In general the term ‘active galactic nucleus’, or AGN, refers to the existence of energetic phenomena in the nuclei, or central regions, of galaxies which cannot be attributed clearly and directly to stars. The two largest subclasses of AGNs are Seyfert galaxies and quasars, and the distinction between them is to some degree a matter of semantics. The fundamental difference between these two subclasses is in the amount of radiation emitted by the compact central source; in the case of a typical Seyfert galaxy, the total energy emitted by the nuclear source at visible wavelengths is comparable to the energy emitted by all of the stars in the galaxy (i.e., ∼ 1011L⊙), but in a typical quasar the nuclear source is brighter than the stars by a factor of 100 or more. Historically, the early failure to realize that Seyferts and quasars are probably related has to do with the different methods by which these two types of objects were first isolated, which left a large gap in luminosity between them. The appearance of quasars did not initially suggest identification with galaxies, which is a consequence of the basic fact that high-luminosity objects, like bright quasars, are rare. One is likely to find rare objects only at great distances, which is of course what happens with quasars. At very large distances, only the star-like nuclear source is seen in a quasar, and the light from the surrounding galaxy, because of its small angular size and relative faintness, is lost in the glare of the nucleus. Hence, the source looks ‘quasi-stellar’.

The narrow-line region (NLR) in AGNs is of interest for at least three interrelated reasons. First, the NLR is the largest spatial scale where the ionizing radiation from the central source dominates over other sources. Second, the NLR is the only AGN component which is spatially resolved in the optical – this is of particular importance as the NLR is clearly illuminated in a non-isotropic manner by the central source. Finally, the NLR dynamics might tell us something about how AGNs are fueled.

Narrow-Line Spectra

As in the case of the BLR, the relative strengths of the emission lines we observe in NLR spectra allow us to discern some of the properties of the ionizing spectrum. Unlike the BLR, the electron densities in the NLR are low enough that many forbidden transitions are not collisionally suppressed. This allows us to use the intensity ratios of certain pairs of forbidden lines to measure the electron densities and temperatures in the NLR gas (§6.2). In comparison to the BLR, the analysis is simplified by the low densities. On the other hand, however, in the case of the NLR an additional complication is introduced into the spectroscopic analysis by significant amounts of dust since the NLR arises outside the dust sublimation radius (eq. 4.15); indeed, it may well be that the radius where dust sublimates provides the fundamental demarcation between the BLR and the NLR.

In contrast to what was believed for the first twenty years of AGN studies, the continuum spectra of AGNs are quite complex. As mentioned in Chapter 1, at least to a low-order approximation the SED of AGNs can be described as a power law of the form Fv ∝ v-α, where α is generally between zero and unity. This led to the initial suspicions that this continuum is non-thermal in origin. It is certainly tempting to attribute the bulk of an AGN spectrum to synchrotron emission, primarily because of the broadband energy characteristics of the emission and because of the similarity of the spectra to known synchrotron sources such as supernova remnants and extended radio sources. By the end of the 1970s, the best working model to produce the broad-band continuum was the synchrotron self-Compton (SSC) mechanism. Given a power-law distribution of energies, relativistic electrons in a magnetic field can produce a synchrotron power law spectrum over many decades of frequency. Moreover, it is possible in principle to produce the higher-energy emission, all the way up to X-rays, via the SSC process; the SSC process becomes important when the synchrotron radiation density becomes sufficiently high that the emitted photons are inverse-Compton scattered off the very electrons which are responsible for the synchrotron radiation. The major difficulty in understanding whether a particular source of radiation is pure synchrotron emission or SSC is that SSC-boosted photons have the same relative energy distribution as the original photons, thus providing no unique indication of the process.

One of the main goals of QSO research is to use these objects as a probe of the history of the Universe. Two specific aims are first, to determine the characteristics of the QSO population as a function of redshift, and second, to find the lookback time at which QSOs first appeared, as this provides some measure of the time scale for galaxy formation in the early Universe. Both of these important aims require large and preferably unbiased samples of QSOs. In this chapter, we consider how large samples of QSOs might be obtained through various survey techniques, and how the samples we obtain might be affected by various biases.

