New insights on unification of radio-loud AGN

The radio-loud AGN unification model associates powerful radio galaxies with radio-loud quasars and blazars. In analogy with the radio-quiet scheme, the nuclear regions of objects showing only narrow emission lines in their optical spectrum are thought to be obscured to our line-of-sight by a geometrically and optically thick dusty "torus". In objects showing broad emission lines we directly observe the innermost parsecs around the central black hole, i.e. the broad line region and the accretion disk. Radiation from the base of the relativistic jet dominates the overall emission of blazars, that are seen almost pole-on. Although the broad picture seems to be well established, there are several fundamental aspects that are still to be understood. HST studies have recently shed new light on many issues, from the properties of the nuclei to the structure of the host galaxies.Comment: Invited review, to appear in Multiwavelength AGN Surveys, ed. R. Maiolino and R. Mujica (Singapore: World Scientific), 2004. 10 pages, 2 figure

The BL Lac heart of Centaurus A

Emission from the nucleus of the closest radio galaxy, Centaurus A, is observed from the radio to the gamma ray band. We build, for the first time, its overall Spectral Energy Distribution (SED) that appears to be intriguingly similar to those of blazars, showing two broad peaks located in the far-infrared band and at ~0.1 MeV respectively. The whole nuclear emission of Centaurus A is successfully reproduced with a synchrotron self-Compton model. The estimated physical parameters of the emitting source are similar to those of BL Lacs, except for a much smaller beaming factor, as qualitatively expected when a relativistic jet is orientated at a large angle to the line of sight. These results represent strong evidence that Centaurus A is indeed a misoriented BL Lac and provide strong support in favour of the unification scheme for low luminosity radio-loud AGNs. Modeling of the SED of Centaurus A also provides further and independent indications of the presence of velocity structures in sub-pc scale jets.Comment: 5 pages, 2 figures, accepted for publication in MNRAS, pink page

Does the unification of BL Lac and FR I radio galaxies require jet velocity structures?

We explore the viability of the unification of BL Lacs and FR I radio galaxies by comparing the core emission of radio galaxies with those of BL Lacs of similar extended radio power, taking advantage of the newly measured optical nuclear luminosity of FR I sources. The spectral properties of complete samples are also studied in the radio-optical luminosity plane: starting from the Spectral Energy Distribution (SED) of BL Lacs, we calculate the predicted luminosity of FR I nuclei in the frame of a simple one--zone model, by properly taking into account the relativistic transformations. We find that the bulk Lorentz factors required by the spread in the observed luminosities in all bands are significantly smaller than those implied by other, both observational and theoretical, considerations. This discrepancy is also reflected in the fact that FR I nuclei are over-luminous by a factor of 10-10^4, with respect to the predictions, both in the radio and in the optical band. In order to reconcile these results with the unification scheme, velocity structures in the jet are suggested, where a fast spine is surrounded by a slow (but still relativistic) layer so that the emission at different angles is dominated by different velocity components: the fast one dominates the emission in BL Lacs while the slow layer dominates the emission in misaligned objects. Furthermore for the lowest luminosity BL Lacs it has to be also postulated that their beaming factor in the radio band is lower than in the optical (and X-ray), as would result from deceleration of the jet. The self-consistency of the unification model therefore requires that both intrinsic differences in the SED and different beaming properties play a substantial role in characterizing the phenomenology of these sources.Comment: 10 pages, 11 figures, revised version, to be published in A&

Radio Loud AGNs are Mergers

We measure the merger fraction of Type 2 radio-loud and radio-quiet active galactic nuclei at z>1 using new samples. The objects have HST images taken with WFC3 in the IR channel. These samples are compared to the 3CR sample of radio galaxies at z>1 and to a sample of non-active galaxies. We also consider lower redshift radio galaxies with HST observations and previous generation instruments (NICMOS and WFPC2). The full sample spans an unprecedented range in both redshift and AGN luminosity. We perform statistical tests to determine whether the different samples are differently associated with mergers. We find that all (92%) radio-loud galaxies at z>1 are associated with recent or ongoing merger events. Among the radio-loud population there is no evidence for any dependence of the merger fraction on either redshift or AGN power. For the matched radio-quiet samples, only 38% are merging systems. The merger fraction for the sample of non-active galaxies at z>1 is indistinguishable from radio-quiet objects. This is strong evidence that mergers are the triggering mechanism for the radio-loud AGN phenomenon and the launching of relativistic jets from supermassive black holes. We speculate that major BH-BH mergers play a major role in spinning up the central supermassive black holes in these objects.Comment: 16 pages, 6 figures, accepted for publication in the Ap