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Astron. Astrophys. 326, 45-50 (1997)

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2. An alternative proposal for the origin of the depolarization asymmetry

Considering the above, it seems desirable to explore an alternative scenario to account for the L-G effect. Here we examine the possibility whereby the required Faraday screen contributing the extra rotation measure (RM) in the foreground of the farther radio lobe is associated not with any putative ICM core around the radio source, but, instead, with a large disk of magneto-ionic medium surrounding the host galaxy and oriented roughly perpendicular to the radio source axis. Prominent tracers of such gaseous disks are the extended dust-lanes seen across many massive nearby elliptical galaxies, including our radio-loud neighbour Centaurus A, as well as 3C 270 (Mahabal et al. 1996) (Sect. 3). It is well known that the dust-lanes tend to align perpendicular to the jets (Kotanyi & Ekers 1979; Moellenhoff et al. 1992). Being often decoupled - kinematically as well as morphologically - from the stars in the associated elliptical galaxy, the disks are generally believed to be of an external origin, probably acquired through merger of a gas-rich galaxy (e.g., Goudfrooij & de Jong 1995). According to recent models, dust-lanes begin to form through dissipative processes after making a few orbits around the accreting galaxy, i.e., [FORMULA] years after the capture (e.g., Steinman-Cameron 1991; Quinn 1991; Rix & Katz 1991). Indeed, according to a long-standing view based on a variety of observational evidence, capture of gas-rich disks by massive ellipticals at [FORMULA] is probably responsible for the intense quasar activity witnessed at those early epochs, including the great abundance of powerful double radio sources (e.g., Fukugita et al. 1996; Lynden-Bell 1996; also, Wilson & Colbert 1995). It has been argued that the process of galaxy capture may be accelerated due to the ram-pressure breaking applied by the gaseous coronae of the accreting ellipticals (Sofue & Wakamatsu 1992). Conceivably, the captured objects could even be damped Lyman-alpha clouds; they are thought to be progenitors of the massive spirals and more abundant at higher redshifts (Wolfe et al. 1995).

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© European Southern Observatory (ESO) 1997

Online publication: April 20, 1998