A Unified Model for AGN

Ryan YamadaAstro 671

March 27, 2006

Overview

• Introduction to AGN• Evidence for unified model• Structure• Radiative transfer models for dusty

torus

Active Galactic Nuclei

• Emission-line spectrum demonstrating widerrange of ionization than HII regions and graterrange of velocities

• Emission line profile showing greater range ofvelocities than in starburst galaxies

• “blue featureless continuum” probably emittedby accretion disk around a supermassiveblack hole and/or synchrotron emission

Radio-quiet galaxies

• Seyfert 1 galaxies– Exhibit narrow and broad line emission– HI, He I, He II emission line FWHM ~ 5,000-

10,000 km/s– Strong keV X-ray emission

• Seyfert 2 galaxies– Narrow line region in flux spectra– Line FWHM are < 1000 km/s– No strong keV emission

Radio galaxies

• Broad-line radio galaxies (BLRGs)– Analogous to Seyfert 1s– Bright, starlike nucleus surrounded by a very faint,

hazy envelope• Narrow-line radio galaxies (NLRGs)

– Analogous to Seyfert 2s– Giant or supergiant elliptical galaxies (cD, D, E)

• May display extended radio lobes, or a coreand halo

Cygnus A

(Perley, Dreher, and Cowan, NRAO/AUI)

Quasars and QSOs

• Quasars - Strong radio sources distantenough that the nucleus appearsstarlike

• Quasi-stellar objects (QSOs) - radioquiet high-luminosity starlike objects

• Both quasars and QSOs distinguishedby large redshift, nebular lines

Centaurus A

Sources of energy

• Accretion– Magnetic field anchored to accretion disk– Induces electric field, emitting synchrotron

radiation that contributes to continuum– Most energetic AGN must accrete 1-10 Msun yr-1

• Blandford-Znajek mechanism– For 108 Msun black hole, Rs = 3 x 1013 cm, B = 104

G,P ~ B2RS

2c = 2.7 x 1045 erg s-1

A unified model?

• Evolutionary relationship betweendifferent AGN types– Quasars and blazars -> radio galaxies ->

normal elliptical galaxies– QSOs -> Seyferts -> normal spiral galaxies

• Role of observing angle in classification

Constraints on unified model

• Torus must be opaque– Only hard x-rays observed in Seyfert 2 galaxies– Continuum in Seyfert 2s weaker than Seyfert 1s

• Broad-line region is close to center andclumpy– Broad-line region responds quickly (1-4 weeks) to

changes in continuum• Narrow-line region is farther away, but still

near nucleus (and also clumpy)– ne ~ 104 cm-3

– T ~ 104 K

C. M. Urry and P. Padovani

Evidence for a unified model

• Antonucci and Miller (1985) observedNGC 1068 (Seyfert 2) in polarized lightand found a Seyfert 1 spectrum withbroad emission lines

• Seyfert 1 nucleus hidden from directview by optically thick material

Evidence for a unified model

• H emission linesappear to be producedby continuum radiationand subsequentrecombination

• common origin foremission lines forSeyfert 1 and 2galaxies, broad- andnarrow-line radiogalaxies, quasars, andQSOs from Shuder (1981)

Unified model

• Powered by a supermassive black hole106-109 Msun and its accretion disk,extending to ~ 1 pc

• Surrounded by a dusty toroidalstructure, extending to > 100 pc

Structure of unified model

• Broad lines (Hα and Hβ) vary on short timescales (~0.1 year), but narrow lines vary little

• Seyfert 2 galaxies found to have Seyfert 1nuclei (Antonucci and Miller, 1985)

• Indicates that broad and narrow lines inspectra originate under different regionsunder different circumstances

Broad line region

• Broad-line region is close to center andclumpy– Responds to changes in continuum (1-4

weeks for Seyfert galaxies, longer forQSOs)

– T ~ 104 K– ne ~ 109-1010 cm-3

– Clumpy; flat distribution of optically thickclouds

AGN in the infrared

• Obscuring dusty torus should reradiateabsorbed nuclear radiation in IR

• Seyfert 2 galaxies exhibit 10µm silicateabsorption feature

• However, Seyfert 1s and quasars donot exhibit the 10µm silicate feature inemission

• One solution: clumps of dust in torus

Modeling a clumpy medium(Nenkova et al. 2002)

• calculated IR emission from clumps inobscuring torus– Included emission from externally illuminated

clumps and cloud shadowing• Model reproduces important IR features

– broad IR bump extending to ~ 100 µm– 10mm feature apparent in equatorial viewing, but

smeared out in axial viewing because of cloudshadowing

• Optical depth of clouds constrained to τV > 60– additional clouds in the outer torus of lower optical

depth would not significantly affect the SED

FromNenkova etal. (2002)

Summary

• Differences between Seyfert and radiogalaxies, QSOs, and quasars appears to bedue to orientation (and possibly age)

• Unified model of AGN is supported andrefined by observations in all wavelengths

• IR observations particularly important fortesting models for dust distribution in outertorus

References

• Antonucci and Miller, ApJ 297, 621, 1985• Antonucci, ARA&A, 31, 473, 1993• Carroll and Ostlie. An Introduction to Modern

Astrophysics. 1996• Nenkova et al., ApJ, 570L, 9, 2002• Osterbrock and Ferland. Astrophysics of

Gaseous Nebulae and Active Galactic Nuclei(second edition). 2006

• Shuder, ApJ, 244, 12, 1981.