Transcript AGN Unification-1

AGN Unification-1
History
The present status
Aims and objectives
• Review the arguments that led to unified
schemes.
• Outline the different schemes, their
strengths and weaknesses.
• Suggest future lines of attack
What’s all this Unification?
• Historically it is attempt to explain as much
as the spread of observational properties as
possible in terms of orientation effects.
– Assume some axis; i.e. rotation
• More generally, it is an attempt to explain
the diversity of observational properties in
terms of a simple model
The AGN Paradigm
Annotated by M.
Voit
Introduction
• AGN are not spherically symmetric and thus what
you see depends on from where you view them.
This is the basis of most unification models.
• It was the discovery of superluminal motion and
the interpretation in terms of bulk relativistic
motion of the emitter that first made people realize
that orientation in AGN was important.
• I will outline the consequences of Doppler
boosting, describe the historical development of
schemes and then review the modern evidence.
– N.B. Relativistic beaming is not the only mechanism
that can make AGN emission anisotropic
Doppler boosting
• When an emitting body is moving relativistically
the radiation received by an observer is a very
strong function of the angle between the line of
sight and the direction of motion.
sobs  sem ( 3   )
– The Doppler effect changes the energy and frequency
of arrival of the photons.
– Relativistic aberration changes the angular distribution
of the radiation.
•  Is the Doppler factor

•
Is the spectral index
Practical consequences of
boosting
• Superluminal motion implies Lorentz factors of 5
to 10 => possible boosting of flux density by
~1000.
• Sources with the strongest cores will be those
viewed with their axes at small angles to the l.o.s.
• The highly boosted sources will only be a small
percentage of the total population.
– But may be a large fraction of a flux limited sample
Parent populations
• To every beamed source there will be many
unbeamed sources – the parent population.
• How to identify the parent population?
– Look at some emission that’s isotropic; e.g.
radio lobe emission, far infrared emission,
narrow-line emission, etc in the beamed
population and look for another population
having the same luminosity function for the
isotropic emission.
History of Unification
• Rowan-Robinson (1976, ApJ, 213,635) tried to
unify Seyfert galaxies and radio sources.
– Mostly wrong – no beaming
– But the importance of dust and IR emission correct.
• Blandford and Rees (Pittsburgh BL Lac meeting
1978) laid the foundations for beaming
unification. (Radio loud only).
History continued
• Scheuer and Readhead (1979, Nature,277,182)
proposed that radio core-dominated quasars and
radio quiet quasars could be unified – the former
being beamed versions of the latter.
• Orr and Browne (1982,MNRAS,200,1067 )
realized the the Scheuer and Readhead scheme
could not work because MERLIN and VLA had
shown that most of the core-dominated quasars
had extended (isotropic) radio emission and thus
their parent population could not be radio quiet.
We looked for a non-radio quiet parent population
– Proposed core-dominated/lobe-dominated unification
for quasars
Radio Galaxy/Quasar Unification
(Both are FR2s)
• Widely discussed before, but first published by Barthel
(1989, ApJ, 336,606) – an extension of coredominated/lobe-dominated quasar unification.
• Quasars have strong continuum and broad lines and radio
galaxies (FR2s) have little continuum (other than starlight)
and no broad lines.
• How could they be the same thing? Only if one could hide
the quasar nucleus with something optically thick (a
molecular torus).
– N.B. In a parallel line of development Antonucci and Miller had
discovered polarized broad lines in the Seyfert 2 NGC1068 which
they interpreted as being scattered nuclear radiation from a hidden
BLR.
The AGN Paradigm
Annotated by M.
Voit
BL Lacs and FR1 RGs
• Similar arguments apply to these intrinsically
lower luminosity objects; BL Lacs are the beamed
cores of FR1 RGs. (Note FR1 RGs generally have
only weak and narrow emission lines and BLLacs
are almost lineless.)
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Blandford and Rees (1978)
Browne (1983, MNRAS,204,23)
Antonucci and Ulvestad (1985,ApJ,294,158)
Padovani and Urry (1991, ApJ,368,373)
Evidence for BL Lac/FR1
unification
• The statistics look ok (Browne; Padovani and
Urry) for reasonable Lorentz factors
• The required relativistic jets are seen in a few
FR1s, most notably in M87 (Biretta AJ,520,621).
• The strength of optical cores in FR1s seems to
correlate with the strength of the radio core
consistent with both being beamed (Capetti
&Celotti,1999,MNRAS,303,434, Chiaberge et al.
2000,A&A,358,104)
=> No hidden BLR in FR1s (but BL Lac has a broad line)
HST Image of jet in M87
• M87 is and FR1 radio
galaxy
• Superluminal motion
has been detected in
both radio and optical
Evidence for superluminal
motion in M87
NGC6251
• HST image of the
optical core.
• Despite dust lane
(dark band) the core is
clearly visible
• The strength of cores
correlated with that of
radio core
Correlation between optical nuclear and radio
core luminosities (Chiaberge et al,A&A,358,104)
Unification across the FR1/FR2
boundary?