The measurable quantity that will result from surveys is the QSO ‘surface density’ dN(F,z)/dΩ i.e., number of QSOs per unit solid angle (square degree) as a function of flux F and redshift z. From this, we can compute the ‘luminosity function’, which is the relative number of AGNs at a given luminosity, and the ‘space density’, which is the total number of sources per unit comoving volume! over some specified luminosity range – when the luminosity function is correctly normalized, the total space density is simply the integral of the luminosity function over its entire range.

The primary goal of QSO surveys then is to determine dN(F,z)/dΩ in an accurate and unbiased fashion. This is a difficult and complicated undertaking because QSOs are faint and their surface density is low; the total surface density of QSOs brighter than B = 21 mag is only ∼40deg-2.

As we pointed out in Chapter 3, the main problem with sustaining an active nucleus by gravitational accretion over its lifetime of at least 108 years is funneling enough mass into the nucleus. Removing a sufficient amount of angular momentum from the gas flowing into the nucleus requires breaking the azimuthal symmetry of the galaxy's gravitational potential. A clear way to do that is by gravitational interactions with other systems, as was originally suggested by Toomre and Toomre (1972) and by Gunn (1979). This provides motivation for examining the nearby environment of AGNs to see if indeed there is evidence for interactions with nearby galaxies. The two specific questions that we want to consider are:

1. What kinds of galaxies harbor AGNs? Are there any discernible differences between galaxies with active nuclei and those without them?

2. Does the presence or absence of companion galaxies have anything to do with whether or not a galaxy harbors an AGN?

We will consider these issues separately, although they are clearly related.

Host Galaxies

The study of the ‘host galaxies’, those galaxies that contain active nuclei, is a very difficult undertaking. The major problems were alluded to at the beginning of this book: the light from the AGN itself often dominates the total light from the galaxy, particularly in the case of the highest-luminosity AGNs, which are spatially rare and thus typically found only at great distances. Consequently the work on the lowerluminosity end of the AGN distribution, i.e., Seyfert galaxies, has tended to yield less ambiguous results.

Like many textbooks, this one arose out of the author's frustration. While I believe that there are many excellent journal articles, scholarly reviews, conference proceedings, and even a few advanced monographs on active galactic nuclei (AGNs), there is no single place where a beginning student can get the very basic background necessary to get the most out of the more research-oriented material. The aims of this book are thus actually twofold: first, I wanted to summarize our basic, if marginal, understanding of AGNs at what I believe is a level of familiarity that should be expected of doctoral-level students in astronomy, and second, I wanted to provide a fairly comprehensive introduction to AGNs that would serve as a gateway to the more specialized review articles and research literature for students who have research ambitions in the field. The intended audience is thus advanced undergraduate and beginning graduate students in astronomy and astrophysics. Fairly complete undergraduate preparation in physics is assumed, as is some basic understanding of extragalactic astronomy.

I have tried to focus on basic issues and avoid minutiae and arcane issues, even though some of these undoubtedly will turn out to be tremendously important in the future. I have attempted to compile the basic background material that is by and- large familiar to researchers in AGNs, although I caution that it is by no means complete: research-level competence in the field of AGNs will require a good deal more background than is given here.

Review of the Intellectual State of the Art

In this book, the concepts and theories behind spacecraft–environment interactions were examined. Each chapter is illustrated with selected examples. On the basis of these examples and theories, a reader should be able to identify and construct simple, first-order estimates of the principal interactions of importance to a specific spacecraft. The discipline of spacecraft–environment interactions represented by this process is, however, continuing to evolve both in its intellectual underpinnings and in its value to the spacecraft designer. In this final chapter, the current state of the art in spacecraft–environment interactions is reviewed and the future direction of progress in the field is predicted.

