INTRODUCTION

In the belief that only unkind gods would arrange two energy sources for planetary dynamos as equally important, this re-exploration of plausible sources seeks to eliminate rotational energy in favor of convection. Recent experiments and theory of the ‘elliptical’ instabilities in a rotating fluid due to precessional and tidal strains provide quantitative results for velocity fields and energy production. The adequacy of these flows to produce a. dynamo on both terrestrial and giant planets is assessed in the context of ‘strong field’ scaling. With little ambiguity it is concluded that Mercury, Venus, and Mars can not have a dynamo of tidal or precessional origin. The case for today's Earth is marginal. Here precessional strains (accidentally comparable to tidal strains) also are potential sources of inertial instabilities. The ancient Earth with its closer Moon, as well as all the giant planets, have tides well in excess of those needed to critically maintain dynamos. Hence the project proposed here proves to be successful only in part – an Earth in the distant future will not be able to sustain the geodynamo with its rotational energy. On the other hand, convection remains a possible dynamo energy source, with such a large number of undetermined processes and parameters that it is unfairly easy to establish conditions for its inadequacy. A large literature explores its adequacy. A brief review of this literature, in both a ‘strong-field’ and ‘weak-field’ context, advances several cautionary restraints to be employed on that day when the limits of validity of a quantitative dynamo-convection theory are to be determined.

INTRODUCTION

Many astrophysical bodies possess magnetic fields that arise from dynamo action. The case of the Earth is a unique one because the observational data available are much more detailed for the Earth than for any other astrophysical body, making possible a rather detailed comparison of geodynamo theory with observations. To meet this unique opportunity we therefore need a geodynamo theory that is very detailed. To develop the fully-fledged theory of such a complicated system as the geodynamo, even with the help of modern computers, it is however necessary to possess a qualitative understanding of its structure. This can be achieved by preliminary ‘scouting’ calculations of some artificially simplified models that are much simpler than the full geodynamo model but nevertheless help to understand it. A kinematic dynamo theory is the first step towards this goal. Kinematic models provide us with an understanding of its electrodynamics (the magnetic field generation process). The next necessary step is an understanding of its mechanics. The model-Z geodynamo emerges as a result of this step of scouting calculations. It may be considered as a specific case of a more general model that we call the nonlinear (pseudo-) axisymmetric dynamo model. This is a natural generalisation of the linear, nearly axisymmetric, kinematic dynamo model (Braginsky 1964a, b, c, d), and it is ‘intermediate’ between the kinematic and the complete theories of the geodynamo.

The nonlinear axisymmetric dynamo model aims at understanding the specific features of the main convective flow and the production of axisymmetric field in the core while the field generation due to the non-axisymmetric motion (a-effect) is considered as given. Another direction for an essential ‘intermediate’ investigation is to explore non-axisymmetric magnetoconvection.

Quasars, which can be a thousand times brighter than an ordinary galaxy, are the most distant objects observable in the Universe. How quasars produce the luminosity of 1013 suns in a volume the size of the solar system continues to be a major question in astronomy. Distant quasars are very rare objects whose study has been blocked by their scarcity. Recent technical advances, however, have opened new paths for their discovery. Forty quasars with redshifts greater than 4 have been found since 1986. Redshift 4 corresponds to a light travel time of more than 10 billion years. As a result, we are now able to probe the epoch shortly after the Big Bang when quasars may have first formed and to study the universe when it was less than a tenth its present age.

Quasars were one of the main discoveries thirty years ago that revolutionized astronomy. While they and the black holes thought to occur in their centers have become household words today, quasars are as enigmatic in many ways as they were when first discovered. Whatever their nature, they offer us views of the Universe never before seen, especially at distances far beyond what astronomers of the previous generation expected to see. In this chapter I wish to review briefly their history, how extraordinary their properties are, and how they serve as probes of the Universe to nearly as far as the visible horizon.

