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In recent years, the discovery of massive quasars at
has provided a striking challenge to our understanding of the origin and growth of supermassive black holes in the early Universe. Mounting observational and theoretical evidence indicates the viability of massive seeds, formed by the collapse of supermassive stars, as a progenitor model for such early, massive accreting black holes. Although considerable progress has been made in our theoretical understanding, many questions remain regarding how (and how often) such objects may form, how they live and die, and how next generation observatories may yield new insight into the origin of these primordial titans. This review focusses on our present understanding of this remarkable formation scenario, based on the discussions held at the Monash Prato Centre from November 20 to 24, 2017, during the workshop ‘Titans of the Early Universe: The Origin of the First Supermassive Black Holes’.
A few comments first on education in schools - this is a special worry here in the UK, where our international rankings are disappointing. An appreciation of science is vital not just for tomorrow's scientist and engineers, but for everyone who will live and work in a world even more underpinned by technology - and even more vulnerable to its failures and misapplications - than the present one. Even more important, the option of higher education in science and technology should not be foreclosed to them. There is widespread concern particularly about the 16-18 age group. Many of us put strong emphasis on broadening the curriculum for this group, which currently enforces unduly early specialisation here in England. Young people opting for humanities should not drop all science when they are 16.
About 107 L⊙ of luminosity, mainly in ionizing flux and infrared radiation, emerges from the central pc3 of our Galaxy. This exceeds the luminosity from the corresponding region of most nearby galaxies, though it is surpassed by M82 and NGC 253 (Reike and Lebofsky 1982), but perhaps involves nothing more exotic than a starburst 106 − 107 years ago. But the manifestations of activity at the Galactic Centre that are unambiguously non-thermal in character are at a much lower level: the γ-ray annihilation line (~ 1038 erg s−1) and the compact radio source (~ 1034 erg s−1). I shall comment briefly on these two phenomena, and also suggest an interpretation of the remarkable pseudo-spiral structures revealed by the NeII infrared and the radio-continuum maps. These phenomena relate to the old question (cf. Lynden-Bell and Rees 1971) of whether our Galaxy has ever experienced a more violent phase, leaving a massive collapsed remnant.
The observed superluminal components have (deprojected) lengths of ~ 1020 cm, and imply relativistic bulk motions on these scales. There are, however, persuasive reasons for attributing the primary energy production to scales 1014–1015 cm. Moreover, the initial bifurcation and collimation must also be imposed on these small scales if the long-term stability of the jet axis in extended sources is due to the gyroscopic effect of a spinning black hole (Rees 1978). The issues I shall address in this talk are: how the jet gets from ~ 1015cm to ~ 1019 cm; and what VLBI data can tell us about the properties of galactic nuclei on scales below ~ 1019 cm — scales where optical and X-ray studies provide some evidence, but where there is no short-term hope of achieving spatial resolution.
There still seem to be three serious contenders for the dark matter in galactic halos and groups of galaxies: (i) very low mass stars, (ii) black hole remnants of very massive stars or (iii) some species of particle (e.g. axions, photinos, etc.) surviving from the big bang. There are genuine prospects of detecting individual objects in all three of these categories, and thereby narrowing down the present range of options. If the Universe has the critical density (Ω = 1), rather than the lower value (Ω = 0.1–0.2) inferred from dynamical evidence, then the galaxies must be more clustered than the overall distribution even on scales 10–20 Mpc. “Biased” galaxy formation could account for this.
The clustering properties of galaxies are incompatible with a cosmological model with Ω = 1 unless the formation process for bright galaxies is ‘biased’ in the sense that their resultant distribution exaggerates the amplitude of large-scale inhomogeneities in the overall mass distribution. The mechanisms for such biasing are intimately connected with the nature of the dark matter. Various possibilities are summarised here.
Unfortunately, there is as yet no direct observation of a forming galaxy, nor of a manifestly “young” galaxy. It is not even clear what such galaxies might look like. There are, however, some lines of attack that will yield indirect information about the environment in which galaxies formed. There is no lack of theories of galaxy formation, many of which provide quite different scenarios for the birth-process. the hope is that theories of galaxy formation may indicate what young galaxies ought to look like and thus guide us in our search for such objects.
The purpose of this article is to review some recent attempts to understand the origin of globular clusters. To put this in perspective, it may help to recall the analogous problem of the origin of galaxies. This splits into two parts. First, given a proto-galaxy with a specified mass and radius, how does it collapse, form stars and settle into a state of dynamical equilibrium? Richard Larson explored these topics in an important series of numerical simulations in the 1970s. Progress in this area brings into sharper focus a second set of questions that really has precedence over the first. Why did proto-galaxies have properties like the initial conditions in the collapse calculations and what distinguishes galaxies from structures on much larger and much smaller scales? Similar questions face us when we consider the origin of globular clusters. First, how did stars form in a proto-cluster, what was the efficiency, the initial mass function and so forth? It is appropriate that Larson has discussed these topics in the preceding article but here we are mainly concerned with the second kind of question: What is special about objects with masses of order 105-106 M⊙ and dimensions of a few tens of parsecs?
