The origin of the dichotomy of radio loudness among quasars can be explained using recent findings that the mass of the central supermassive black hole (SMBH) in extended radio-loud quasars is systematically a few times that of their counterparts in radio-quiet quasars. This sensitive dependence of radio jet ejection upon SMBH mass probably arises from the blockage of jets by the presence of substantial quantities of gas tidally stripped from stars by the central BH. This disruptive gas, however, will only be available around BHs with masses less than $M_c ~\gtrsim ~10^8\Msun$, for which the tidal disruption radius lies outside the SMBH's event horizon. Consequently, we find that AGN with $M_{BH} > M_c$ can successfully launch jets with a wide range of powers, thus producing radio-loud quasars. The great majority of jets launched by less massive BHs, however, will be truncated in the vicinity of the SMBH due to mass loading from this stellar debris. This scenario also can naturally explain the remarkable dearth of extended radio structures in quasars showing broad absorption line spectra.
We investigate whether models based on the assumption that jets in quasars are powered by rotating black holes can explain the observed radio dichotomy of quasars. We show that in terms of the ``spin paradigm'' models, radio-loud quasars could be objects in which the black hole's rotation rate corresponds to an equilibrium between spin-up by accretion and spin-down by the Blandford-Znajek mechanism. Radio-quiet quasars could be hosting black holes with an average spin much smaller than the equilibrium one. We discuss possible accretion scenarios which can lead to such a bimodal distribution of…
Recent claims by Ivezic et al. (2002) that the distribution of the radio-to-optical flux ratio, R, for quasars is bimodal (the so-called quasar radio dichotomy) were questioned on statistical grounds by Cirasuolo et al. (2003). We apply the approach suggested by Cirasuolo et al. to a sample of \~10,000 objects detected by SDSS and FIRST, and find support for the quasar radio dichotomy. The discrepancy between the claims by Cirasuolo et al. and the results presented here is most likely because 1) the ~100 times larger sample based on two homogeneous surveys that is used here allows a direct det…
We investigate correlations between the direction of the optical linear polarization and the orientation of the host galaxy/extended emission for type1 and type2 radio-loud and radio-quiet quasars. We have used high resolution Hubble Space Telescope data and a deconvolution process to obtain a good determination of the host galaxy/extended emission (EE) position angle. With these new measurements and a compilation of data from the literature, we find a significant correlation, different for type1 and type2 objects, between the linear polarization position angle and the orientation of the EE, s…
The observation that the extremely broad, blueshifted absorption troughs which characterize broad absorption line quasars (BALQs) occur exclusively in radio-quiet quasars (RQQs) suggests that this class of active galactic nuclei (AGN) may offer important clues to the radio-loud/radio-quiet (RL/RQ) dichotomy in quasars. Interestingly, there is also substantial observational evidence for similar, but lower velocity intrinsic absorption outflows in some Seyferts and radio-loud quasars (RLQs) as well. Theoretically, however, it is difficult to interpret this broad range of mass ejection phenomena …
Much evidence has been presented in favor of and against the existence of two distinct populations of quasars, radio-loud and radio-quiet. The SDSS differs from earlier optically selected quasar surveys in the large number of quasars and the targeting of FIRST radio source counterparts as quasar candidates. This allows a qualitatively different approach of constructing a series of samples at different redshifts which are volume-limited with respect to both radio and optical luminosity. This technique avoids any biases from the strong evolution of quasar counts with redshift and potential redsh…
Most quasars are known to be radio quiet and according to the ``spin paradigm'', which connects their radio-loudness with the value of the black hole spin, they must harbor very slowly rotating black holes. On the other hand, quasars are powered by accretion which tends to increase the spin of the black hole. We show that the Blandford-Znajek mechanism is not efficient enough to counteract the spinning up of black holes by an accretion disk, regardless of the accretion rate. We establish conditions under which it is possible to obtain low values of the black hole spin, in the scenario involvin…
The results of a comprehensive study of the cluster environments and black-hole masses of an optically matched sample of radio-loud and radio-quiet quasars are presented. No evidence is found for a difference in large-scale environments, with both quasar classes found to be located in clusters of Abell class $\sim0$ . Conversely, virial black-hole mass estimates based on H$_β$ line-widths show a clear difference in the quasar black-hole mass distributions. Our results suggest that a black-hole mass of $\sim10^{9}\Msolar$ is required to produce a powerful radio-loud quasar, and that it is black…