This book originated as a symposium at the American Association for the Advancement of Science annual meeting in San Francisco in 1989. The topic, The Farthest Things in the Universe, suggested itself to me as the most interesting and significant topic that people could hear about. An earlier AAAS Symposium had led to a book, The Redshift Controversy, that was still in use, and we hope that this volume will prove itself of similarly lasting interest.

Two of the original speakers, Hyron Spinrad of the University of California at Berkeley, and Patrick Osmer, then of the National Optical Astronomy Observatories, revised their pieces to bring them up-to-date for inclusion in this book. Further, Ed Cheng of the COBE Science Team and NASA's Goddard Space Flight Center agreed to write a new piece for inclusion in the book. We appreciate his taking time during the period of his duties as Chief Scientist for the Hubble Space Telescope's repair mission to complete his piece. During the interval from the time of the symposium to the present, the Cosmic Background Explorer spacecraft was launched and has had its tremendous successes in showing that the Universe has a blackbody spectrum and in finding ripples in space that may be the seeds from which galaxy-formation began. Thus this book appears at an optimum time.

The technical ability of astronomers to obtain images and spectra of very faint galaxies has improved greatly over the last decade. Since galaxies are vast collections of gas and stars, they must physically evolve with time. We should be able to directly observe the time-evolution of galaxies by studying very distant systems; the look-back internal corresponding to the mostdistant galaxies known in 1992 now approaches 15 billion years (80% of the total expansion age of the Universe)!

The line spectra of these faint galaxies are invaluable for redshift determination and physical study. The realization that Ly α (121.6 nm), formed in neutral hydrogen gas, is a strong emission line in most active galaxies and perhaps normal star-forming galaxies also, has helped us measure much larger redshifts in 1987–92 than was previously possible. Recall that this wavelength is in the ultraviolet; it can be observed only by satellites. But when galaxies are very far away, their Doppler effect shifts this spectral line into the region of the spectrum that we can observe with large telescopes on Earth. The largest redshifts for radio galaxies now approach z=3.8. Differing selection effects control which galaxies can be seen/isolated that far away. At least some red galaxies must form at redshift zf>5 (where the subscript f stands for the epoch of star formation).

When we look out into space at night, we see the Moon, the planets, and the stars. The Moon is so close, only about 380000 kilometers (240000 miles) that we can send humans out to walk on it, as we did in the brief glorious period from 1969 to 1972. Even the planets are close enough that we can send spacecraft out to them, notably the Voyager spacecraft, one of which has passed Neptune. Whereas light and radio signals from spacecraft take only about a second to reach us from the Moon, the radio signals from Voyager 2 at Neptune took several hours to travel to waiting radio telescopes on Earth. We say that the distance to the Moon is 1 light-second and the distance to Neptune is several light-hours.

Aside from our Sun, the nearest star at 8 light-minutes away, the distances to the stars are measured in light-years. The nearest star system is Alpha Centauri, visible only in the southern sky, and the single nearest star is known as Proxima Centauri, about 4.2 light-years away. We know so little about the stars that new evidence in 1993 indicates that Proxima Centauri might not be a member of a triple-star system along with the other parts of alpha Centauri, as has long been thought. The speeds at which those stars are moving through space may be sufficiently different that Proxima is only temporarily near Alpha's components.

Looking up at the clear night sky, it is hard to avoid wondering about the many objects that we can see. It is simple to recognize with the naked eye that there are planets, countless stars, and the band of light from the disk of our own Galaxy, the Milky Way. With the help of binoculars or a small telescope, the complexity of the scene increases dramatically, and it becomes apparent that the glow of the Milky Way is the light from many faint stars. We also start to notice that there are numerous faint and fuzzy objects which are the nearby galaxies and the star-forming regions in our own Galaxy. Probing with more and more sophisticated instruments, the level of detail and structure that can be resolved using visible light increases until the light becomes so exceedingly faint that even the best detectors on the largest telescopes see only darkness. This is the regime of the farthest objects in the Universe.

