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.

– 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

)3( emobs ss

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 core-dominated/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.)

• 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

Seyfert 1

The Structure of AGN

Torus

Seyfert 2

Central Engine:Accretion Disk+Black Hole

Broad Line Region

Narrow 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 un-

obscured 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!