Neutrals

The primary interaction concerns for the neutral atmosphere are drag, atomic oxygen erosion, glow, and contamination. Although the overall processes associated with spacecraft drag in LEO or polar orbits are reasonably well understood, the detailed effects are often hard to predict accurately. The neutral environment for the Earth and its reaction to the Sun are, in principle, moderately well understood. Indeed, statistical models have been built that offer reasonable accuracy. However, there are still outstanding questions to be answered as to how solar and geophysical activity couple to the atmosphere and how to model the often almost impulsive atmospheric responses to sudden changes in these parameters. Errors as high as factors of 10 to 100 in predicting the density along an orbit are not unusual. Once the neutrals strike the spacecraft surface, the accommodation of the neutrals on the surface becomes an issue.

α Cygni variables

According to the nomenclature of the GCVS, luminous variable B and A supergiants are called α Cygni variables, and are classified among the pulsating variables. The class also includes massive O and late type stars, since these belong to the same evolutionary sequence. In the MK spectral-classification system, they have luminosity classes Ib, Iab, Ia and Ia+ (in increasing order of luminosity). The most luminous supergiants are also called ‘hypergiants’ - these are, in fact, Luminous Blue Variables (LBVs). Ia supergiants are pre-LBV objects, therefore we also refer to Section 2.1 for all details that are related to both groups of variables. All OBA supergiants are variable (Rosendhal & Snowden 1971, Maeder & Rufener 1972, Sterken 1977). The amplitudes of the most luminous supergiants resemble the microvariations observed in LBVs during quiescence, the level of variability increases towards higher luminosities for all spectral classes.

Pulsational instability accounts to some extent for the semi-regular variations (Leitherer et al. 1985, Wolf 1986) - it should also be noted that the β Cep instability strip widens into the supergiant region. The amplitudes of the variations seem to increase with the time scales at which they occur.

HD 57060 = UW CMa and HD 167971 are two interesting cases of microvariations. HD 57060 (Fig. 3.1) is a binary consisting of an O8 supergiant star and an O or B type main-sequence star in synchronous revolution with a period of 4d39.

Variable stars

All stars display variations of brightness and colour in the course of their passage through subsequent stages of stellar evolution. As a rule, however, a star is called variable when its brightness or colour variations are detectible on time scales of the order of the mean life time of man. The variations may be periodic, semi-periodic or irregular, with time scales ranging from a couple of minutes to over a century. It is this kind of variable star which is the topic of this book. The typical time scale, the amplitude of the brightness variations, and the shape of the light curve can be deduced from photometric observation, and those quantities place the star in the appropriate class. For example, a star of the UV Ceti type typically has brightness variations (the so-called flares) of several magnitudes in an interval of time as short as a few minutes, whereas a Cepheid shows periodic variations of about one magnitude in a time span of several days. However, spectral type, luminosity class and chemical composition are complementary important spectroscopic parameters that are needed for classifying variable stars according to the origin of their variations.

At the very beginning of the space age, spacecraft designers learned that the effects of the space environment on a spacecraft's systems would be vital factors in spacecraft design and operation. Since those early years, the topic of spacecraft–environment interactions has developed into a multidisciplinary field involving engineers and scientists from all over the world. Traditionally, engineers have been interested in spacecraft design and operational issues, and scientists have concentrated on the fundamental physics and chemistry associated with the interactions. These diverse interests have led to numerous books and conferences. The field has grown substantially in the past decade with the advent of the Shuttle and the ability to perform repeatable, in-situ experiments. The authors therefore concluded that, with the growth of the field and the expanding interest in it, it was timely to prepare a comprehensive book summarizing the many recent discoveries. In particular, since the field has evolved in a way that has been driven by mission and spacecraft requirements rather than as a specific discipline, a book would be a valuable step in integrating the field intellectually. Such a book would also serve as an introduction to the discipline for graduate students and professionals. For specific applications, these individuals could then turn to one of the handbooks or collections of conference papers referenced throughout the book.

This book is the direct outgrowth of courses that the authors have taught.