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Active galaxies

Active galaxies. • Overview • Black holes and accretion • AGN components • Unification model • Quasars and cosmology. Overview. Discoveries 1939 : Grote Reber, a pioneer of radioastronomy, discovers the first radiogalaxy : Cygnus A. Overview - 2.

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Active galaxies

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  1. Active galaxies • Overview • Black holes and accretion • AGN components • Unification model • Quasars and cosmology

  2. Overview Discoveries 1939: Grote Reber, a pioneer of radioastronomy, discovers the first radiogalaxy: Cygnus A.

  3. Overview - 2 1943: Carl Seyfert publishes a paper on a class of spiral galaxies with a very bright nucleus and a spectrum displaying strong emission lines. M77 (NGC 1068), the closest Seyfert galaxy

  4. Overview - 3 1940s-50s: the first radio surveys reveal, in addition to galactic sources and radiogalaxies, a class of objects that appear star-like in the optical but whose spectrum does not correspond to any known object. They are named quasars, `quasi-stellar radio sources´. 1963: Maarten Schmidt identifies the emission lines of 3C273 as those of hydrogen, but with a redshift of 0.158 → very distant objects → very luminous Quasar 3C 273

  5. Overview - 4 Astronomers progressively realize that these apparently very different objects belong to a same class that they name AGNs: Active Galactic Nuclei

  6. Overview - 5 Classification Influenced by the historical circumstances of their discoveries QSO = blasars

  7. Overview - 6 The separation between some classes is historical: – Seyfert galaxies: one observes the galaxy and then realizes that it has a bright nucleus with emission lines – QSOs (Quasi Stellar Objects = radio-quiet quasars): one observes the very bright nucleus and only after realizes that it lies inside a galaxy → to avoid ambiguity, one defines (± arbitrarily) that QSOs/quasars are AGNs whose absolute magnitude is brighter than MV = –23 and nuclei of Seyfert galaxies as AGNs fainter than that limit MV = –23 → L ≈ 1044 erg/s → `typical´ galaxy → in a quasar, the nucleus is generally brighter then the rest of the galaxy

  8. Overview - 7 Spectral distribution Galaxy: ≈ sum of stellar spectra (≈ black bodies) → limited spectral range (UV + visible + IR) AGN: covers the whole spectral range from X rays to radio waves Lradio / Lbol (AGN) > ~10 Lradio / Lbol (normal galaxy) LX / Lbol (AGN) → ~10 000 LX / Lbol (normal galaxy) Non thermal origin of a large fraction of AGN emission Often approximated by a power law Fν ≈ ν–α(but α depends on the frequency range)

  9. Overview - 8 Spectral characteristics: • power law in radio – microwaves – far UV • IR Bump: thermal emission of dust grains (T ~ 50–200 K) • ± power law in the optical • Big Blue Bump: thermal emission from the accretion disk • power law in X rays + emission lines on top of the continuum (gas clouds excited / ionized by the main radiation source) IR Bump

  10. Overview - 9 Broad lines • hydrogen: Balmer series + Lymanα • ionizedmetals: Mgii, Feii, Ciii, Civ… • FWHM = Full Width at Half Maximum, after subtraction of the continuum Doppler broadening caused by gas motion: FWHM ~ 2000 – 10000 km/s

  11. Overview - 10 Narrow lines • hydrogen: superimposed on the broad lines • ionized metals: mostly forbidden lines* in the optical/UV • FWHM ~ 400 km/s (broader than emission in `normal´ galaxies) * forbidden lines: radiative transition probability very low → de-excitation by collisions in the laboratory

  12. Overview - 11 Radio emission 2 classes: • Fanaroff-Riley type I (= FRI): – brighter at the center – Ln (1.4 GHz) < 1032 ergs-1Hz-1 • Fanaroff-Riley type II (= FRII): – brightness increases in outer parts – often jets + lobes – structure variable with ν – Ln (1.4 GHz) > 1032 ergs-1Hz-1

  13. Overview - 12 Origin of radio emission: • Fν ≈ ν–αwith α ~ 0 for the compact nucleus and α ~ 0.7 for the extended parts • radiation linearly polarised (at least 30%, which is a lot) → synchrotron radiation emitted by electrons in relativistic motion in a magnetic field For an electron of energy the characteristic emission frequency is if B in Gauss

  14. Overview - 13 • for ν < νc: Fν ~ ν1/3 • for ν > νc: Fνexponentially decreases → in (very) 1st approx., emission from an e– is ~ monochromatic → the emission spectrum reflects the energy spectrum of the e– • a radio emission at λ ~ 1 cm with a magnetic field B ~ 10–4 Gauss requires γ ~ 105 → v ~ 0.99999 c • the measured polarization depends on the orientation of the magnetic field with respect to the line-of-sight

