INTRODUCTION to AGN.ppt

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1、INTRODUCTION to AGN,“Sometimes you cant stick your head in the engine, so you have to examine the exhaust” - D. E. Osterbrock,Astro 596G Spring 2007,In 2006, there were 2151 papers in ADS with “quasar” or “AGN” in their abstracts,Key Questions,Growth of SMBHs Due to mergers, accretion, stellar captu

2、res? SMBHs and Galaxy/LSS formation Were BHs seeds or byproducts of galaxy formation? How important is is the feedback from SMBHs in structure formation? Jets in AGN Are jets matter or Poynting flux dominated? Why are some AGN radio-loud, some radio-quiet? Accretion Physics Is our basic cartoon of t

3、he central engine correct? Is the unified model viable? Useful? Have we included the essential physics?,AGN ULIRGS (Ultraluminous IR Galaxies)Refn: An Introduction to Active Galactic Nuclei, by Brad Peterson,Why study quasar emission lines?Probe inner few pcs of the AGNLearn about the unobservable U

4、V continuumc.f. Zanstra temperaturesUV originates in the inner region of the accretion diskUV photons from quasars an important contributor to the metagalactic UV backgroundInteresting line-formation phenomena because conditions are extremeWant to understand the source of the gas which fuels the cen

5、tral engineUnique probe of abundances at z=5-6Possible uses as cosmological probes (e.g. Baldwin effect)Use to measure central black hole masses in AGN,Broad red wing of Fe Kalpha emission reflection of X-ray continuum off inner accretion disk,Nandra et al. ASCA composite for Sy1s,Radio: Synchrotron

6、 (relativistic electrons in B-field) Mm-1 micron: Dust emission 1 micron - .2 keV: Thermal emission from optically thick accretion disk X-rays: Synchrotron, Inverse Compton, Hot corona + reflection,Continuum Spectral energy distributions:,Quasar and Seyfert 1 Spectra:Broad permitted lines (widths 10

7、,000 km/sec)Narrow forbidden lines (widths only few hundred km/sec), e.g. OIII 4959, 5007CIII 1909 is broad,Forbidden Lines:O IIISemi-Forbidden:C IIIRecombination: Ly alpha, Halpha,Aside of forbidden v. permitted lines, recombination lines:,In photoionized gas, the lines of hydrogen and helium are “

8、recombination” lines, and to first order do not depend on physical conditions like density or temperature.,e-,Photon of Ly alpha, Halpha, etc,1,2,Most metals are collisionally excited (the first excited level is low enough that for nebular temperatures, collisions can populate the upper levels)Wheth

9、er or not the electron de-excites via emitting a photon or by a collision with a free electron depends on(1) density of particles, and (2) the Einstein A for the radiative de-excitation transition.Permitted lines: Radiative de-excitation is fast (Einstein A is high) , soatom radiatively de-excites b

10、efore a collision Forbidden lines: Radiative de-excitation is not likely. If the densityis low enough, a collision will not happen, and the atom will de-exciteradiatively we see the line.If the density is greater than the “Critical density” then theatom collisionally de-excites before it emits a pho

11、ton no line,Ionization Parameter,Emission Lines arise in 2 separate “regions”Narrow Line Region (NLR) Extended spatially (kpc) in nearby Seyferts Low densityn 10 3-6 cm-3 since you see forbidden lines like OIII FWHM n(critical) they must be collisionally de-excited, although you do see broad CIII so

12、 nn(critical) for CIIINLRG, Seyfert 2s: NLR only, BLR obscured or absent Seyfert 1s, Quasars, BLRG: NLR and BLR visible,Unified Models for Sy1s and Sy2s:Antonucci & Miller 1985 ApJ 297, 621Antonucci 1993 ARAA Vol. 31, p. 473Spectropolarimetry of the Seyfert 2 galaxy NGC 1068:,Polarized Flux shows br

13、oad permitted lines: Looks like a Sy 1 in polarized light,H,OIII,Unified Model:Sy 1s and Sy 2s are the same object, seen at different aspect angles. Polarized light is Thomson scattered BLR,Obscuring Torus,BLR light scattered by electrons to us,BLR,Scattering electrons,Although local Seyferts, image

