117 research outputs found
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
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