Introduction

The detection of dark matter in astronomy has a long history. In past years it was called “the astronomy of the invisible”. The story begins in 1844 when, by chance, two different dark matter problems were identified. In that year it was noted that the planet Uranus had moved away from its calculated position by as much as two minutes of arc. In the same year F. W. Bessell drew attention to the sinuous motion of the star Sirius, the brightest star in the sky.

The subsequent development of the Uranus problem led to one of the most famous stories in the history of astronomy. In 1845 J. C. Adams, who had just ceased to be an undergraduate at Cambridge University, succeeded in calculating fairly accurately the position of a hypothetical planet whose gravitational effect on Uranus might be responsible for its disturbed motion. He attempted unsuccessfully to interest the Astronomer Royal G. B. Airy in this prediction. Apparently Airy had attributed the discrepancy to a departure from Newton's law of gravity. Perhaps also he was unimpressed by the student's youth.

Independently of Adams, in 1846 the Frenchman Le Verrier calculated the position of the hypothetical planet with a precision of 1 degree. (For a much shortened version of the needed calculations see Lyttleton 1958). Le Verrier contacted a German astronomer, Galle, at the Berlin Observatory, who rapidly succeeded in observing a new planet (Neptune) within 1 degree of the predicted position. The discovery of this planet (no longer “dark”) is widely considered to be a triumph of nineteenth century science, and naturally became the subject of chauvinistic controversy.

Introduction

We saw in the last chapter that the Milky Way contains diffuse ionised gas (DIG) with a large scale height. We also saw that there is strong, but not decisive, evidence that conventional sources in the Galaxy are not adequate to account for the observed ionisation. What seem to be needed are sources which are smoothly distributed, so that the opacity of the neutral hydrogen can be overcome, and which possess a large enough scale height to account for the large scale height of the DIG. Dark matter neutrinos in the Galaxy would be expected to possess both these properties, as we discuss in detail in the next part of this book. If the radiative decay of these neutrinos is to be a serious candidate for the ionization source of the DIG in our Galaxy, we would expect to find the same ionisation problems in nearby galaxies whose structure is similar to ours. This is the subject of the present chapter.

There is one advantage and one disadvantage in studying the ionisation in other galaxies. The advantage is that by observing from a point outside the galaxies it is easier to discover the global properties of the ionisation. The disadvantage is that pulsars are not observable in other galaxies (except the Magellanic Clouds), so that we cannot use the pulsar dispersion measure to determine the distribution of the electron density, and have to rely on measurements of Hα and other emission lines. As we shall see, it has been possible by these means to observe the DIG in nearby galaxies and to discover that conventional ionisation sources in these galaxies again seem to be inadequate.

Introduction

In this chapter we study the implications of the neutrino decay theory for the reionisation of the universe and the consequent suppression of fluctuations in the microwave background. We saw on page 48 that we expect the early high temperature universe to have become neutral at a red shift ∼ 1000, when it had cooled down to a temperature ∼ 3000 K. On the other hand we know from considerations of the Gunn-Peterson effect that the intergalactic medium is highly ionised at redshifts between zero and 4.9. The questions then arise, at what red shift between 4.9 and 1000 did the reionisation occur, and by what process?

These questions, and the general thermal history of the universe, have been much discussed. They are obviously relevant to our understanding of the processes of galaxy formation. In addition it has long been realised that they play a crucial role in determining the present anisotropy ΔT/T of the microwave background on small angular scales. As has often been discussed (e.g. Efstathiou 1988 and references cited therein), if the postrecombination universe had been reionised so early that its optical depth for Thomson scattering exceeded unity, then the ΔT/T induced by fluctuations associated with galaxy formation after recombination at z ∼ 1000 would have been severely attenuated by z = 0. This is an important question because the present stringent observational limits on ΔT/T at small angular scales would impose severe constraints on several theories of galaxy formation in the absence of a scattering screen.

Introduction

The evidence which has been accumulated in this book relating to our neutrino decay hypothesis is strong but circumstantial. It is crucially important to test the validity of the hypothesis by attempting to make a direct detection of the postulated radiation. Fortunately the kinematics of the decay imply that the emitted photons are monochromatic, so that the radiation from a given source, if strong enough to be detected, would show up as an unidentified line broadened by the velocity dispersion of the neutrinos in the source. Had the emission possessed a continuous spectrum it would have been much more difficult to distinguish it convincingly from radiation of a conventional origin.