• There does seem to be a real distiction between
FR1s and FR2s:
–
–
–
–
Radio structure
Radio luminosity
Optical emission line properties (but remember BL Lac)
Cosmological evolution
• But the non-thermal emission is similar in both
• Also FR2s could possibly evolve into FR1s
– There is no strong evidence against this (Unification by
time?)
FR2s evolving into FR1s?
• Assume:
– FR2s are objects with relativistic jets that reach the full
extent of the radio source
– That the distance that jets can travel at relativistic
speeds depends on jet power; high power jets make it
further out.
• Then young small sources of a given jet power
will be FR2s, but as they grow and get older they
will become FR1s
Some crossing of the FR boundary with time for
lower-power objects.
(N.B. There are some FR2s with weak emission lines
which when beamed may become BL Lacs)
Tests of radio galaxy/quasar
unification
• The relative numbers of FR2 RGs and Qs (about
2:1 => half-cone angle of ~45 degrees) should be
related to the size of the un-obscured cone angle
hence can calculate by what factor the radio sizes
of Qs should be smaller than RGs.
– The results are mixed but do not rule anything out.
• If the quasar nucleus is hidden by dust the
intercepted energy should be re-radiated in the
FIR. Qs and RGs should have same FIR
luminosity.
– Seems just about ok
Tests continued
• Broad lines should be detectable in narrow line
RGs – either in scattered polarized light or in the
IR.
– Some examples of both are seen as well as some UV
broad lines (e.g. Cygnus A)
• Narrow emission lines well away from the torus
should have the same luminosity in RGs and Qs of
intrinsically the same power.
– [OIII] is stronger in Qs (Jackson and Browne)
– [OII] is the same (Hes et al.)
• The Q luminosity function should be a “beamed”
version of the RG one (Urry and Padovani)
– This works
Orientation indicators in
radio-loud objects
• The ratio of an isotropic emission to a
beamed emission should be an indicator of
orientation.
– R = Radio core/radio extended
(Hine and Longair; Orr and Browne)
– R5000 = Radio core/5000 Angstrom continuum
(Wills & Brotherton, 1995,ApJ,448,81 )
• Can we use these to deduce something about the
inner regions of AGN?
Correlations– Emission lines
(Wills; Baker; Corbin; Barthel; Brotherton, Jackson, Browne and others
have had fun in this area)
• The goal is to use correlation to test models and, more
important, to learn about the inner regions of radio galaxies
and quasars.
• What has been learnt?
– H-beta FWHM anti-correlates with R => disk-like BLR (Wills and
Browne). (Also some broadlines have disk-like profiles)
– [OII] and [OIII] equivalent widths suggests extinction even in the
inner NLR (Baker, Barthel, Jackson & Browne)
– Even the thermal (disk) continuum is orientation dependent in
quasars.
• I cannot make sense of the wealth of information!
Correlations -- Radio
• If jets are relativistic, some “unification” is
inevitable. What’s the evidence for relativistic
jets?
– Superluminal motion (rarely measurable in RGs)
– Jet asymmetry (X-ray jets seen with Chandra need
relativistic motion to give enough IC emission)
– Laing– Garrington effect
• Even in radio galaxies, the side of the source with
the jet is less depolarized
=> Jet asymmetry arises from orientation and hence they
are relativistic.
Radio map of 3C175
CHANDRA X-Ray Jet in
Pictor-A
Wider Unification
• Stimulated by the discovery of polarized broad
lines in a Seyfert 2 (narrow-line Seyfert) by
Antonucci and Miller (1985,ApJ,297,621), in the
mid 1980s the optical community realized that
AGN were not spherically symmetric and that
orientation effects were important.
• There emerged the standard model the key
ingredient of which is the “obscuring torus” which
hides the inner part of all AGN (BLR plus disk
emission), both radio-quiet and radio-loud
The Structure of AGN
Seyfert 1
Narrow Line Region
Torus
Central Engine:
Accretion Disk+Black Hole
Seyfert 2
Broad Line Region
Evidence for the standard model
• More hidden BLR seen in scattered
(polarized) light.
• Ionization cones.
– Though many claimed not many are convincing
• Photoionization considerations – some
Seyfert 2s do not have enough ionization
photons seen to give the NLR luminosity
• Molecular disks, particularly NGC4258
Ionization cone in NGC 5728
• If ionizing photons are
blocked by the torus
then one expects to see
cones delineating the
boundary.
Conclusions about RG/Q
unification
• Some radio FR2 galaxies have hidden Qs
• The simplest picture where there is a single unobscured cone angle for all objects needs
elaboration.
• Perhaps the Andy Lawrence (MN, 252,586) and
Heino Falcke idea of a cone angle that depends on
intrinsic luminosity (receding torus) is one of the
most promising.
• Unified schemes seem to have run out of
predictive power!