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Unified Schemes for Extragalactic Radio Sources

Published online by Cambridge University Press:  25 May 2016

Gopal Krishna
Gopal Krishna*
Affiliation:
NCRA-TIFR, Poona University Campus, PUNE-411007, India

Extract

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The most widely discussed class of unified schemes for radio-loud extragalactic sources attempts to interpret their seemingly disparate types as the same objects seen from different directions. The orientation dependence is attributed to relativistic beaming of the nonthermal jet and, possibly, anisotropic obscuration/re-radiation of the nuclear emission by a circum-nuclear distribution of dusty material with polar openings, possibly a torus. Although alternative approaches have been mooted for unifying radio galaxies (RGs) and quasars (QSRs) by incorporating a strong jet-environment interaction, or temporally decaying nuclear prominence, the orientation based unified scheme, thanks to its rich predictive potential, has been subjected to a multitude of observational tests and its pros and cons have been discussed extensively in recent reviews. Here we briefly address some recent developments, including the claim that the radio size measurements of powerful RGs and QSRs are incompatible with orientation being the primary distinction between them. On balance, it seems that while the basic orientation picture can broadly explain the bulk of the observations, its viability could be much enhanced by taking into consideration the (inevitable) temporal evolution of radio sources.

Type
Unification Issues
Information
Symposium - International Astronomical Union , Volume 175: Extragalactic Radio Sources , 1996 , pp. 373 - 378
Copyright
Copyright © Kluwer 1996 

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