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Transcript
AGN • The term ``Active Galactic Nucleus’’ (AGN) refers to the existence of energetic phenomena in the nuclei of galaxies that cannot be attributed directly to stars • AGN are classified based (in large part) on their luminosity in different wavebands (especially the optical and radio) Seyfert Galaxies • In 1943, Carl Seyfert identified a class of spiral galaxies w/unusually bright central regions (stellar appearing cores), whose spectra are dominated by high-excitation nuclear emission lines Seyfert galaxies make up about 1% of spirals, so there are numerous nearby examples; however, at the time, they were viewed as oddities Seyfert Galaxy NGC 7742 Discovery of Quasars • In late 1950s, w/development of radio telescopes, some celestial objects were found to emit profusely • Catalogs compiled, largely at Cambridge (e.g., 3rd such catalog known as “3C,” & objects in it given numerical designation) • The positions of some of these radio sources were found to be coincident with objects that looked like stars on normal photographs The nature of these bluish-looking “radio stars” (a typical star in the sky is red) was very uncertain (they became known as quasi-stellar radio sources) No known mechanism by which stars could produce so much radio radiation. Moreover, the spectra showed very strong broad emission lines at unidentified wavelengths • Maarten Schmidt made a breakthrough in 1963 – realized emission lines seen in the spectrum of 3C 273 were the hydrogen Balmer-series emission lines – but at the uncommonly large redshift z=0.158 • More disturbing than this vast distance was the enormous luminosity implied – 3C 273 was and remains the brightest known quasar (B=13.1 mag) – It is about 100 times as luminous as normal bright spirals like the Milky Way or M31 • Probable importance of quasars was recognized immediately, providing strong motivation to find more • Subsequent searches for blue stars revealed a class of “radio-quiet” quasars – Optical spectra similar to those of quasars, yet radio emission weak or absent – Now called QSOs (“quasi-stellar objects”), but common to use terms interchangeably – About 10x more numerous than radio-loud quasars AGN SEDs cannot be described as blackbodies Reflects amount of energy emitted in each equally spaced interval on the logarithmic frequency axis Quasars are among the most luminous objects at every wavelength at which they have been observed Working model for AGN phenomenon is a `central engine’ that consists of a hot accretion disk surrounding a supermassive black hole. Energy is generated by gravitational infall of material, which is heated to high temperatures in the disk Radiation Force • The energy produced propagates outwards & can interact with infalling matter by absorption or scattering • Through this interaction, the momentum of the radiation is transferred to the matter, i.e., the infalling matter experiences an outward-directed radiation force • For matter to fall onto the SMBH at all, this radiation force needs to be smaller than the gravitational force Source of Radiation • Assume -Spherically symmetric emission -Completely ionized hydrogen gas • Outward energy flux at distance r from center is S=L/4πr2 • The momentum carried by a photon (energy E=hν) is E/c, so the outward momentum flux, or pressure, is S/c= L/4πr2 c • The outward radiation force on a single electron is then obtained by multiplying by the cross-section for interaction with a photon (Thomson scattering), frad= σTL/4πr2c Eddington Limit • Outward radiation force "T L f rad = 4!r 2c • Force due to gravity per electron-proton pair (ignore electron mass because nearly a factor of 2000 smaller than proton mass) GM • m p f grav = r2 • Radiation pressure balances gravity when f rad = f grav Critical luminosity is called the Eddington limit (bolometric, since σT is photon freq-independent) ! T L GM • m p = 2 4"r c r2 If L is larger than this value, the pressure due to 4"Gcm p radiation exceeds the gravitational force at all radii, LEdd = M Edd and gas will be blown away !T LEdd M• " 1.3 !10 ergs / s M Sun 38 • For accretion to occur at all, we need L<LEdd • We can turn the above argument around: if a luminosity L is observed, we conclude LEdd>L, or M • > M Edd = $T L LEdd " 8 !105 44 M Sun 4#Gcm p 10 ergs / s • Thus, a lower limit for the mass of the SMBH can be derived from the luminosity. For QSOs, typical masses are >108 MSun, while Seyfert galaxies have lower limits of ~106 MSun Fueling AGN • The fundamental process at work in AGN is the conversion of mass to energy, which is done with some efficiency ε • The energy available for emission in radiation from a mass