Abstract. A significant fraction of extended radio sources presents a peculiar X-shaped radio morphology: in addition to the classical double lobed structure, radio emission is also observed along a second axis of symmetry in the form of diffuse wings or tails. We re-examine the origin of these extensions relating the radio morphology to the properties of their host galaxies. The orientation of the wings shows a striking connection with the structure of the host galaxy as they are preferentially aligned with its minor axis. Furthermore, wings are only observed in galaxies of high projected ellipticity. Hydrodynamical simulations of the radio-source evolution show that X-shaped radio-sources naturally form in this geometrical situation: as a jet propagates in a non-spherical gas distribution, the cocoon surrounding the radio-jets expands laterally at a high rate producing wings of radio emission, in a way that is reminiscent of the twin-exhaust model for radio-sources.

We search for a dichotomy/bimodality between radio-loud (RL) and radio-quiet (RQ) type 1 active galactic nuclei (AGN). We examine several samples of Slogan Digital Sky Survey (SDSS) quasi-stellar objects (QSOs) with high signal-to-noise ratio optical spectra and matching Faint Images of the Radio Sky at Twenty-cm/NRAO VLA Sky Survey (FIRST/NVSS) radio observations. We use the radio data to identify the weakest RL sources with a Fanaroff-Riley type II (FR II) structure to define a RL/RQ boundary which corresponds to log L 1.4 GHz = 31.6 erg s −1 Hz −1 . We measure the properties of broad-line Hβ and Fe II emission to define the optical plane of a 4DE1 spectroscopic diagnostic space. The RL quasars occupy a much more restricted domain in this optical plane compared to the RQ sources, which a 2D Kolmogorov-Smirnov test finds to be highly significant. This tells us that the range of broad-line region kinematics and structure for RL sources is more restricted than for the RQ QSOs, which supports the notion of dichotomy. FR II and CD RL sources also show significant 4DE1 domain differences that likely reflect differences in line-of-sight orientation (inclined versus face-on, respectively) for these two classes. The possibility of a distinct radio-intermediate (RI) population between RQ and RL source is disfavoured because a 4DE1 diagnostic space comparison shows no difference between RI and RQ sources. We show that searches for dichotomy in radio versus bolometric luminosity diagrams will yield ambiguous results mainly because in a reasonably complete sample, the radio brightest RQ sources will be numerous enough to blur the gap between RQ and RL sources. Within resolution constraints of NVSS and FIRST, we find no FR I sources among the broad-line quasar population.

We explore the properties of the Hβ emission line profile in a large, homogeneous and bright sample of N ∼ 470 low-redshift quasars extracted from Sloan Digital Sky Survey (Data Release 5). We approach the investigation from two complementary directions: composite/ median spectra and a set of line diagnostic measures (asymmetry index, centroid shift and kurtosis) in individual quasars. The project is developed and presented in the framework of the so-called 4D Eigenvector 1 (4DE1) parameter space, with a focus on its optical dimensions, full width at half-maximum of broad Hβ [FWHM(Hβ)] and the relative strength of optical Fe II [R Fe II ≡ W(Fe II4434-4684 Å)/W(Hβ)]. We reenforce the conclusion that not all quasars are alike and spectroscopically they do not distribute randomly about an average typical optical spectrum. Our results give further support to the concept of two populations A and B [narrower and broader than 4000 km s −1 FWHM(Hβ), respectively] that emerged in the context of 4DE1 space. The broad Hβ profiles in composite spectra of Population A sources are best described by a Lorentzian and in Population B by a double Gaussian model. Moreover, high-and low-accretion sources (an alternative view of the Population A/B concept) not only show significant differences in terms of black hole (BH) and Eddington ratio L bol /L Edd , but they also show distinct properties in terms of line asymmetry, shift and shapes. We finally suggest that a potential refinement of the 4DE1 space can be provided by separating two populations of quasars at R Fe II ∼ 0.50 rather than at FWHM(Hβ) = 4000 km s −1 . Concomitantly, the asymmetry and centroid shift profile measures at 1/4 fractional intensity can be reasonable surrogates for the FWHM(Hβ) dimension of the current 4DE1.