Publications of the Astronomical Society of the Pacific
107: 803-845, 1995 September
For a PDF version of the article, click
here.
For a Postscript version of the article, click
here.
UNIFIED SCHEMES FOR RADIO-LOUD ACTIVE GALACTIC NUCLEI
C. Megan Urry
Space Telescope Science
Institute, 3700 San Martin
Drive, Baltimore, Maryland, 21218
Electronic mail: cmu@stsci.edu
Paolo Padovani
Dipartimento di
Fisica, II
Università di Roma "Tor Vergata" Via della Ricerca Scientifica
1, I-00133 Roma, Italy
Electronic mail:
padovani@roma2.infn.it
ABSTRACT. The appearance of active galactic nuclei (AGN) depends
so strongly on orientation that our current classification schemes are
dominated by random
pointing directions instead of more interesting physical properties.
Light from the centers of
many AGN is obscured by optically thick circumnuclear matter,
particularly at optical and ultraviolet wavelengths. In radio-loud AGN,
bipolar jets emanating
from the nucleus emit radio through gamma-ray light that is relativistically
beamed along the jet axes. Understanding the origin and magnitude of
radiation
anisotropies in AGN allows us to unify different classes of AGN; that is, to
identify each single, underlying AGN type that gives rise to different
classes through different orientations. This review describes the
unification of radio-loud AGN, which include
radio galaxies, quasars, and blazars. We describe the classification and
general
properties of AGN. We summarize the evidence for anisotropic emission
caused by circumnuclear obscuration and relativistic beaming. We outline
the two most plausible unified schemes for radio-loud AGN,
one linking the high-luminosity sources (quasars and luminous radio
galaxies)
and one the low-luminosity sources (BL~Lac objects and less luminous
radio galaxies). Using the formalism appropriate to samples biased by
relativistic beaming, we
show the population statistics for two schemes are in accordance with
available data. We analyze the possible connections between low- and
high-luminosity
radio-loud AGN and conclude they probably are powered by similar physical
processes, at least within the relativistic jet. We review potential
difficulties with unification and conclude that none currently
constitutes a serious problem. We discuss likely complications to
unified schemes that are suggested by realistic physical considerations;
these will be important to consider when more comprehensive data for
larger complete samples become available. We conclude with a list of the
ten questions we believe are the most pressing in this field.
KEY WORDS: Active galaxies : radio-loud - Unification - BL Lac
objects - Quasars
Table of Contents
- INTRODUCTION
- OBSERVED PROPERTIES AND EMPIRICAL
CLASSIFICATION OF AGN
- ANISOTROPIC RADIATION FROM OBSCURATION
- 3.1. Polarimetric Evidence for Hidden
Nuclear Regions
- 3.2. Infrared and X-Ray Evidence for Hidden
Nuclear Regions
- 3.3. Anisotropic Illumination of Narrow
Emission Line Gas
- ANISOTROPIC RADIATION FROM RELATIVISTIC
BEAMING
- 4.1. Evidence for Relativistically
Beamed Gamma-Rays
- 4.2. Superluminal Motion in Radio Jets
- 4.3. Observed Radio Jet Asymmetries
- 4.4. Brightness Temperature Calculations
and SSC Models
- BASIS FOR UNIFICATION SCHEMES
- 5.1. History of Radio-Loud Unification
Schemes
- 5.2. Distinction between FR I and FR II
Radio Galaxies
- 5.3. Isotropic Properties of Quasars and FR
II Galaxies
- 5.3.1. Extended Radio Emission
- 5.3.2. Narrow Emission Lines
- 5.3.3. Infrared Properties>
- 5.3.4. Host Galaxies
- 5.3.5. Environments
- 5.3.6. Cosmic Evolution
- 5.4 Isotropic Properties of BL Lac
Objects and FR I Galaxies
- 5.4.1. Extended Radio Emission
- 5.4.2. Narrow Emission Lines
- 5.4.3. Host Galaxies
- 5.4.4. Environments
- 5.5. The Myth of Unbiased Selection
- 5.6. Effect of Relativistic Beaming on
Number Statistics
- STATISTICAL UNIFICATION OF RADIO-LOUD AGN
- 6.1. Unification of Radio Quasars and FR
II Galaxies
- 6.1.1. Content of the 2 Jy Sample of
Radio Sources
- 6.1.2. Observed LFs of High-Luminosity
Radio Sources
- 6.1.3. Beamed LFs of High-Luminosity
Radio Sources
- 6.2. Unification of BL Lac Objects and FR
I Galaxies
- 6.2.1. X-Ray and Radio Samples of
BL Lac Objects
- 6.2.2. Properties of X-Ray-Selected and
Radio-Selected BL Lac Objects
- 6.2.3. Population Statistics for X-Ray
Samples
- 6.2.4. Population Statistics for Radio
Samples
- 6.2.5. Relation of X-Ray- and
Radio-Selected BL Lac Objects
- 6.2.6. New Terminology and a New
Connection Between BL Lac Classes
- 6.3. Independent Estimates of
Relativistic Beaming Parameters
- RELATION OF QUASARS AND BL LAC OBJECTS
- 7.1. Low- and High-Redshift BL Lac Objects
- 7.2. Possible Connections between BL Lac
Objects and FSRQ
- 7.2.1. The Evolutionary Connection
- 7.2.2. Multiwavelength Spectral
Continuity of BL Lac Objects and FSRQ
- 7.2.3. BL Lac Objects as Gravitationally
Micro-Lensed FSRQ
- THE VIABILITY OF UNIFIED SCHEMES
- 8.1. Potential Problems with Unification
- 8.1.1. Linear Sizes of Blazars and Radio
Galaxies
- 8.1.2. Dependence of Quasar Fraction on
Redshift
- 8.1.3. Absence of Superluminal Motion in
Radio Galaxies
- 8.1.4. The Parent Population of BL Lac
Objects
- 8.2. Possible Complications for
Unification
- 8.2.1. Properties of the Obscuring Torus
- 8.2.2. Cosmic Evolution of Radio-Loud
AGN
- 8.2.3. Parameterization of Relativistic
Beaming
- 8.2.4. Compact Steep-Spectrum and
Gigahertz Peaked-Spectrum Sources
- 8.2.5. Selection Effects in the
Identification of Quasars
- 8.2.6. Extended Continuum Emission in
Type 2 AGN
- THE TEN MOST IMPORTANT QUESTIONS
- RELATIVISTIC BEAMING PARAMETERS
- DOPPLER ENHANCEMENT
- RATIO OF CORE- TO EXTENDED-FLUX
- GLOSSARY OF ACRONYMS
- REFERENCES