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4. Quasars & Blazars (AGN – Part 2) Some radio sources were not associated with galaxies, but with stellar-like sources on optical images In 1963, Maarten Schmidt made the breakthrough that the spectrum of 3C273 had nebular emission lines at the then unheard of redshift of z=0.158 Soon afterward, 3C48 was found to be at a redshift of z=0.367 (thus began the race for the highest redshift quasar!) z = 0.2 and z = 1.0 shifts……. Quasars are very luminous – L ~ 1013-1015L These objects are AGN whose nuclear light dominates so that we can’t distinguish the host galaxy These “3C” objects are known as quasi-stellar radio sources (quasars); there are also quasi-stellar objects (QSO’s) identified via the spectra that are radio quiet Radio quiet QSO’s outnumber quasars by 10-30, the term quasar is often used for both Quasars are variable over small time scales – hours to days! Current record for the highest redshift quasar is z=6.4 found in the Sloan Digital Sky Survey: PKS 1117-248, optical : Some quasars display extended radio structures: The entire SED of radio loud (top) and radio quiet quasars (lower panel): Quasar host galaxies: Many are interacting – do interactions trigger AGN activity? Promote fueling gas into centres? Quasar Evolution Note: not all galaxies with SMBHs (Super-Massive Black Holes) are AGN (even the Milky Way) Quasars were much (~1000 times) more numerous at z~3 than today: Is an AGN a requirement for galaxy formation? Does every galaxy have a SMBH? (Remember -- lots have been discovered in nearby ellipticals and bulges!) More interactions in the past: Quasars were also much more luminous in the past There is probably some combination of luminosity and density evolution. Here are the Luminosity Functions for quasars at different redshifts (the numbers actually start to drop off above z=2). (This is the co-moving space density) Co-moving volume = 1/(1+z)3 2df quasar luminosity function: 5. Blazars: BL Lacs Catalogued as variable stars (e.g., BL Lacertae) Highly variable, highly polarized Variability occurs on the time scale of days Virtually featureless continuous spectra Also: optically violent variables (OVV) quasars Collectively OVV’s and BL Lac’s are known as blazers 6. Interpretation of AGN: accretion 1. Size. Variability on short time scales – hours - implies a small region: R = ct ~ 7-10 AU !! (for timescale variability on the order of hours) 2. Mass. Eddington luminosity: the largest luminosity that can pass through a layer of gas in hydrostatic equilibrium, supposing spherical symmetry. Using the mass-luminosity relation, it can be used to set limits on the mass. For an AGN with an observed (bolometric) luminosity L, can estimate the minimum mass of the black hole involved. Suppose the gas around the black hole is: • Spherically symmetric • Fully ionized hydrogen At distance r, flux is: F L / 4r2 This is flux of energy. Since momentum of a photon of energy E is E / c, momentum flux due to radiation is: Prad L/ 4r2c This is the pressure that would be exerted on a totally absorbing surface at distance r from the source. Force exerted on the gas depends upon the opacity (i.e. the fraction of the radiation absorbed per unit mass of gas). Minimum force is given by the absorption due just to free electrons. This is given by the Thomson cross-section: T 83)e2mec2)26.65 1025 cm2 Outward radiation force on a single electron is: Frad LT / (4r2c) Inward force due to gravity of a central point mass M is: Fgrav GM(mp me )r2 = GMmpr2 Note: include proton mass in Fgrav since electrons and protons coupled due to electrostatic forces. Setting Frad = Fgrav, and solving for L: Ledd 4GMm p c / T Ledd 1.3 1038 M M ergs 1 3.3 10 4 Lsun M sun M sun This luminosity is known as the Eddington limit. It is the maximum luminosity of a source of mass M, which is powered by spherical accretion of gas. Can invert the argument. If a source with observed luminosity L is radiating at the Eddington limit, the mass would be a minimum mass - source could actually be radiating at much less than the Eddington limit. Need a large mass in a small space Therefore, probably powered by a black hole!! Check