14、d with HST, often show disk-like, dusty structures in their cores on relatively large scales, direct evidence for an obscuring torus, required by the Unified Models, doesnt really exist.Theoretically, its not clear how youd “make” a torus and why it would be where it is,WFPC images of Seyfert Nuclei

15、,Another part of the puzzle:Broad Absorption Line Quasars (BAL QSOs)Warm (i.e. ionized) UV and Xray absorbersAssociated AbsorbersAll these objects have outflowing, radiatively driven winds In a few cases, the absorption varies with time must be veryclose to the central engineTypically see very high

16、ionization states, not typical of ISM clouds,e.g. NV, OVI, OVII etc,Although BAL winds are thought to be radiatively driven, its hard to have enough radiation force without totally ionizing the material and making it Impossible to radiatively drive out,Perhaps the winds and the BLR are one and the s

17、ame,Double-peaked Balmer-line profiles are seen in a few AGNShape and variability of line profile suggest an origin for Hbeta in a rotating disk,NGC 1097 Storchi-Bergmann et al. (2003),Reverberation Mapping Brad Peterson and many collaborators,Powerful probe of BLR structure and kinematicsMeasure th

18、e emission-line response to continuum variationsNeed good enough time sampling to derive time lags between continuum and emission line variations unambiguously Emission lines respond to changes inthe continuum flux with a “lag” corresponding to the light travel time from the ionizing source to the B

19、LRFirst Campaign: NGC 5548Obs. With IUE every 4 daysin 1988-1989 season,Continuum,Emission line,NGC 5578,“Isodelay Surfaces”,All points on an “isodelay surface” have the same extra light-travel time to the observer, relative to photons from the continuum source., = r/c,Courtesy B. Peterson,Key Assum

20、ptions,Continuum originates in a single central source. Continuum source (101314 cm) is much smaller than BLR (1016 cm) Continuum source not necessarily isotropicLight-travel time is most important time scale. Cloud response instantaneous rec = ( ne B)1 0.1 n101 hr BLR structure stable dyn = (R/VFWH

21、M) 3 5 yrsThere is a simple, though not necessarily linear, relationship between the observed continuum and the ionizing continuum.,Reverberation Mapping Results,Reverberation lags have been measured for 36 AGNs, mostly for H, but in some cases for multiple lines. AGNs with lags for multiple lines s

22、how that highest ionization emission lines respond most rapidly ionization stratification,No significant lag is seen between the optical and UV continuumThe delays for the different emission lines are smaller than models predicted The lag increases with decreasing ionization one zone models are out,

23、 must have radial stratification of BLRRevised BLR models since the BLR gas is closer to the ionizing photon source than previously thought, so density must be higher to keep the ionization parameter constant n 1011 cm-3 Really optically thick,Time-Variable Lags in NGC 5548,14 years of observing the

24、 H response in NGC 5548 shows that lags increase with the mean continuum flux. Measured lags range from 6 to 26 days Best fit is lag Lopt0.9 Structural changes inthe BLR, Lopt0.9,Optical luminosity,Hbeta lag,Locally Optimally Emitting CloudsBaldwin, Ferland, Korista & Verner 1995 ApJ 455, L119Fergus

25、on+ 1997 ApJ 487, 122All quasar emission line spectra “look alike”i.e. A single-zone model is pretty successful with a narrow range of density and ionization parameter implausible “fine-tuning” of BLR parametersUse CLOUDY to make a huge grid of BLR models as a function ofdensity and ionization param

26、eter (i.e. distance from the central engine)For each emission line, there is a narrow range of density & ionization parameter where the line formation is “maximally efficient”and most of the line emission is formedSo you can have a completely chaotic BLR with no preferred density, etc. The observed

27、spectrum is some average of the “full family” of models,Locally optimally-emitting cloud (LOC) model,The flux variations in each line are responsivity-weighted. Determined by where physical conditions (mainly flux and particle density) give the largest response for given continuum increase. Emission in a particular line comes predominantly from clouds with optimal conditions for that line.,Ionizing flux,Particle density,