Since the line is predicted to have an energy Eγ ∼ 15 eV, the problem of detectability is tied up with the high opacity of the interstellar medium for radiation of this energy. This problem is a natural one since the opacity is mainly due to the photoionisation of neutral hydrogen, the very process which originally led to the postulate that the decay radiation lies in this energy region. It does mean that care must be taken to choose a suitable observing target.

For example, a number of attempts were made to detect decay photons from dark matter in the Virgo and Coma clusters under the stimulus of the earlier neutrino decay theories of Cowsik (1977) and de Rujula and Glashow (1980). These attempts were made by Shipman and Cowsik (1981), Henry and Feldman (1981) and Holberg and Barber (1985).

Introduction

We saw in chapter 6 that some nearby spiral galaxies contain diffuse ionised gas (DIG) reminiscent of the Reynolds layer in our Galaxy. This DIG has been studied in particular detail in NGC 891. It was found difficult to account for the DIG observed in that galaxy several kiloparsecs from its plane in terms of known sources of ionisation. The observers concerned therefore concluded that a new galactic source is required, a conclusion which is reminiscent of the situation prevailing for the Reynolds layer in our Galaxy. In this chapter we examine the hypothesis (Sciama and Salucci 1990) that the new source required is decaying dark matter neutrinos with the same properties as we have already invoked in discussing the Reynolds layer in the previous chapter.

This hypothesis has been criticised by Dettmar and Schulz (1992) on the grounds that the decay photons would not heat the gas to the temperature required to account for the emission line ratios [NII]/ Hα and [SII]/ Hα which they observed. This criticism suffers from the defect that in their calculation they assume that the only heat source for the gas is the decay photons themselves. Since in the decaying neutrino theory Eγ is close to 13.6 eV, it is true that there is not much heat input associated with each ionisation. Indeed this point is relevant to our discussion of the temperature of Lyman α clouds in chapter 11. However, in the present case one would expect that other heating processes should be important.

Introduction

The idea that there may be significant quantities of dark matter distributed smoothly throughout the universe as a whole developed gradually. In its modern formulation the idea is based on a number of considerations. The first concerns the role played by the mean density of the universe in the cosmological models of general relativity. These models have a fundamental status in discussions of cosmological dark matter, and so we devote much of this chapter to an account of them.

The second consideration concerns the mean density of ordinary matter in the universe. By “ordinary matter” I mean atoms, neutral or ionised, which are collectively referred to as baryonic. Estimates have been made of the contribution of visible baryons to the mean density of the universe using direct astronomical measurements. An estimate has also been made, using indirect arguments, of the total contribution of baryons, visible and invisible, to the mean density. This estimate is based on a comparison of the measured abundances of certain light elements (D, He3, He4 and Li7) with the calculated output of thermonuclear reactions occurring in the “first three minutes” after the hot big bang origin of the universe. These arguments will also be described in this chapter. We shall find that, according to modern estimates, the mean density in visible baryons is significantly less than the total mean density in baryons. If these estimates are correct, an appreciable number of baryons must be dark.

The third consideration concerns the contribution of nonbaryonic matter to the mean density of the universe. Various forms of more or less “exotic” matter have been proposed under this heading.

Abstract

Balmer emission line variations of the integrated flux as well as profile variations for the Seyfert galaxy NGC4593 are presented. This active galaxy was observed in an international monitoring campaign. The broad emission line profiles consist of at least three components varying in an independent way. The FWZI of the broad line profiles of the Hα line remained constant during the intensity variation indicating a turbulent velocity field of the BLR.

Introduction

It has been shown by simple kinematical model calculations that the response of the broad line profiles to an outburst of the central continuum source of an active galactic nucleus (AGN) is different for radial, rotational, and turbulent motions (Pérez, Robinson, de la Fuente 1992, Welsh, Home 1991). The detailed study of variations of the broad emission line profiles provides a powerful tool to investigate the structure and the kinematics of the central broad-line region (BLR), which cannot be directly resolved. Therefore, the investigation of the temporal evolution of broad emission line profiles with respect to continuum variations yields information for distinguishing between these possible types of the kinematics of the BLR gas. The analysis of the variability of emission line profiles requires spectra with high spectral resolution and high signal-to-noise ratio monitored over at least several months with a sampling rate of a few days.