for consistency: Schwarzschild radius is the radius where the “escape velocity” equals the speed of light around a black hole of mass M: RS = 2GM/c2 It relates mass and size! Accretion: Continuum emission powered by gas falling onto central black hole, At a rate of one solar mass per year, losing potential energy and radiating Since gas has angular momentum it forms an accretion disk: an accretion disk around a supermassive black hole Gas clouds in surrounding galaxy are ionized by this central engine. Clouds closest to the central engine are dense and have high velocities (otherwise they would fall in!). So emission lines from this region are broad (~2000 km/s) – the broad line region (BLR) Clouds farther away from the central engine have a lower density and lower velocities. Line emission is narrow (5001000 km/s) – the narrow line region (NLR) Dust torus surrounds the central engine and BLR in the same plane as the accretion disk. The orientation of the dust torus relative to the line of sight affects the appearance of the AGN. How fast must gas be supplied to the black hole to produce typical AGN luminosities (1044 - 1046 erg s-1)? L M˙ c2 Accretion rate: units g s-1 or Solar masses per year A mass m of gas at infinity has zero potential energy. Energy available if the gas spirals in to radius r is: E GMBHm / r L GMBH (dm/dt) / r Really an upper limit - not all the potential energy will be radiated as the gas falls in. Standard estimate is to sustain a quasar is: w needed 1046 erg s-1 c21026 g s-1 Mass supply needed is fairly modest… -1 Summary Seyfert 1: Broad/narrow emission, weak radio, strong Xray,spirals, variable Seyfert 2: narrow emission, weak X-ray, spirals, not variable Quasars QSR: broad/narrow, strong radio, FRII, polarisation, variable Quasars QSO: broad/narrow polarisation, variable lines, weak radio, weak Blazars BL Lacs: strong polarisation, absent/weak lines, strong radio, highly variable, mainly ellipticals OVV quasars: powerful BL Lacs but with emission lines Radio Galaxies BLRG: Broad/narrow lines, strong radio emission, FR II, weak polarisation, elliptical variable, variable Radio Galaxies NLRG: narrow lines only, strong radio, FR I/II, no polarisation, elliptical, not variable In Seyfert 1s we are observing the active nucleus directly. In Seyfert 2s we observe it through an obscuring structure which prevents us getting a direct view of the optical continuum: At higher luminosities, quasars take the place of Seyfert 1s, but the corresponding 'quasar 2s' are elusive at present. If they do not have the scattering component of Seyfert 2s they would be hard to detect except through their luminous narrow-line and hard X-ray emission. High-luminosity radio-loud quasars can be unified with narrow-line radio galaxies in a manner directly analoguous to the Seyfert 1/2 unification. X-ray evidence, where available, supports the unified picture: powerful radio galaxies show evidence of obscuration from a torus, while quasars do not. The population of radio galaxies is completely dominated by lowluminosity, low-excitation objects. There is no evidence for a torus in these objects at all. Most likely, they form a separate class in which only jet-related emission is important. At small angles to the line of sight, they will appear as BL Lac objects 7. Intervening gas: Lyman-alpha forests: the Ly-alpha (1215A at rest) peak shifted to 3730A : Ly-alpha wavelength of the neutral H in intervening galaxies lies between 1215A and the longer wavelength. Narrow absorption lines: low column, intergalactic clouds at various redshifts. Damped Ly absorbers – intervening high density (N(HI) > 2 x 1020 cm2) gas along the line of sight to a distant (and bright!) quasar Probably due to the outer disks of galaxies High density implies the absorption line is optically thick (“damped”) We can trace the evolution of HI in damped Ly absorbers We can also measure the metal abundances of DLAs, Lyman break galaxies, the Ly forest (intergalactic medium) and quasars. Next, a Lyman break galaxy (utilises 912A also, below which HI can be ionized from the ground state: These breaks can be utilized to derive ‘photometric’ redshift using UBVRI photometry. THE END