Unified Schemes for Extragalactic Radio Sources

Abstract

My own suspicion is that the universe is not only queerer than we suppose, but queerer than we can suppose -J.B.S. Haldane OverviewThis review aims to provide 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. The asymmetries addressed here are related not only to the jets, and 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 constraints can be further sharpened by exploiting the more recently discovered asymmetry properties of the extended optical emission-line regions (EELR) associated with powerful radio sources, including the giant halos of Lyman-α 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 some important types of asymmetries, namely, 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 the observed one-sidedness of the extended radio jets even in 'weak-headedquasars', we also propose a scenario for the late evolutionary stage of double radio sources. We end by providing a list of key questions relating to radio source asymmetries which still require final answers.

The infrared jet in Centaurus A: multiwavelength constraints on emission mechanisms and particle acceleration

“…An alternative model is that the jet interacts at the flare point with some large-scale external feature that is able to affect the whole jet. Gopal-Krishna & Saripalli (1984) have pointed out the coincidence between the radio and X-ray flare point and an optical 'shell', seen in deep images, which they consider to be a remnant of a cannibalized galaxy. A series of shocks caused by such an interaction would be equally consistent with what we observe, if they can be distributed over the entire region of X-ray emission or, again, if the magnetic field is sub-equipartition.…”

A search for distant radio galaxies from SUMSS and NVSS - III. Radio spectral energy distributions and the z- correlation

“…As ex-pected, there is very little difference between the two distributions, with the only exception being a steepening in the rest frame for three sources in the sample with flattening SEDs. Whilst many authors have shown that a k-correction is a credible explanation for the z-α correlation (Gopal-Krishna 1988;Lacy et al 1993;Carilli et al 1999), some of these same studies also show that an apparent -yet weaker -correlation still exists between redshift and rest-frame (k-corrected) radio spectral index (Lacy et al 1993; Blundell et al 1999;Carilli et al 1999). These samples were all selected at frequencies in the range 38 ν 408 MHz, at least a factor of 3 lower in finding frequency than the SUMSS-NVSS USS sample, and at least an order of magnitude brighter in radio luminosity.…”

Intranight polarization variability in radio-loud and radio-quiet AGN

“…Different models exist that explain variability in jet emission, for example through lighthouse effects (see e.g. Gopal-Krishna & Wiita 1992), shock fronts in jets (see e.g. Marscher & Gear 1995;Marscher et al 2008) or certain physical conditions in the jet plasma (see e.g.…”