Before discussing these objects in any detail, I would like to take a brief moment and address the question of how we can possibly know about things so remote in both distance and experience. After all, we invent and test the physical sciences here on Earth by making experiments, interacting with the world around us, and creating a system of beliefs (theories) that ties all these experiments together into a consistent and testable story.

This contribution summarises our studies on the emission line profiles from compact Supernova Remnant shells and how they might be related to the broad line profiles in active galaxies. The emphasis is on the theoretical problems associated with radiative transfer effects in spherical and irregularly shaped shells. Line profiles from systems containing many compact remnants are also calculated with the aim of comparing the results to luminous active nuclei, where several remnants are expected to coexist. The observed diversity of profile characteristics in QSOs and the consequences it has to the starburst model are discussed.

Introduction

Line profiles of any astrophysical object, from stellar atmospheres to the Broad Line Region of active galaxies, provide valuable information on the physical and dynamical conditions which may be used to constrain or even reject theoretical models for such objects. Our goal in this work is to develop models for the emission line profiles in compact Supernova Remnants (cSNR) and Active Galactic Nuclei (AGN). The presence of both cSNR and AGN in the same title can only mean that we are talking about the starburst model for AGN of Roberto Terlevich and collaborators (see Terlevich et al. 1992 as well as Franco's, Plewa's and R. Terlevich's papers in this volume). Indeed, the main idea here is to see how well the starburst model does regarding the broad lines in active galaxies.

Collisions of galaxies are often observed to produce increases in the far-IR flux and star formation rates, as compared to those seen in isolated galaxies. It is expected that the star formation taking place in these systems occurs under more violent circumstances than those found in the spiral arms of disk galaxies, for example. We will present some results from a study in which we have produced combined N-body/Smooth Particle Hydrodynamics simulations of collisions of galaxies, looking for regions in which shocks develop and where the gas density gets high enough that new star formation might be expected to take place. These results give us a preliminary idea of what the properties of the shocked, high density regions might be and provide a basis for the future inclusion of violent star formation into such calculations with a view to eventually explaining the observed enhancements.

Introduction

In seeking out places in the present Universe where star formation might be expected to occur under more violent circumstances than those that occur in the arms of spiral galaxies one is drawn to consider the interiors of gas-rich galaxies that are undergoing a collision with another galaxy. Although not extremely common at this epoch, such systems of galaxies have been detected and extensively studied during recent years. There are now considerable data available to suggest that these systems commonly experience increased star formation, this taking place most often in the nuclei of the disk galaxies, rather than in their spiral arms.

The star formation history in the nuclei of late-type spiral galaxies is compared between a sample in a high galaxy density medium (HDS) and a control sample (CS) of isolated galaxies. We have observed 20 HDS and 18 CS galaxies selected from a larger list generated by the application of a group-finding algorithm to the SSRS survey. Using equivalent widths of absorption lines and the continuum distribution, we determined the nuclear stellar population types, from those dominated by old populations to those containing star formation bursts of different ages and intensities. The HDS and CS stellar population type histograms are similar, suggesting that environmental influences, at least for the present samples, do not substantially affect the nuclear stellar population. However, the nuclear emission lines indicate that, in the BPT diagnostic diagrams, there is an excess of HDS galaxies located within or close to the AGN loci. For 6 HDS and 2 CS galaxies it was possible to determine Oxygen (O/H) and Nitrogen (N/H) abundances. The samples present similar (O/H) values, but in the CS galaxies the (N/O) ratio is lower at equal galaxy luminosity.

Introduction

Evidence of several environmental effects that affect galaxy properties have been reported recently: (a) the morphology-density relation (fractional increase of early-type galaxies towards regions of high concentration, Dressier 1980; Postman & Geller 1984; Giovanelli, Haynes & Chincarini 1986; Maia & da Costa 1990); (b) the morphology-clustercentric radius relation (Whitmore, Gilmore & Jones 1993).

