Asymmetries in Powerful Extragalactic Radio Sources

This review provides the background and current perspective of some of the striking asymmetries exhibited by extragalactic double-lobed radio sources, paying special attention to the reported interrelationships among some of them. We particularly aim to highlight some less discussed aspects pertaining to the radio lobes and hot-spots for which any relativistic beaming effects are milder than those imposed on the jets. Various attributes of these features, such as their geometry, intensity, spectrum, and polarization provide important underpinnings to models of radio galaxies and quasars, as well as to theories that attempt to unify them. These contraints are sharpened by exploiting the asymmetry properties of the extended optical emission-line regions associated with powerful radio sources, including the giant halos of Lyman-alpha emission associated with them. We suggest that highly extended disks of dusty gaseous material, which could also be parts of cosmic filaments, around the host galaxies of radio sources may hold the key to understanding the correlated radio--optical asymmetry and perhaps also the depolarization asymmetry of the twin radio lobes. We also discuss the nature and possible implications of the extreme type of asymmetry witnessed in the enigmatic quasar 3C273, whose entire radio emission is observed on just one side of the nucleus. In this context, theoretical scenarios for the production of one-sided jets are also briefly recounted. Taking a clue from theobserved one-sidedness of the extended radio jets even in `weak-headed-quasars', we also propose a scenario for the late evolutionary stage of double radio sources. We end with a list of key questions concerning radio source asymmetries which still require final answers. (Abridged)