Published in The Astronomical Journal, Volume 144, Issue
3, article id. 85, 9 pp. (2012).
For a PDF version of the article, click here.
astro-ph/1206.6893
Abstract: The simple, yet profoundly far-reaching classification scheme based on extended radio morphologies of radio galaxies, the Fanaroff-Riley classification has been a cornerstone in our understanding of radio galaxies. Over the decades since the recognition that there are two basic types of radio galaxy morphologies there have been several findings in different wavebands that have reported properties on different scales. Although it is realized that there may be intrinsic as well external causes an overarching view of how we may understand the two morphological types is missing. With the radio power-absolute magnitude relation (the Owen-Ledlow diagram) as backdrop we review and develop an understanding of the two radio galaxy types in the light of what is known about them. We have for the first time included the dust properties of the two FR classes together with the relative orientations of dust, host major axis and the radio axis to present a qualitative framework within which to understand the conditions under which they form. We discuss how the host elliptical and its history can explain the distribution of radio galaxies in the Owen-Ledlow diagram. Mass of the host elliptical galaxy is a crucial player in deciding what type of a radio galaxy it can host in what conditions. Benign conditions, characterized by natural evolutionary processes, most easily give rise to FR-I type sources in ellipticals of all mass regimes where as with FR-IIs we reason that it is hard to form them without mergers. In undisturbed conditions elliptical galaxies appear to acquire stable states where the black hole axis settles along the host minor axis. With the steady conditions and the continuous supply of ambient gas, the FR-Is in principle, may be powered for a long time. Aided by mergers and interactions low mass ellipticals more easily form FR-II type sources than more massive hosts. LEG FR-IIs may include dying as well as restarted FR-II radio galaxies.
Subject headings : galaxies: elliptical and lenticular, cD - galaxies: active - radio continuum: general - galaxies: jets - galaxies: nuclei
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