For Seyfert galaxies, the AGN unification model provides a simple and well established explanation of the Type 1/Type 2 dichotomy through orientation based effects. The generalization of this unification model to the higher luminosity AGNs that are the quasars remains a key question. The recent detection of Type 2 Radio-Quiet quasars seems to support such an extension. We propose to further test this scenario. On the basis of a compilation of quasar host galaxy position angles consisting of previously published data and of new measurements performed using HST Archive images, we investigate the…
We investigate correlations between the optical linear polarization position angle and the orientation of the host galaxy/extended emission of Type 1 and Type 2 Radio-Loud (RL) and Radio-Quiet (RQ) quasars. We have used high resolution Hubble Space Telescope (HST) data and deconvolution process to obtain a good determination of the host galaxy orientation. With these new measurements and a compilation of data from the literature, we find a significant correlation between the polarization position angle and the position angle of the major axis of the host galaxy/extended emission. The correlati…
We present a detailed analysis of a sample of radio-detected quasars, obtained by matching together objects from the FIRST and 2dF Quasar Redshift Surveys. The dataset consists of 113 sources, spanning a redshift range 0.3 < z < 2.2, with optical magnitudes 18.25 < b_J < 20.85 and radio fluxes S_{1.4 GHz} < 1 mJy. These objects exhibit properties such as redshift and colour distribution in full agreement with those derived for the whole quasar population, suggestive of an independence of the mechanism(s) controlling the birth and life-time of quasars of their level of radio emission. The long …
We present a new approach to tackle the issue of radio loudness in quasars. We constrain a (simple) prescription for the intrinsic distribution of radio-to-optical ratios by comparing properties of Monte Carlo simulated samples with those of observed optically selected quasars. We find strong evidence for a dependence of the radio luminosity on the optical one, even though with a large scatter. The dependence of the fraction of radio loud quasars on apparent and absolute optical magnitudes results in a selection effect related to the radio and optical limits of current surveys. The intrinsic d…
We search fora dichotomy/bimodality between Radio Loud (RL) and Radio Quiet (RQ) Type 1 Active Galactic Nuclei (AGN). We examine several samples of SDSS QSOs with high S/N optical spectra and matching FIRST/NVSS radio observations. We use the radio data to identify the weakest RL sources with FRII structure to define a RL/RQ boundary which corresponds to log L$_{1.4GHz}$=31.6 ergs s$^{-1}$ Hz$^{-1}$. We measure properties of broad line H$β$ and FeII emission to define the optical plane of a 4DE1 spectroscopic diagnostic space. The RL quasars occupy a much more restricted domain in this optical…
We present preliminary results from HST imaging of radio-loud quasar hosts, covering a ~x100 range in radio luminosity but in a narrow redshift range (0.5 to 0.65). The sample was selected from our new, spectroscopically complete 7C survey and the 3CRR catalogue. Despite the very large radio luminosity range, the host luminosities are only weakly correlated (if at all) with radio power, perhaps reflecting a predominance of purely central engine processes in the formation of radio jets, and hence perhaps also in the radio-loud/-quiet dichotomy at these redshifts. The results also contradict nai…
We present new clues to the problem of the radio loudness dichotomy arising from an extensive search for intranight optical variability in seven sets of optically luminous radio-quiet quasars and (radio-loud) BL Lacertae objects, which are matched in optical luminosity and redshift. Our monitoring of radio-quiet quasars has for the first time clearly detected such intranight variability, with peak-to-peak amplitudes ~1%, occurring with a duty cycle of ~ 1/6. The matched BL Lacs have both higher variability amplitudes and duty cycles when observed in the same fashion. We show that the much less…
The radio luminosity function (RLF) of radio galaxies and radio-loud quasars is often modelled as a broken power-law. The break luminosity is close to the dividing line between the two Fanaroff-Riley (FR) morphological classes for the large-scale radio structure of these objects. We use an analytical model for the luminosity and size evolution of FRII-type objects together with a simple prescription for FRI-type sources to construct the RLF. We postulate that all sources start out with an FRII-type morphology. Weaker jets subsequently disrupt within the quasi-constant density cores of their ho…
Using a sample of more than 6000 quasars from the Sloan digital sky survey (SDSS) we compare the black-hole mass distributions of radio-loud and radio-quiet quasars. Based on the virial black-hole mass estimator the radio-loud quasars (RLQs) are found to harbour systematically more massive black holes than radio-quiet quasars (RQQs) with very high significance (>>99.99%), with mean black-hole masses of <\log(M_{bh}/\Msun)>=8.89\pm0.02 and <\log(M_{bh}/\Msun)>=8.69\pm0.01 for the RLQs and RQQs respectively. Crucially, the new RLQ and RQQ samples have indistinguishable distributions on the redsh…
