A Unified Model for AGN Ryan Yamada Astro 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 wider range of ionization than HII regions and grater range of velocities Emission line profile showing greater range of velocities than in starburst galaxies blue featureless continuum probably emitted by accretion disk around a supermassive black 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 core and halo
Cygnus A (Perley, Dreher, and Cowan, NRAO/AUI)
Quasars and QSOs Quasars - Strong radio sources distant enough that the nucleus appears starlike Quasi-stellar objects (QSOs) - radio quiet high-luminosity starlike objects Both quasars and QSOs distinguished by 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 M sun yr -1 Blandford-Znajek mechanism For 10 8 M sun black hole, R s = 3 x 10 13 cm, B = 10 4 G, P ~ B 2 R S2 c = 2.7 x 10 45 erg s -1
A unified model? Evolutionary relationship between different 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 and clumpy 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) n e ~ 10 4 cm -3 T ~ 10 4 K
C. M. Urry and P. Padovani
Evidence for a unified model Antonucci and Miller (1985) observed NGC 1068 (Seyfert 2) in polarized light and found a Seyfert 1 spectrum with broad emission lines Seyfert 1 nucleus hidden from direct view by optically thick material
Evidence for a unified model H emission lines appear to be produced by continuum radiation and subsequent recombination common origin for emission lines for Seyfert 1 and 2 galaxies, broad- and narrow-line radio galaxies, quasars, and QSOs from Shuder (1981)
Unified model Powered by a supermassive black hole 10 6-10 9 M sun and its accretion disk, extending to ~ 1 pc Surrounded by a dusty toroidal structure, extending to > 100 pc
Structure of unified model Broad lines (H α and H β ) vary on short time scales (~0.1 year), but narrow lines vary little Seyfert 2 galaxies found to have Seyfert 1 nuclei (Antonucci and Miller, 1985) Indicates that broad and narrow lines in spectra originate under different regions under different circumstances
Broad line region Broad-line region is close to center and clumpy Responds to changes in continuum (1-4 weeks for Seyfert galaxies, longer for QSOs) T ~ 10 4 K n e ~ 10 9-10 10 cm -3 Clumpy; flat distribution of optically thick clouds
AGN in the infrared Obscuring dusty torus should reradiate absorbed nuclear radiation in IR Seyfert 2 galaxies exhibit 10µm silicate absorption feature However, Seyfert 1s and quasars do not exhibit the 10µm silicate feature in emission One solution: clumps of dust in torus
Modeling a clumpy medium (Nenkova et al. 2002) calculated IR emission from clumps in obscuring 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 cloud shadowing Optical depth of clouds constrained to τ V > 60 additional clouds in the outer torus of lower optical depth would not significantly affect the SED
From Nenkova et al. (2002)
Summary Differences between Seyfert and radio galaxies, QSOs, and quasars appears to be due to orientation (and possibly age) Unified model of AGN is supported and refined by observations in all wavelengths IR observations particularly important for testing models for dust distribution in outer torus
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.