  15. Overview - 14 Auto-absorption: • the synchrotron radiation can itself be absorbed by the e– in relativistic motion (auto-absorption) • efficiency of this auto-absorption maximum at low frequencies → flattening of spectrum at low frequencies • extended parts (radio lobes) `optically thin´ (τ << 1) → no auto-absorption → α ≈ 0.7 • compact nucleus `optically thick´ (τ > 1) → auto-absorption → α ≈ 0 or even < 0 • synchrotron emission → kinetic energy loss for the e– but characteristic time for this loss generally > system lifetime

  16. Overview - 15 Polarization • basically all AGNs are weakly polarized (~0.5 à 2%) (but more than stars for which light gets polarized when it crosses dust clouds) • that polarization is linear, its orientation is variable • some AGNs reach polarizations ~10% : – highly variable objects or – objets without broad emission lines → property that will be explained by the unification model

  17. Overview - 16 Variability • most AGNs are variable • amplitude ~ 0.1 – 1 mag • non periodic variations • variability tends to increase with the observed frequency (radio → X) Light curve of the quasar WFI J2033–4723 over a period ~ 3 years (relative magnitudes)

  18. Black holes and accretion Radiation pressure We assume spherical symmetry (not realistic!) Impulse carried by a photon: p = E/c Radiation pressure = flux of photon impulse = 1/c× energy flux Radiative force: Frad = Prad × σe (σe = cross section interaction e– – γ) Gravitational force on a hydrogen atom:

  19. Black holes and accretion - 2 Eddington limit Stable structure if Frad < Fgrav→Eddington limit: Frad = Fgrav → Eddington luminosity: → maximum L for given M or minimum M for given L → Eddington mass: with L44 = luminosity in units of 1044 erg/s Ex: for a typical quasar (L ~ 1046 erg/s), one gets:

  20. Black holes and accretion - 3 Feeding the black hole Conversion of mass into energy with efficiency η with M = mass accreted by the black hole Luminosity: → accreted mass: Accretion models give η~ 0.1 → for a typical quasar, Eddington accretion rate (necessary to maintain LE) : (↔ maximum accretion rate)

  21. Black holes and accretion - 4 Energy production mechanism The gas has a non-zero angular momentum around the SMBH → it cannot fall radially onto the SMBH → it starts orbiting Friction between particles → gas concentrates into a disk Friction forces < gravitational forces → keplerian motion → differential rotation → maintains friction → heating → friction dissipates kinetic energy → spiral towards the SMBH → friction → temperature increases → radiation more and more energetic closer to the SMBH

  22. Black holes and accretion - 5 Structure of the accretion disk Simplifications: – transparent medium – energy of a particle dissipated locally → emission of a black body with variable T as a function of distance to the SMBH → emitted flux = superposition of Planck functions Depends: – on the magnetic field – on the accretion rate – on the presence of jets Viscosity not well modelled → complex structure

  23. Black holes and accretion - 6 Emission spectrum of the accretion disk Rate of available potential energy: Virial theorem → half of it is converted in kinetic energy the other half in radiation (since 2 surfaces of the accretion disk)

  24. Black holes and accretion - 7 Results: For an accretion disk around a BH of 108 MO, with an Eddington accretion rate, the maximum of emission is located around 100 Å (FUV or soft X rays)

  25. Black holes and accretion - 8 Disk characteristics depending on the accretion rate 1. Weak accretion Thin disk (thickness << radius) → internal radiation flux << perpendicular radiation flux → spectrum = superposition of `local´ spectra at different T° 2. Strong accretion Radiation does not escape easily → thickening of the disk (~ torus) Energy transported inside the disk faster than radiation can evacuate it → internal transfer no more negligible → homogeneization of T° → spectrum ~ black body of T ~ 104 K

  26. Black holes and accretion - 9 Superluminal motion Radio or optical jets coming from AGNs sometimes seem to move at speeds > c This is due to projection effects: jets moving towards us at a speed close to c can have an apparent transverse velocity > c → observing this phenomenon implies ejection velocities close to c

  27. Black holes and accretion - 10 Observations at t1 and t2: The observer does not perceive Δly The interval Δtobs observed between emissions in t1 and t2 is < Δt= t2 – t1

  28. AGN components

  29. AGN components - 2 Broad Line Region – BLR Line width: If thermal broadening → T ~ 1010 K → atoms fully ionized → no spectral lines → broadening must be due to the motion of the gas clouds Let’s assume the clouds in circular orbits around the central mass:

  30. AGN components - 3 We know that an e− on an excitedlevelcanlooseitsenergy by radiation or collision If by radiation → emission line Permitted line: high transition probability (lifetime of the excited state Δt ~10−8 s) Forbidden line: low transition probability (Δt ~1 s) → de-excitation by collision unless the gasdensityisverylow Semi-permitted line: intermediate case Notations: Caii (permitted) Ciii] (semi-permitted) [Civ] (forbidden)