The vertical distribution of molecular complexes along the Carina-Sagittarius arm has been studied on the basis of the giant molecular cloud (GMC) data compiled by Myers et al. (1986). The analysis indicates that the CO complexes are preferentially located below the formal Galactic plane. A separation of the sample into two groups: (a) GMCs associated with HII regions, and (b) GMCs without associated HII regions, establishes that group (a) shows, in average, larger z departure and mass than group (b). This result seems to suggest that the star formation activity in this major arm displays a vertical asymmetry which opens up interesting questions about the triggering mechanisms of star formation in spiral arms.

The density and location of young stars in major spiral arms and the relation to their parent molecular clouds are important to the understanding of how molecular clouds evolve and form stars in our Galaxy. In previous work (Alfaro et al. 1992, 1993) we analyzed the vertical structure of young open clusters (YOCs) along the optical segment of the Carina-Sagittarius arm, and its connection with the density of YOCs as representative of star formation activity. The main conclusions of that work can be summarized as follows:

1. A clear correlation between YOC density and z-departure from the formal Galactic plane is found when this density distribution is compared with the vertical structure. The cores of both supercomplexes are closely coincident with the two minima of the vertical profile, and the regions of lowest star-forming tracers appear associated with the relative maximum of z.

Super-associations and star complexes

The concept of a “superassociation” was first introduced by Baade (1963) in his Harvard lectures in 1958. He gave this name to a region about 500 pc across around the giant HII region 30 Dor in the LMC which is full of OB-associations. The same region was the first example of a super-association given by Ambartsumian (1964). Altogether, 19 OB-associations and young clusters here form a morphological unit of 1 kpc in diameter with evident hierarchical structure (Efremov 1988, 1989).

One may say that a “super-association” is the counterpart of a hydrogen emission nebula (HII region) in B, V etc. broad bands. Wray and de Vaucouleur (1980) have shown that in the B bandpass the continuum-to-emission ratio is always greater than 10:1. Thus in this band one deals mainly with the star population of a super-association.

Nevertheless, the diameter of the 30 Dor HII region is only 250 pc and it occupies less than 0.1 of the total area of the super-association, the remaining HII regions here being much smaller. This may well also be the case for extragalactic super-associations – giant HII regions. In bright star cloud NGC 206 in M31, named by Baade (1963) as a real super-association, there are only a few small HII regions, not seen at all on the B plates.

In many respects super-associations (SAs) are similar to common star complexes (Efremov 1978, 1993), the main difference being the richness of an SA in HII gas and OB stars that causes the high total luminosity.

As part of the GEFE collaboration, observations of star-forming regions with high spectral resolution and long-slit sampling are being undertaken. 2D maps of physical parameters like density, excitation, extinction…etc. have been produced with 1″ spatial resolution and 2″ spatial sampling. Some preliminary results on the giant HII Region NGC 5471 and the irregular galaxy NGC 4214 are presented. Very high velocity components have been detected at some particular positions on the nebulae, as well as other peculiar kinematical structures (redshifted secondary emission peaks, line splitting etc.). The whole emitting area of NGC 5471 behaves as a unique entity with respect to excitation, with no correlation with the emitting knots. On the other hand, differentiated star-forming regions can be identified on NGC 4214. Finally, the distribution of dust particles seems to be rather inhomogeneous and anticorrelated with the distribution of emission-line intensities.

Introduction: aim and targets

The ultimate aim of the GEFE collaboration is to determine which are the physical parameters that control the formation of a violent burst of star formation. Within this framework and in order to fulfil this main objective it is important to know the physical properties of star-forming regions with high enough spatial resolution as to determine variations of the measurable parameters within the emitting nebulae. We aim to use measurements of age, excitation degree, velocity dispersion and chemical composition to know whether we are dealing with single star-forming regions or with well differentiated physical entities within a patch of ionized gas, which cause misinterpretation in our understanding of the “physical object” (Muñoz-Tuñón et al. 1993).