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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.
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