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Ay 127!Quasars & AGN:!Their Phenomenology, Physics, and Evolution!Quasars and AGN!• They are highly energetic manifestations in the nuclei of galaxies, believed to be powered by accretion onto massive black holes!• Empirical classification schemes and various types have been developed, on the basis of the spectra; but recently, various unification schemes have been developed to explain AGN as different appearances of the same underlying phenomenon!• Quasars/AGN are observed to evolve strongly in time, with the comoving densities of luminous ones increasing by ~ 103 from z ~ 0 to z ~ 2!• At z ~ 0, at least 30% of all galaxies show some sign of a nuclear activity (mostly low level); ~ 1% can be classified as Seyferts (moderately luminous), and ~ 10-6 contain luminous quasars!• However, we think that most or all non-dwarf galaxies contain SMBHs, and thus probably underwent at least one AGN phase!AGN,!an artist’s view!Obscuring dusty torus!Central black hole!Relativistic jet!Accretion disk!Illumination cone!Black hole: R ~ 10-6 - 10-5 pc!Accretion disk: R ~ 10-3 - 10-2 pc!Broad line region: R ~ 0.1 - 1 pc!Narrow line region: R ~ 10 - 102 pc!Obscuring torus or disk: R ~ 102 - 103 pc!Narrow line region!Broad line region!Observable Properties of AGN!• Energy emission over a broad range of frequencies, from radio to gamma rays!– Nonthermal radio or X-ray emission is a good way to find AGN!– Generally bluer spectra than stars: “UV excess”!– Colors unlike those of stars, especially when modified by the intergalactic absorption!• Presence of strong, usually broad emission lines in their spectra!• Can reach large luminosities, up to ~ 1015 L!• Strong variability at all time scales!– Implies small physical size of the emission region!• Central engines unresolved!• Zero proper motions due to a large distances!All of these have been used to devise methods to discover AGN, and each method has its own limitations and selection effects!UV-Optical Spectra of Quasars!Strong, broad emission lines:!Balmer lines !of hydrogen!Prominent lines of abundant ions!Explaining the Broad-Band Spectral Energy Distribution in AGN!Synchrotron jet (radio)!Synchrotron jet (X-ray)!Thermal emission from dusty!Torus (FIR)!Thermal emission from accretion!Disk (UV)!AGN Classification!• According to radio emission:!– Radio loud: radio galaxies (RGs) and quasars; F-R types I and II!– Radio quiet (but perhaps not entirely radio silent)!• According to optical spectrum:!– Narrow-line RGs, Seyfert 2’s; Liners!– Broad line RGs, Seyfert 1’s, quasars!• According to optical luminosity:!– Seyfert to quasar sequence, range of radio powers, etc.!• Special types:!– Blazars (aka BL Lac’s) and optically violently variable (OVV) objects!• These classifications are largely parallel!• Some distinction may reflect real, internal physical differences, and some may be simply orientation effects!– This is the central thesis of the AGN unification models!Types of Seyfert Galaxies!Type 1 Seyfert galaxies have in their spectra:!• Narrow emission lines, with a width of several hundred km/s!• Broad emission lines, with widths up to 104 km/s!They also have brighter and bluer nuclei!Type 2 Seyfert galaxies have only the narrow line component:!Both types have high ionization, forbidden lines!(= transitions not easily observed in the lab) !Types of Seyfert Galaxies!4000! 6000!5000!λ (Å)!Spectroscopic Diagnostics!Intensity ratios of various emission lines depend on the spectrum of the ionizing continuum radiation: to get lines from high energy levels!(e.g., ionizing potentials of tens of eV), one needs “hard” spectra with lots of high energy (UV / soft X-ray) photons.!Accretion disks can provide those in AGN, while objects powered by star formation have much “softer” spectra!Radio Galaxies: Typical Examples!Centaurus A!Fornax A!Radio overlayed on optical images!Energy stored in radio lobes can reach ~ 1060 - 1061 erg. If jet lifetime is ~ 108 yrs, the implied mechanical luminosities are ~ 1012 - 1013 L!Radio Source Classification!Fanaroff-Riley Type I (FR I): Separation between the points of peak intensity in the lobes < 1/2 the largest size of the source!Edge darkened radio jets, slower jet speeds, lower radio power!Fanaroff-Riley Type II (FR II): Separation between the points of peak intensity in the lobes > 1/2 the largest size of the source!Edge brightened radio jets, speeds ~0.1c, higher radio power!FR I: 3C272.1! FR II: 3C47!BL Lacs (Blazars) and OVVs!Named after the prototype BL Lacertae. They have strong, blue, variable continua, and lack strong emission or absorption lines in their spectra:!Related class are optically violent variables. All AGN are variable, but OVVs show large variations (> 0.1 mag) in optical flux on short timescales (< day), and much stronger at longer time scales!Quasar Surveys!• In order to study QSOs (and other AGN), we first have to find them, in large numbers, and hopefully in a systematic fashion!– This is especially important for studies of their evolution!• Recall that each discovery method has its own biases!• Nowadays the most popular technique is to use colors to separate QSOs from normal stars!– In optical, one can also use slitless spectroscopy, variability, and zero proper motions !• Soft X-ray (up to a few keV) and optical selection find the same types of relatively unobscured objects; hard X-ray selection and FIR/sub-mm detect more obscured populations; radio finds both!• Next: multi-wavelength, survey cross-matching in the Virtual Observatory framework - will help with the selection effects!Quasar Counts!For the unobscured, Type 1 QSOs; they may be outnumbered by the obscured ones. Down to ~ 22th mag, there are ~ 100 deg-2; down to ~ 29th mag, probably a few hundred more  a total of a few × 107 over the entire sky, or ~ 1 per 1000 faint galaxies!Redshift range z ~ 0.3 - 2.3!Redshift range z ~ 2.3 - 3.3!SDSS Quasar Survey!z!i!r!g!u!Ratios of fluxes in different survey filters (=colors) are in general different for QSOs and for stars - even though both look “stellar” on the images. The colors will change with redshift as different features (emission lines, continuum breaks) shift from one filter to another. For each redshift range, a different filter combination would be the


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CALTECH AY 127 - Quasars & AGN

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