Abstract

Thirteen years of IUE observations of the Seyfert 1 galaxy Fairall-9 have shown a large variability in the UV continuum and UV emission lines, Lyαλ1216 and CIVλ1550. The relation between UV continuum and X-ray (no delay) is similar to that found for lower luminosity objects and suggests that X-ray reprocessing causes the UV continuum. We also use the line variability to support and refine a gaussian decomposition of the profiles of Lyα and CIV, based on a previous study of Hβ. The decomposition of Lyα and CIV shows a well identified component structure valid for both lines. The components respond very differently to the changes in the ionizing continuum brightness, confirming the different physical nature of the material associated with them.

The UV-X ray continuum relation

We show in figure 1 (1a and 1b) the UV and X-ray light curves of F-9, while fig. 1c and 1d show relations between these two quantities. Although the time resolution in the CCF (1d) is of course limited by the sparse X-ray sampling no evidence is found for delays of the size suggested by the IR (400 days). We see here also that at high UV brightness the rather tight correlation between the 2-10 keV flux and F(1338Å) breaks down. Although the details of this behaviour are currently not fully understood it has been suggested that at low levels the UV continuum is the result of reprocessing of X-rays emitted above the disk while the huge UV variations could be associated with major accretion events.

Abstract

This is a short summary of several detailed calculations of strong radiative cooling behind supernova shock waves evolving in a high density medium. These lead to definite predictions about the lag, the observed delay between sudden changes in the continuum ionizing radiation followed, after some time, by changes in the intensity of the emmision lines from the broad line region of AGNs. A full description of these results is due to appear soon in a journal.

Introduction

In the starburst model of AGNs, sometimes viewed as exotic and/or unconventional, the applied physics are in fact most conventional, as it uses the little, or the lot, that we know about real events: the physics of stars and stellar evolution and their interaction with the surrounding gas, and with these predictions are made. In this model, the observed broad emission lines and their variability, are generated by “compact”, strongly radiative supernova remnants which are expected to occur in the central regions of early type galaxies undergoing a violent nuclear burst of star formation. The activity of all stars and the frequency of supernova explosions soon establishes a high pressure region around the cluster. This high pressure, as it acts upon the slow winds from the massive stars (M ≥ 8M⊙), leads to the development of a high density circumstellar medium (n0 ≥ 107 cm−3) around each of the potential supernova stars.

Abstract

We describe constraints on the metallicity of quasar broad line region gas. The overall emission line spectrum is surprisingly insensitive to order of magnitude changes in the global metallicity Z. Indirect methods, employing photoionization models and explicit stellar chemical evolution models of the selective enrichment of the elements, must be used to infer the metal enrichment. Two line ratios, both involving NV λ1240, are developed to measure Z. The first is the ratio of NV to the collisionally excited line CIV λ1549. The second is the ratio of NV to the recombination line HeII λ1640. Both indicate nitrogen enhancements exceeding an order of magnitude above solar. These results imply high metallicities in high redshift quasars, a property they have in common with the cores of massive galaxies.

Introduction

One of the longest standing goals of AGN emission line research has been to use these lines to measure the composition of the emitting gas. This could then test models of both the quasar phenomenon and stellar nucleosynthesis (Davidson and Netzer 1979; Shields 1976). Fundamental uncertainties concerning the nature and geometry of the BLR have made this work difficult.

Variability studies have rejuvenated interest in the BLR by providing methods to directly measure quantities such as the distance between continuum source and emitting clouds. These have shown that the clouds are both denser and exposed to a far more intense radiation field than had been inferred (Peterson 1992; Ferland and Persson 1989; Ferland et al. 1992).

Abstract

The polarization properties of a two-phase model, recently proposed to explain the X-ray emission of Active Galactic Nuclei, have been calculated for different values of the model parameters. An important signature of the model is the orthogonality between the UV/soft X-ray and hard X-ray polarization.

Recently, a two-phase model in which hot, thermal electrons in an optically thin layer comptonize the soft photons coming from an underlying cold, optically thick accretion disc, has been proposed to explain the X-ray emission of Active Galactic Nuclei.

