Book contents
- Frontmatter
- Contents
- Preface
- Acknowledgments
- Acronyms and abbreviations
- 1 HISTORICAL BACKGROUND
- 2 THE COSMOLOGICAL FRAMEWORK
- 3 RADIATIVE PROCESSES–I
- 4 RADIATIVE PROCESSES–II
- 5 THE STANDARD MODEL
- 6 SURVEYS
- 7 LUMINOSITY FUNCTIONS
- 8 THE CONTINUUM
- 9 RADIO PROPERTIES
- 10 X-RAY EMISSION
- 11 X-RAY AND GAMMA-RAY SPECTRA
- 12 UNIFICATION
- 13 QUASAR ABSORPTION LINES
- 14 GRAVITATIONAL LENSING
- 15 PROBLEMS AND CONTROVERSIES
- References
- Books, reviews and proceedings
- Author index
- Subject index
12 - UNIFICATION
Published online by Cambridge University Press: 05 June 2012
- Frontmatter
- Contents
- Preface
- Acknowledgments
- Acronyms and abbreviations
- 1 HISTORICAL BACKGROUND
- 2 THE COSMOLOGICAL FRAMEWORK
- 3 RADIATIVE PROCESSES–I
- 4 RADIATIVE PROCESSES–II
- 5 THE STANDARD MODEL
- 6 SURVEYS
- 7 LUMINOSITY FUNCTIONS
- 8 THE CONTINUUM
- 9 RADIO PROPERTIES
- 10 X-RAY EMISSION
- 11 X-RAY AND GAMMA-RAY SPECTRA
- 12 UNIFICATION
- 13 QUASAR ABSORPTION LINES
- 14 GRAVITATIONAL LENSING
- 15 PROBLEMS AND CONTROVERSIES
- References
- Books, reviews and proceedings
- Author index
- Subject index
Summary
Introduction
We have seen that there are many kinds of quasars and AGN, with observed properties so different that they could belong to fundamentally different populations. However, seemingly different kinds of object sometimes share many properties which clearly set them apart as a class from other kinds of object. It is tempting to think of them as belonging to a single population, the differences arising because of different values taken on by one or more basic parameters.
An example of this kind of behaviour is provided by radio-quiet and radio-loud quasars. We have seen in Section 9.8 that only a small fraction of all quasars are radio-loud. The ratio of radio to optical luminosities has a bimodal distribution, with quasars termed as radio-loud having distinctly higher values of the ratio relative to the radio-quiet variety. However, the emission line properties of these two types of object are so similar that it is very difficult to tell them apart from spectroscopic criteria alone. Radio-loud quasars have higher X-ray luminosity on average than radio-quiet quasars, and flatter X-ray spectra below a few keV, but there is a great deal of overlap in the distribution of these properties. It is tempting therefore to imagine that these radioloud and radio-quiet quasars belong to a single population. It was in fact suggested by Scheuer and Readhead (1979) that all radio-quiet quasars have relativistic radio jets in their cores, and that they appear to be luminous flat spectrum radio sources when the jet makes a small angle with line of sight to the observer.
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- Quasars and Active Galactic NucleiAn Introduction, pp. 357 - 382Publisher: Cambridge University PressPrint publication year: 1999
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