In this talk I shall address three different processes relevant to continuum variability in AGNs. The first two refer to the physical conditions in the regions responsible for the non-thermal emission, and the implications of high brightness temperatures. The third is the distinctive type of flare that results when a star is tidally disrupted by a massive black hole; this process, which merits much further study, it likely to be specially important as a diagnostic of physical conditions in low-luminosity nearby nuclei.
It is now 22 years since quasars were discovered. When the ageing veterans of those pioneering investigations think back over two decades of boisterous debate, their reactions are probably rather mixed. Wonderment at the range and variety of novel phenomena revealed in all wavebands must be tinged with disappointment that we seem so slow in grasping what is really going on. Our understanding has advanced slowly, through many small steps — forward steps preponderating (fortunately) over backward ones.
Quasars offer three types of clue to galactic evolution. (i) The formation of massive black holes in the centres of young galaxies; (ii) Clues to subgalactic structure from quasar absorption spectra; (iii) Implications for redshifts higher than 5. These are briefly summarised in this text.
The clearest evidence for the ‘hot big bang’ is of course the microwave background radiation. Its spectrum is now known, from the FIRAS experiment on COBE, to be a very precise black body – indeed, the deviations due to high-z activity, hot intergalactic gas, etc are smaller than many people might have expected. Also the light element abundances have remained concordant with the predictions of big bang nucleosynthesis, thereby giving us confidence in extrapolating back to when the universe was a few seconds old (see Copi, Schramm and Turner 1994 for a recent review). These developments give us grounds for greater confidence in this model than would have been warranted ten years ago. Several things could have happened which would have refuted the picture, but they haven't happened. For instance:
(i)Objects could have been found where the helium abundance was far below 23 per cent.
(ii)The background spectrum at millimetre wavelengths could have been weaker than a black body with temperature chosen to fit the Rayleigh-Jeans part of the spectrum.
(iii)A stable neutrino might have been discovered in the mass range 100eV-1MeV.
In this contribution I shall first briefly review the arguments suggesting that most galaxies pass through a quasar or AGN phase early in their lifetime, perhaps concurrent with bulge formation. Then, as a footnote to the paper by van der Marel, I will discuss how relic black holes in quiescent galaxies could reveal themselves by the occasional flares resulting from tidal disruption of stars. In conclusion, I shall briefly discuss the phenomena associated with binary black holes, arguing that these are the likely outcome of galaxy mergers.
The radio structures mapped by aperture-synthesis techniques with the VLA, and discussed earlier at this conference by Fomalont, are energised by activity in galactic nuclei. The compact radio sources resolvable by VLBI techniques (Pauliny-Toth, these proceedings) are on scales ~105 times smaller; but even this still far exceeds the scale where - according to most theoretical ideas - the primary power production is concentrated. This central “core” - involving a large concentrated mass, probably a black hole of ~ 108 M⊙ accreting material from its surroundings - is the primary origin of the power for all categories of activity associated with active galaxies and quasars. In this paper, I shall principally discuss how this “core” could give rise to the fast-moving plasma which emerges in directed beams and energises the radio sources; I shall also comment on some interesting features of radio source structure.
The evidence is now compelling that “jets” delineate the channels along which power is supplied from galactic nuclei into extended radio sources — this accumulating evidence, reported by many speakers, has been one of the major themes of this conference. Jets (often apparently one-sided) have been discovered inside many symmetrical double sources. and M87, familiar optically as a “one-sided jet” for over 60 years, is now found to have weak double radio lobes. The VLA has resolved ∼ 70 jets in extended sources; there are now many instances where small jet-like structures are found on the VLBI scale; indirect arguments (some of which I'll mention) indicate that there is directed outflow on still smaller scales, and that the primary collimation may occur right down in the central “powerhouse” (scales ∼< 1015cm). The length-scales relevant to jet production and propagation thus span 9 orders of magnitude; the physical processes and conditions may vary widely over this vast range of scales. This paper will deal briefly with two aspects of jet physics: firstly, some direct inferences from radio maps; and, secondly, some possible mechanisms in galactic nuclei that could set up a collimated outflow.
This paper will be concerned with three topics relevant to the X-ray background: (i) X-ray emission mechanisms in quasars; (ii) the contributions to the X-ray background from quasars, clusters of galaxies, intercluster gas, young galaxies, etc; and (iii) the use of X-ray background observations as a probe for large-scale density irregularities in the Universe.
Observations of SS433 are consistent with the view that the Doppler-shifted line emission originates in a pair of oppositely-directed, precessing jets in which a gas outflow is maintained at the remarkably time- and space-invariant speed of 0.26c. A radiative acceleration mechanism is described for the jets and a detailed, numerical, relativistic flow calculation presented which explain this terminal velocity as the result of “line-locking”. The “line-locking” mechanism suggested here for SS433 may be important as well in extra-galactic radio sources in which the radio luminosity is similarly weak compared with the kinetic energy and optical luminosities.