Assuming a plane-parallel geometry, and isotropic and unpolarized disc thermal radiation, we have calculated the polarization properties as a function of the energy and of the inclination angle, for different values of τ0, the optical depth of the hot phase (which, in the adopted model, is related to the electron temperature). This was done by solving the well-known equation of radiative transfer by separating the different scattering orders. The polarization of the X-rays reflected from the disc has also been taken into account. In the figure we show the degree of polarization as a function of the energy for different values of the inclination angle (at the two extremes of the energy range ∣P∣ increases with it). The assumed energy shape of the thermal radiation is a black-body with T=50eV. Note that the hard X-rays have a negative polarization (i.e. the polarization vector lies in the meridian plane), while the polarization of the UV/soft X-rays is positive (i.e. the polarization vector is perpendicular to the meridian plane).

Abstract

The strong evolution of the host object population postulated in hierarchical models for structure formation is invoked to explain the observed strong evolution of the space density of quasars. The quasar activity is interpreted as marking the advent of a new step in the hierarchic build-up of bigger and bigger dark matter halos. The Press–Schechter formalism within the CDM scenario is used to estimate the number of newly forming dark matter halos. Pronounced peaks are found in the number density of newly forming massive black holes, capable of explaining the short time scale of the evolution of the quasar population. A gratifying fit to the observed luminosity function is obtained.

Quasar evolution in the CDM scenario

Soon after the discovery of the first quasars it was noticed that quasars are a strongly evolving population of objects. With the increasing number of known intermediate redshift quasars it became possible to determine the time evolution of the luminosity function of quasars. The main feature of the luminosity function is a characteristic break luminosity which decreases with time. The quasar luminosity function is most naturally interpreted as a superposition of many generations of short-lived quasars with a life time ∼ 108yr and a characteristic mass that decreases with time as ∼ (1 + z)3.

In hierarchical models for structure formation, such as the CDM scenario, larger and larger structures build up by merging of smaller structures and the smaller structures are at least partially erased.

Abstract

Short-wavelength IUE archival data for NGC 3783 were re-extracted and analysed to constrain numerical modelling parameters in detailed photoionization analyses. The He IIλ1640, C IVλ1549, and C IIIλ1909 line intensities and trends can be reasonably well reproduced by a two cloud component model. In order to produce satisfactory line intensities or trends for other higher ionization lines and lower ionization lines, still more gas components are necessary with gas density ranging from roughly 1011 cm−3 to 109 cm−3 or less. In going from the inner to outer clouds, the optical depth increases and the gas density decreases approximately as r−2.

The amount of dust obscuration along the line of sight, as required by the models, is consistent with reddening estimates from HeII line ratios, and CNO abundance ratios derived from intercombination line intensities suggest abundances of carbon, nitrogen and oxygen lower by a factor of about 2 relative to solar. The HeII line rest equivalent widths from the models suggest a gas covering factor of order 0.25.

Introduction

Although sophisticated numerical photoionization models have been employed for two decades in investigations of the broad–line–emitting regions (BLRs) of Seyfert 1 galaxies, some of the most fundamental BLR properties such as the intrinsic ionizing radiation field, the ionization parameter, emitting gas density, chemical abundances, and the “covering factor” (the fraction of solid angle occupied by optically thick gas around the ionizing radiation source) are still open to question.

Abstract

A realistic stellar cluster potential may have an observable effect on the thermal structure of an AGN accretion disc. Optimization of model parameters has found an extremely good fit to the broad-band spectrum of 3C 273 for reasonable assumptions about the central black hole and surrounding cluster.

Introduction

Various authors have shown that compact stellar clusters around AGN Black Holes are important in determining the properties and evolution of Active Galactic nuclei, e.g.. We describe a way of directly determining the mass and size of such clusters in certain cases.

Radiation from an AGN accretion disc derives, at least in part, from the local release of binding energy through viscosity. Here we assume that accretion is the sole energy source: the spectrum of the disc is determined by the potential into which it is falling. We derive an estimate of the disc spectrum by assuming black-body emission.

In a forthcoming paper, we find the spectrum expected from this model AGN, in which a dense, young stellar cluster surrounds a massive Black Hole. Here, we show a fit of this model to the continuum of 3C 273 (in its low state).

A Model of 3C 273

In Perry & Williams, we derive cluster parameters from the parameters of the fit given by. We have optimized the fit of our model to the data, (Fig. 1). In this figure, the different symbols identify different observations; the points near 1021 Hz are upper limits.