, when they do oscillate, become overdamped. Hence they do not suffer Silk damping on galactic and subgalactic scales. The smallest scale which survives damping depends on the field strength and is of order a dimensionless Alfven velocity times the usual baryon-photon Silk damping scale. After recombination, nonlinear effects can convert the Alfven mode into compressional, gravitationally unstable waves and seed cosmic structures if the cosmic magnetic field is sufficiently strong., 45 pages, Tex file, no figures

An inhomogeneous cosmological magnetic field will create Alfven-wave modes that induce a small rotational velocity perturbation on the last scattering surface of the microwave background radiation. The Alfven-wave mode survives Silk damping on much smaller scales than the compressional modes. This, in combination with its rotational nature, ensures that there will be no sharp cut-off in anisotropy on arc-minute scales. We estimate that a magnetic field which redshifts to a present value of $3\times 10^{-9}$ Gauss produces temperature anisotropies at the 10 micro Kelvin level at and below 10 arc-min scales. A tangled magnetic field, which is large enough to influence the formation of large scale structure is therefore potentially detectable by future observations., Comment: 5 pages, Revtex, no figures

If a compact double (CD) is caused by lensing, the orientation and/or flux ratio of its components can substantially change in ≈ 1 year.

We determine the magnetic helicity, along with the magnetic energy, at high latitudes using data from the Ulysses mission. The data set spans the time period from 1993 to 1996. The basic assumption of the analysis is that the solar wind is homogeneous. Because the solar wind speed is high, we follow the approach first pioneered by Matthaeus et al. (1982, Phys. Rev. Lett. 48, 1256) by which, under the assumption of spatial homogeneity, one can use Fourier transforms of the magnetic field time series to construct one-dimensional spectra of the magnetic energy and magnetic helicity under the assumption that the Taylor frozen-in-flow hypothesis is valid. That is a well-satisfied assumption for the data used in this study. The magnetic helicity derives from the skew-symmetric terms of the three-dimensional magnetic correlation tensor, while the symmetric terms of the tensor are used to determine the magnetic energy spectrum. Our results show a sign change of magnetic helicity at wavenumber k~2 AU^{-1} (or frequency nu~2 uHz) at distances below 2.8 AU and at k~30 AU^{-1} (or nu~25 uHz) at larger distances. At small scales the magnetic helicity is positive at northern heliographic latitudes and negative at southern latitudes. The positive magnetic helicity at small scales is argued to be the result of turbulent diffusion reversing the sign relative to what is seen at small scales at the solar surface. Furthermore, the magnetic helicity declines toward solar minimum in 1996. The magnetic helicity flux integrated separately over one hemisphere amounts to about 10^{45} Mx^2/cycle at large scales and to a 3 times lower value at smaller scales., 8 pages, 6 figures, ApJ (in press)

If a compact double (CD) is caused by lensing, the orientation and/or flux ratio of its components can substantially change in ≈ 1 year.

A model for the triple quasar Q1115+080 A, B, C based on a single gravitational lens is proposed. It is demonstrated that the resulting five-image configuration produced by a lens galaxy having a spheroidal mass component is in reasonable accord with the observed features of the triple quasar. The model naturally leads to the resolution of the brightest member into two closely separated components of approximately equal intensity. The mass and redshift of the lens galaxy and time delays from various images are calculated as a function of the core radius of the galaxy.

The twin quasar QSO 0957 + 561 A,B along with bright radio arches and VLBI structures is modeled using a gravitational lens consisting of the giant elliptical galaxy and the background cluster. The effective time-delay between components A and B comes out to be around one year, which is significantly different from the value obtained by Young et al. for a similar set of lens parameters. It is demonstrated that the observed intensity variation in component B between 1979 and mid-1981, reported by Keel (1982) can result from minilensing by low mass stars.

On etablit les conditions qui determinent si une lentille peut donner naissance a des images multiples. On presente les conditions necessaires pour les lentilles a symetrie spherique ou a symetrie elliptique

Recent work on galaxy formation scenarios has raised the interesting possibility that there could be many galactic halo-like objects in which luminous galaxies have failed to form. Whether dark galactic halos which lack compact luminous components will reveal themselves by multiply imaging distant quasars is considered. A single such dark halo is unlikely to do so, but two dark halos coming along the line-of-sight can form multiple images of distance sources. If the density in such dark halos is a significant fraction of the closure density of the universe, such double halo lensing is found to be on the threshold of detectability. The double dark halo lensing scenario can naturally account for multiply-imaged systems with large image separations but no detectable lens. 2345 + 007, the lens system with the largest observed image separation of 7.3 arcsec, and 1635 + 267 are suggested to be good candidates for being quasars multiply imaged by two dark galactic haloes. It is also suggested that observation of the missing third image in these systems by, say, the Space Telescope could give credence to the operation of lensing by dark galactic haloes.