We link the lack of FR I type structure among quasars to the void of radio loud quasars below a critical disk luminosity of ~10^46 erg/sec in the PG sample. We argue that the opening angle of the obscuring torus in radio loud quasars depends on the power of the central engine, approaching the jet's beaming angle near the FR I/FR II break. Consequently, low power radio quasars would either be classified as radio galaxies (FR I) or strongly core-boosted (BL Lac) object, depending on the aspect angle, and no conspicuous transitional population would be expected for FR I sources. A closing torus w…
We present a technique to explore the radio sky into the nanoJansky regime by employing image stacking using the FIRST survey. We first discuss the non-intuitive relationship between the mean and median values of a distribution that is dominated by noise, followed by an analysis of the systematic effects present in FIRST's 20cm VLA snapshot images. Image stacking allows us to recover the properties of source populations with fluxes a factor of 30 or more below the rms noise level. Mean estimates of radio flux density, luminosity, etc., are derivable for any source class having arcsecond positi…
We present VLBA observations at 15 GHz of ten GHz Peaked Spectrum (GPS) radio sources. The cores are often difficult or impossible to locate. When likely cores are found, they account for a small fraction of the flux density in GPS galaxies - around or below 2%, while in GPS quasars they can account for more than 20% of the total flux density. We detect low polarization in the GPS sources -- i.e., typically less than a few percent and often less than one percent. This establishes that low polarization in the parsec scale structure is an important defining characteristic of the GPS sources. The…
We discuss the optical and radio properties of 30,000 FIRST sources positionally associated with an SDSS source in 1230 deg$^2$ of sky. The majority (83%) of the FIRST sources identified with an SDSS source brighter than r=21 are optically resolved. We estimate an upper limit of 5% for the fraction of quasars with broad-band optical colors indistinguishable from those of stars. The distribution of quasars in the radio flux -- optical flux plane supports the existence of the "quasar radio-dichotomy"; 8% of all quasars with i<18.5 are radio-loud and this fraction seems independent of redshift an…
At least 10-15% of nearby sun-like stars have known Jupiter-mass planets. In contrast, very few planets are found in mature open and globular clusters such as the Hyades and 47 Tuc. We explore here the possibility that this dichotomy is due to the post-formation disruption of planetary systems associated with the stellar encounters in long-lived clusters. One supporting piece of evidence for this scenario is the discovery of freely floating low-mass objects in star forming regions. We use two independent numerical approaches, a hybrid Monte Carlo and a direct $N$-body method, to simulate the i…
Typical blazars seem to be associated with FR I and FR II radio galaxies and radio-loud quasars. However, what happens at lower powers? Do blazars exist in low-luminosity AGN or do they exist in radio-quiet AGN? A recent detection of superluminal motion in a supposedly radio-quiet Seyfert raises the question whether beaming can play an important role in some of these objects as well. Moreover, VLBI observations of nearby low-luminosity AGN reveal compact flat-spectrum radio cores very similar to those in bright radio-loud blazars. Furthermore, with the detection of X-ray emission from the leas…
In order to further test our hypothesis that jets and disk around compact accreting objects are symbiotic features we investigate the newly discovered superluminal galactic radio jets GRS 1915+105 and GRO J1655-40 and the two famous galactic radio jets 1E1740-2942 and SS 433 within the framework of our couple jet/disk model developed initially for active galactic nuclei (AGN) and the galactic center source Sgr A*. By comparing the ``disk'' and radio core luminosity of those galactic jet sources with our model prediction we can show that they can easily be understood as AGN-like jets where the …
A catalogue of 14453 radio-loud AGN with 1.4 GHz fluxes above 3.5 mJy in the redshift range 0.4<z<0.8, has been constructed from the cross-correlation of the NVSS and FIRST radio surveys with the MegaZ-LRG catalogue of luminous red galaxies derived from Sloan Digital Sky Survey imaging data. New techniques were developed for extending the cross-correlation algorithm to FIRST detections that are below the nominal 1 mJy S/N limit of the catalogued sources. We estimate a reliability of ~98.3%, and completeness level (for LRGS) of about 95% for our new catalogue. We present a new determination of …
We present two-dimensional slab-jet simulations of jets in inhomogeneous media consisting of a tenuous hot medium populated with a small filling factor by warm, dense clouds. The simulations are relevant to the structure and dynamics of sources such as Gigahertz Peak Spectrum and Compact Steep Spectrum radio galaxies, High Redshift Radio Galaxies and radio galaxies in cooling flows. The jets are disrupted to a degree depending upon the filling factor of the clouds. With a small filling factor, the jet retains some forward momentum but also forms a halo or bubble around the source. At larger fi…