M is E=εMc2 dE 2 & L = = ! M c • The rate at which energy is emitted, dt gives the mass accretion rate L $ L !3 ' !1 & M = 2 ) 1.8 (10 % M yr " Sun 44 *c * 10 ergs / s & # • For ε~0.1, the accretion rate for even fairly high-luminosity sources, say 1046 ergs/s, is only about 2 Msun/yr Zoo of AGN • Luminous AGN are classified as • Seyfert galaxies (Type 1 and Type 2) • Quasars • BL Lacs • Radio galaxies (‘broad line’ and ‘narrow line’ varieties) • All are powered by accretion onto supermassive black holes, so why are there so many classes? Are these all physically distinct objects? • Unified model seeks to explain the different classes as being due to different orientations of intrinsically similar systems to the observer’s line of sight The Unified Model The type of AGN that you see in the optical depends on your line of sight Around the accretion disk are relatively dense, fastmoving clouds of hot gas responsible for the broad emission lines seen in unobscured AGN Emission lines are “broad” due to differential Doppler shifts from the bulk motions of the individual fastmoving line-emitting clouds But, if you are viewing the AGN through a dust torus that hides the accretion disk, then only slowermoving clouds farther from the black hole are visible, and the emission lines are narrow If you are viewing the disk face-on & a beam of radiation is being produced, then the active galaxy is a BL Lac object (also know as a blazar), which is featureless No absorption or emission lines AGN Evolution • We now know that dormant supermassive black holes reside at the centers of almost all present-day galaxies • Their presence has been detected through their gravitational influence on neighboring stars and gas, which orbit the holes Milky Way R. Genzel • We also now know from local observations that a tight relation exists between – the mass of the black hole & the spread of stellar velocities within the host galaxy M• " ! 4 – or, less perfectly, the mass of the black hole & the mass of the host galaxy Mbh / Mbulge = 0.001-0.002 This hints at a deep connection between the processes that formed the stars in the host and the processes that formed the central black hole When Did These SMBHs Form? Historic Answer: Number of optically selected quasars peaks at z=2-3 We are here! Time after Big Bang (Gyr) Where Did They Go? • Galaxies in the past were closer together collisions were more common, pushing gas into the black hole • Galaxies in the past had more gas (not yet incorporated into stars) feeding frenzy for the black hole! Indeed, often the galaxies hosting quasars are observed to be interacting or merging w/other galaxies • However, as a quasar’s central black hole consumed most of its surrounding gas, the quasar probably faded w/time • But the supermassive black hole itself cannot be destroyed! Quasar Normal Galaxy V (Very Feeble) So, did all the action really take place at early times? Problem Here! Optical surveys may miss a lot of sources due to obscuration, which moves the light into the far-infrared rather than the optical Thus, the optical results may not be the whole story Accretion onto supermassive black holes can be obscured by dust & gas that absorbs optical light; why not try using X-rays? C. Done Giacconi’s 1962 rocket mission to study X-rays from the Moon found a diffuse uniform X-ray “glow” from the sky This X-ray Background (XRB) radiation was the first cosmic background radiation discovered Only a small fraction of the total energy produced in the universe emerges in X-rays, but X-ray surveys are a window on black hole evolution The Great Escape • Early X-ray telescopes could only produce images in low-energy (soft; 0.5-2 keV) X-rays • Unfortunately, even soft X-ray surveys cannot find all of the sources that make up the X-ray background due to obscuration • Thus, observations are also needed in high-energy (hard) X-rays • Hard X-ray photons can penetrate the dust and gas cocoon that may be obscuring a black hole Modellers predicted what one should find: a population of Type 2 QSOs at z=2-3 Schmidt 1998 Fundamental goal of the Chandra X-ray Observatory: Resolve the sources of the 2-8 keV ‘hard’ XRB XMM-Newton also observes at these high energies: poorer angular resolution than Chandra, but larger telescope collecting area Essential Instrumentation Chandra/XMM have revolutionized distant AGN studies Now possible • to map the history of the AGN population using hard X-ray surveys, and • for the first time, to compare high-redshift & low-redshift samples chosen in the same restframe hard energy (2-8 keV) band Hard X-rays • Can directly probe AGN activity • Are uncontaminated by star formation processes at the X-ray luminosities of interest (I will consider any source with L2-8 keV>1042 ergs s-1 to be an AGN) • Can detect all but the most absorbed sources (may miss sources with NH>1024 cm-2) Chandra or XMM? The advantage of Chandra for this kind of work is its high positional accuracy, which minimizes misidentifications of faint optical galaxy counterparts (Even with Chandra, about 20% of faint-end optical identifications [R=24-26] will be spurious) Moreover, Chandra does not reach the confusion limit, even with the deepest observations CDF-N CDF-N 12 Ms Ms 503 sources RED: 0.5-2 keV GREEN: 2-4 keV BLUE: 4-8 keV D. sources Alexander 370 0.13 sq deg Differential X-ray Number Counts CDF-N + HAWAII S COMBINED -1.63 >80-90% of the 2-8 keV X-ray background has been resolved into discrete sources Cowie et al. 2002 CDF-S S -2.57 Uncertainties in XRB measurement are at 10-20% level, so we cannot accurately determine the resolved fraction of the XRB. Problematic for population synthesis modelers! Hickox & Markevitch 2006 Contributions to the 2-8 keV XRB Unexpected result: large field-to-field variations in the X-ray number counts on the scale of a Chandra image COMBINED CDF-N CDF-S SSA13 Need many Chandra fields to get average true number counts Importance of Wide-Area Surveys Large areas also needed to get necessary volume to • Map low-redshift luminosity function • Sample small number of very high-z sources Can be done w/serendipitous pointings (ChaMP strategy; e.g., Kim et al. 2004; Green et al. 2004) or w/contiguous fields Advantage of contiguous approach is that one can also study hard X-ray source clustering CLASXS 400 ks 525 sources 0.36 sq deg Field-to-field variance of ~50% on solid angles of ~240 arcmin2 Yang et al. 2003 CDF-N SSA13 All 9 LH fields CDF-S Yang et al. 2003 A Multiwavelength Approach • X-ray observations are essential for finding hidden black holes, but they are not sufficient for • fully understanding the nature of the sources • determining their distances (redshifts) • We need observations at many different wavelengths for this Striking how modest the number of X-ray sources is compared to the number of optical sources Diverse assortment of counterparts to hard X-ray sources Redshift Distribution of the X-ray Sources Above f(2-8 keV)~10-14 ergs cm-2 s-1, 80-90% of hard X-ray sources have redshifts, while below this flux, ~60% ASCA CDF-N CLASXS CDF-S Barger et al. 2005 All All z HEX BLAGN Lower X-ray flux sources are often optically faint and red and hence difficult to measure redshifts for A2390 CISCO + HST I, J, H 20’ x 20’ shown RJK NIR Photometric Redshifts With the addition of our NIR data and MIR data from Spitzer, we are able to make reasonable photometric redshift estimates However, they do not tell us the spectral type of the galaxy producing the X-ray light Spectroscopic samples are highly complete to R=24.5 Photometric redshifts increase overall identified fraction to about 85% Photometric redshifts Spectroscopic redshifts Barger et al. 2005 Spectroscopic incompleteness at z>1.2 due to absence of strong spectral features & faintness of sources at these zs Photometric redshifts (open) Barger et al. 2005 Large Scale Structure in CDF-N Barger et al. 2002 Averaged Out When Include CLASXS Data Steffen et al. 2004 Spatial Correlation Function How correlated are the X-ray sources? Do the hard X-ray sources predominantly lie in highly clustered regions? How does the correlation strength compare to that of massive galaxies? (Alison Coil colloquium) AGN Steffen et al. 2004 Some Striking Results First striking result: many of these X-ray identified AGN could not have been picked out w/optical spectra alone---no AGN signatures! Barger et al. 2002 Second striking result: median z~1, not the z~2-3 expected, and lower X-ray flux sources are not QSOs (i.e., Lx>1044 ergs/s) Uncertainties are 1σ median ranges Redshift distribution in 3 xStandard redshift-luminosity relation ray flux bins. for a source with rest-frame 2-8 keV 44 ergs s-1 Notice that the median and luminosity of 10 range do not change much with x-ray flux Solid squares--spectroscopic medians for full sample Open squares--spectroscopic +photometric medians for CDF-N & CDF-S only Individual sources in the CDF-N (8’ radius), where we have spectroscopically identified 70% of the sources Lrf(2-8 keV)=1044 ergs/s 1043 ergs/s Red=spec-zs Purple=phot-zs Diamonds=unid We now know the historic answer is not the full answer! Newly discovered moderate luminosity X-ray sources peak at lower z, and there are many more of them 1043-1044 ergs/s 1044-1045 ergs/s (X-ray analog of optical quasars) Barger & Cowie 2005 Hard X-ray Luminosity Functions When computing rest-frame hard (2-8 keV) X-ray luminosity functions, one