The possible consequences of asymmetry in dark galactic haloes for galaxy formation are examined, by adopting an idealized model of protogalactic collapse. The protogalaxy can lose a significant fraction of its initial angular momentum to the dark halo if it initially rotates about an axis close to the halo’s middle axis. We speculate that elliptical galaxies form when a protogalaxy loses most of its initial angular momentum to the dark halo. Asymmetric halo cores may also lead to triaxial bulges and assist the formation of central bars and compact objects.

The conditions for the production of multiple images by an arbitrary (thick or thin) gravitational lens are studied. The authors show that the necessary and sufficient conditions for the production of multiple images by a lens is the following: The lens should produce a point conjugate to the observer, along some null geodesic, at an affine distance smaller than that of the source. It is shown that previous results on multiple imaging by thin lenses can be obtained as a special case. The authors also show that a thick lens cannot be more efficient than a suitably designed thin lens for the production of multiple images.

The study of large-scale structures in the universe is often based on the observed density distribution of matter smoothed by a suitable filter function. The probability distribution for this smoothed density field in the nonlinear regime is studied using the Zel'dovich approximation. When the shear term of the velocity field is not too large, one can obtain a reasonably good analytic approximation to this probability distribution. The properties of this distribution are discussed and compared with other attempts along similar lines.

A generalized version of the Zel'dovich approximation which is applicable in both the radiation-dominated and the matter-dominated epochs is presented. This approximation allows one to follow the growth of inhomogeneities from the time the mode enters the Hubble radius until it turns around. Comparison of the results with the standard spherical model shows that the analytic approximation is quite good even in the nonlinear regime. Detailed application to cold dark matter models and seeded models are given.

Gravitational lens models involving extended mass distributions generally predict an odd number of images1, with one of the images close to the centre of the principal lens galaxy. In all the lens systems observed up to now, only an even number of images have been unambiguously detected. A number of suggestions2–4 have been advanced to explain the absence of the ‘odd’ image, but none of these is sufficiently compelling. We suggest here that the presence of a compact nucleus with mass in the range 5 × 109 to 5 × 1010 solar masses (M⊙), contained within a region of size ≲50 pc at the centre of a lens galaxy, would dim the odd image significantly without affecting the rest of the image configuration.

The viability of the hypothesis that the redshift of a quasar is due entirely to the Doppler effect which arises from its high ejection speed in a nearby center of explosion is examined in light of data on the aligned triplets of quasars discovered by Arp and Hazard (1980). In view of a physical association of the quasars in a triplet, the computation of the various parameters of the Doppler problem is illustrated and constraints are placed on quasar ejection scenarios in order to critically examine Holyle's recent (1980) hypothesis that quasars emit the bulk of their radiation in a specified backward cone. It is found that the four triplets provide prima facie evidence for such a hypothesis, and further checks on the Doppler model are suggested. A very small, but nonzero fraction of blueshifted quasars is predicted by the model.

We present two sets of models for the newly reported gravitational lens system MG 2016+112, one involving a galaxy in a rich cluster and the other made up of two galaxies at different redshifts. They suggest possible tests of the model, specifically in relation to the location and characteristics of the third image and the time delay between images.

This paper provides an astrophysical scenario for Hoyle's conjecture that quasars ejected at high velocities from active galactic nuclei radiate predominantly in a narrow backward cone. Backward emission is essential in a local Doppler theory in order to explain why no blueshifted quasars are seen.

The gravitational influence of minilenses which are located in lens galaxies and clusters and which intercept the light paths from multiply imaged quasars is investigated by developing an analytical framework. The mathematical formulation is adopted for deriving flux variations due to minilensing, and explicit light curves are computed numerically for a variety of minlens paths coming close to the images of the twin quasar 0957+561 A,B. It is demonstrated that under certain circumstances the operation of minilensing can lead to the vanishing of an image. This feature could provide a diagnostic probe of quasar structure and dark matter in the lenses and also serve as a test of gravitational lens models. Subject headings

Il semble que les objets compacts de masse M c , 1≤M c ≤3•10 4 M ○. , ne sont probablement pas des constituants de la matiere sombre dans la galaxie et l'amas lentilles correspondant au quasar double 0957+561 et dans l'eventuelle galaxie lentille correspondant a 1115+080

In the light of recently reported observations of the triple radio source 2016+112 by Schneider et al. (1985, 1986), gravitational lens models of this system are constructed based on two lens galaxies and a cluster. Various observations of this multiply imaged quasar system, including some of the features accompanying object C, are reasonably well reproduced. The admissible lens models involve a five-image configuration with two merging images rather than just three images. In this respect this lens system is analogous to the triple quasar 1115+080. In the models the emission line regions accompanying images A and B arise from two distinct sources. A measurement of time delays, a relation between conceivable VLBI structures associated with images, and further radio observations of the object C as possible tests of the lens models are suggested. 14 references.