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Astron. Astrophys. 363, 507-516 (2000)

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1. Introduction

A quarter of a century ago, using the first set of synthesis maps of 3CR radio sources, Fanaroff & Riley (1974) demonstrated that the morphology of extended double radio sources undergoes a relatively sharp transition across a critical radio luminosity, [FORMULA], corresponding to [FORMULA] W Hz-1 sr-1. Most sources below this luminosity exhibit FR I type structures, which are distinguished by diffuse radio lobes having their brightest regions within the inner half of the radio source. Such edge-dimmed radio sources include: fairly symmetrical twin-jets; fat doubles; Wide Angle Tail; and Narrow Angle Tail (or head-tail) sources. On the other hand, the more powerful FR II type double sources show less bending, and their brightness peaks occur near the outer edges of the two radio lobes, which are often identified as hot-spots. Interestingly, [FORMULA] was found to lie near the observed break in the radio luminosity function of elliptical galaxies (Meier et al. 1979) and also to correspond to a transition in the properties of nuclear optical emission lines (Hine & Longair 1979). More detailed studies based on improved radio maps later established that the radio luminosity separating FR I from FR II sources is actually a rising function of the optical output of the parent elliptical galaxy, [FORMULA] (Owen & White 1991; Ledlow & Owen 1996).

The origin of the FR I/FR II dichotomy continues to be a much debated outstanding issue in the astrophysics of extragalactic radio sources (e.g., Scheuer 1996). Several authors have linked the morphological differences primarily to the transition of an initially supersonic, but relatively weak, jet to a transonic/subsonic flow decelerated substantially though entrainment of thermal plasma within the inner ([FORMULA] 1 kpc) region of the host elliptical galaxy (e.g., De Young 1993; Bicknell 1984 , 1994 , 1995; Komissarov 1994; Bowman et al. 1996; Kaiser & Alexander 1997). In contrast, several others have argued in favor of more fundamental differences existing between the two classes, involving the nature of the central engine (e.g., Baum et al. 1992; Baum et al. 1995; Reynolds et al. 1996a; Meier et al. 1997, Meier 1999), or the possibility of composition of jet plasma being different, with [FORMULA]-[FORMULA] plasma inferred for FR I sources (Reynolds et al. 1996b), while [FORMULA]-p jets may be preferred for FR II sources (Celotti et al. 1997). In our earlier work we had attributed the FR I/FR II dichotomy primarily to differences in the jet's power/thrust which, together with the properties of the circumgalactic medium, would determine how soon the advance of the hot-spot becomes subsonic relative to the ambient medium; this would lead to the disruption of the jet's collimation due to its weakened Mach disk (Gopal-Krishna & Wiita 1988; Gopal-Krishna 1991; also, Blandford 1996). Further evidence for this scheme, which assumes no fundamental differences between either the central engines or the jets of FR I and FR II sources, was presented by Gopal-Krishna et al. (1996), based on a representative set of Weak Headed Quasars . A growing body of VLBI observations also indicates similar jet velocities in FR I and FR II type sources close to the galactic nucleus (e.g., Giovannini et al. 1995; also Parma et al. 1987).

The goal of this communication is to present a new type of evidence in favor of environmental factors being the primary determinant of the FR dichotomy. In order to do so, it is necessary that we first present perspectives of the observational status (Sect. 2) and theoretical inferences (Sect. 3) bearing on the question of the FR I/FR II dichotomy. We then endeavor to constrain some of the proposed theoretical explanations by introducing a new class of double radio sources, where the two lobes on the opposite sides of the galactic nucleus exhibit clearly different FR morphologies (Sect. 4). Although rare, these HYbrid MOrphology Radio Sources , or HYMORS, provide a valuable probe of the physical origin of the Fanaroff-Riley morphological divide. A brief report on this class was made in Gopal-Krishna & Wiita (2000). Our main conclusions are summarized in Sect. 5.

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Online publication: December 11, 2000