of the interesting things is to make a comparison with optically-selected quasar samples To do that, one needs to use optical spectroscopic classifications to determine the broad-line AGN luminosity functions separately Optical Spectral Classifications of the X-ray Sources Most groups use the four optical spectral classes of Szokoly et al. 2004, which are crude by the standards of optical AGN specialists: absorbers, star formers, high-excitation sources (HEX), broad-line AGN (BLAGN; FWHM>2000 km/s) The HEX sources are quite easily distinguished from the BLAGN and from the low excitation sources weak Hβ narrow CIV Cowie & Barger Noise is dominated by the noisiest spectrum The star formers have strong Hβ and do not show signs of [NeV] or of broad underlying Balmer lines 68 sources show no emission lines at all (absorbers) strong Hβ These would not have been picked out as AGN from their optical spectra What are the X-ray colors by optical spectral class? High-Energy Spectra • X-ray power-law fits are generally of the form "! "! PE ( photons / s / keV ) # E # $ since photons per second is close to the measured quantity of counts per second • In units of energy flux F" % PE ( photons / s / keV ) & h" (ergs / photon) % " #$+1 % " #! α is usually referred to as the `energy index’ and Γ (= α+1) is called the `photon index’ • AGNs are well-fit by a photon index of 1.8-2 BLAGN are nearly all soft and show essentially no visible absorption in X-rays, consistent w/our understanding of them as unobscured 2 x 1021 cm-2 ! = 1.8 Barger et al. 2005 All the other AGN are well-described by a power-law spectrum with photoelectric absorption spread over a wide range of NH 3 x 1022 cm-2 ! = 1.8 Open squares---absorbers and star formers Solid squares---high-excitation signatures Triangles---unidentified sources Barger et al. 2005 ALL LOCAL BLAGNs (RXTE) Sazonov & Revnivtsev (2004) Barger et al. 2005 As move to lower z, all the sources are decreasing in L while the LFs are maintaining the same shapes; if drifted the x-axis, plots would look very much the same from z=1.2 to z=0 (RXTE) Sazonov & Revnivtsev (2004) Sanity Check Good agreement between the optical and X-ray selected LFs! Our optical spectroscopic classification of BLAGN is consistent with that of groups doing direct optical selection Richards et al. 2005 Higher Redshift Intervals Incompleteness larger here, but phot-zs (triangles) indicate unids mostly lie in z=1.5-3 interval Shapes no longer wellrepresented by the maximum likelihood fits to the z=0-1.2 HXLFs computed at z=1 (blue curves): Thus, PLE does not continue beyond z=1.2 There are fewer low-L sources than one would expect; thus, the light density is more dominated by higher L sources at these z The Steffen Effect BLAGN dominate the number densities at the higher X-ray luminosities This therefore says that almost all luminous objects are unobscured BLAGN Since there is a substantial fraction of non-BLAGN sources at the lower luminosities, there must be some luminosity dependence on the obscuration: the simple unified model is not adequate Accretion History of the Universe We can now infer the present-day supermassive black hole mass density accreted by AGN and compare it with the locally measured value (Soltan 1982) (From the standard theory of accretion onto black holes, a maximum efficiency of 6% for accretion onto a non-rotating BH is derived) BCs 2-8 keV comoving energy density production rate drops rapidly from z=1 to z=0; peak is in interval z=0.8-1.2 Open=spectroscopic sources Solid=all, including phot-zs; ‘no ids’ put at z=3 Negative values mean nuclear dominated Cumulative growth of AGNs from Chandra (red curve) compared with the cumulative SFH Both form most of their mass at late times (z~1). If AGN feedback has a significant effect, the relative histories can help diagnose that • Thus, there is room for the obscured accretion that we see with the Chandra observations (only ½ to ¼ of the SMBH mass density was fabricated in broad-line AGN) • One remaining issue is whether there additional high-obscured sources that we are missing in the 2-8 keV sample • Within the uncertainties, there could be a comparable contribution from this type of source, though not much more Summary • X-ray data are consistent with many of the non-BLAGN (the dominant population) having high column densities • AGN evolve very rapidly to z~1.2, consistent with pure luminosity evolution to that redshift • z~1 is where the bulk of the supermassive black hole population forms • Simple unified model is not correct, whether one uses X-ray color or optical spectral classification---there are far fewer low X-ray luminosity unobscured sources than obscured sources The End