  31. AGN components - 4 • Lack of broad forbidden lines + presence of some semi-permitted lines → estimate of the density in the BLR: ne ~ 109 cm−3 • Ionization stages of different atoms → estimate of temperature: T ~ 20 000 K • Nature of the BLR: gas clouds heated by radiation from the accretion disk and cooled by emission of broad spectral lines • Size of the BLR: estimated by the reverberation mapping method: – variation of the UV continuum → variation of the ionization state of the BLR → variation of broad lines with a delay Δt ~ r/c with r = distance from the center

  32. AGN components - 5 Result : size of the BLR strongly correlated with the AGN UV luminosity: r ~ 0.05 to 200 light-days ~ 10 AU to 0.5 light-years

  33. AGN components - 6 Narrow Line Region – NLR Line width ~ 400 km/s Forbiddenlines→ lowdensityne ~ 103 cm−3,T ~ 16 000 K (densities comparable to Hiregions and molecularclouds, but muchhighertemperatures) Extend over hundreds (or eventhousands) pc Structure oftencone-like (regionreached by the ionizing radiation)

  34. AGN components - 7 Host galaxy • Generally: Seyfert = spiral galaxies quasars in elliptical galaxies … but there are exceptions • Ellipticals with AGN have on average more gas than inactive ones • Frequent signs of gravitational interactions → carry matter in to feed the AGN (but still subject to some debate) • Relation between AGN and star formation (starburst) → common cause? feedback ?

  35. AGN components - 8 •Elliptical hosts on average bluer than inactive ellipticals •Spirals hosts on average redder than inactive spirals → tendency to occupy an intermediate position in the color-magnitude diagrams (green valley)

  36. AGN components - 9

  37. AGN components - 10 Radio galaxies

  38. AGN components - 11 Quasar host galaxies

  39. AGN components - 12 Quasar host galaxies HST image deconvolved image

  40. AGN components - 13 Black hole mass • reverberation mapping → size r of the BLR •width of emission lines (BEL) → velocity dispersion σ in the BLR •if we assume keplerian motion of the clouds → one finds the same correlation between black hole mass and bulge* mass as in inactive galaxies •this correlation is still valid at high redshift (→ z ~ 2) * or mass of the whole elliptical galaxy

  41. Unification

  42. Unification - 2 Common aspects • supermassive black hole at the center of a galaxy • accretion of matter through a disk Two activity modes • radiative: strong accretion intense high-energy radiation → Seyferts, quasars matter ejections • kinematic: low accretion radio jets → radiogalaxies massive hosts with little gas

  43. Unification - 3 Unification model Components: • supermassive black hole • accretion disk • dust torus • BLR • NLR • radio jet +viewing angle

  44. Unification - 4 Direct observation of the dust torus: NGC 4261: active elliptical galaxy at 30 Mpc

  45. Unification - 5 Size of the dust torus – large enough to hide the BLR – smaller than the NLR (2) If the torus hides the BLR → weak continuum and no broad lines → type 2 Seyferts or quasars (1) If line of sight ± perpendicular to torus → one observes the BLR and the accretion disk → type 1 Seyferts or quasars 1 2

  46. Unification - 6 Polarized broad lines Type 2 Seyferts do not display any significant broad lines However, broad lines appear in polarized light → the BLR is not directly observed but in scattered light → polarized twice as many type 2 than type 1 → the torus covers ± 2/3 of the solid angle

  47. Unification - 7 Ultraluminous infrared galaxies (ULIRG) = galaxies emetting more than 1013 LO in far IR (Ltot ± as quasars) • generally present signs of violent gravitational interactions → dust heated by: – starbursts – AGN (2 often linked phenomena) → very young AGNs still buried in a dust cocoon

  48. Unification - 9 Blazars(BL Lac, OVV) • highly variable AGNs • polarized light • very weak emission lines, barely detectable → explained by beaming effect: particles in relativistic motion that emit isotropically in their reference frame → anisotropic emission directed forwards in the observer’s frame → magnification in the direction of motion

  49. Unification - 10 • if the jet is relativistic and directed towards the observer • and if the emitted radiation extends to the optical / UV range → magnification of the synchrotron radiation (continuum) can mask the other spectral components (e.g. emission lines) • strong variability explained by small variations in particle speed and direction in the jet • that beaming also explains the intensity difference between the jet directed ± towards the observer (magnified) and the opposite jet that is directed away (counterjet, attenuated) in other types of AGNs

  50. Unification - 11 Evolution AGN hidden by dust AGN appears AGN cleans its environment SMBH runs out of matter inactive SMBH

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