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Active Galactic NucleiLight of normal galaxies is the sum of the star’s blackbodyspectraStellar temperatures range 3000-40,000 K so galaxyspectrum is supposition of order of mag. range intemperature of BB spectraYields spectrum peaking in range 4000-20,000 AngstromSome galaxies show an excess of light overthat from the stars, covering a much broader frequencyrange - originating from small nuclear region - ActiveGalactic NucleiActive galaxies then split into several different “types”depending on the exact observed propertiesWe will review:-History of AGN-Important observed “identifying” properties of AGN-The Physics behind an AGNHistory•Fath, working at Lick observatory ~1908was taking spectra of the nuclei of spiral“nebulae” and noticed NGC 1068 hadstrong emission lines•Slipher took some higher quality spectraat Lowell in 1917 and noted those lineswere like ones seen in planetary nebulae(ie from excited gas)•1926 - Hubble noticed 3 objects hadsimilar strong emission lines, NGC 1068,NGC 4051, NGC 4151•By 1943 Carl Seyfert concluded theseobjects -having strong and broad high-ionization emission lines and brightnuclei, formed a distinct class of galaxies•Found in spiral galaxiesThree images of NGC 4151 with increasing exposure timeQuasars• In the early 1960s, Maarten Schmidt identified theradio source 3C 273 with a faint, blue star.• the “star’s” spectrum displayed emission lines• the wavelengths of these lines matched no knownelement• Schmidt realized that the emission lines belongedto Hydrogen, but they were highly redshifted(z=0.158)• This object is very (> 1010 light years) far away.• other such objects were subsequently discovered• they were called quasi-stellar radio sources orquasars for shortQuasarsMore radio sources werefound to be associated withpoint-like stellar objects withno associated galaxyBecause of this the sourceswere called quasi-stellar radiosources - quasarsorquasi-stellar objects - QSOsQuasar Spectra• Point-like objectswhich:• have spectra that looknothing like a star• highly redshifted• emit a lot of poweracross all wavelengthsSpectral Energy DistributionHubble ST confirms that quasarsdo live in galaxies…they areActive Galactic Nuclei!• So -quasars are observed with high occurrence rates inthe distant past (high redshift/far away).• this implies that many galaxies had bright nuclei early in theirhistories, but those nuclei have since gone dormant and thatactive nuclei are a phase in the life of an AGN• So many galaxies which look “normal” today havesupermassive black holes at their centers that have juststopped accreting• such as Andromeda and Milky WayLINERS• Low Ionization Nuclear Emission Regions• Low(er)ionization gasemitting the lines• No evidence for anon-thermalcontinuum• Less nuclearpower??Blazars• BL Lacs - cataloged as variablestars, like BL Lacertae• Optically Violent VariablesOVVs• Featurelessoptical spectraBUT• Highly variable‘continuum’emission• High polarization• Rapid variabilityStarburst Galaxies• An average of 1 new star peryear forms in the Milky Way.• We observe some galaxieswith a star-forming rate of100 per yr.• We call them starburstgalaxies.• infrared image of Arp220• They look normal in visible light (1010 L like Milky Way).• but they are 100 times brighter in infrared light• molecular clouds block the visible/UV light from new stars• dust in the clouds absorbs this light and reemits the energy as infrared light• With such a fast rate of star formation, the galaxy will use up its gas..• in only a few 100 million years• starburst phase is temporary in light of fact that galaxy is billions of years oldStarburst Galaxies• 100 times star-forming rate alsomeans 100 times supernova rate.• ISM is full of hotsuperbubbles• supernovae continue topump energy into thesuperbubbles• The hot (107–108 K) gas breaksout• and a galactic wind streamsfrom galaxy• NGC 1569 (X-ray–green;visible–red)• Starburst galaxies are irregular in type.• lots of dusty molecular clouds and usually two distinct clumps of stars• This suggests that the starburst is caused by the collision of two spiral galaxies.• although a close encounter could trigger starburst, e.g. Large MagellanicCloudStarburst Galaxies• There is some speculationthere is a relation betweenthe starburst and AGNphenomena• May be that the extra gasthat ‘lands’ near thequiescent nuclear black holeduring a merger lights upthe activity again for a‘dead’ black hole• Thus mergers would triggerboth starburst activity andactive nucleiRedshiftSmall z (<0.1), low velocity approximation onlyz =!obs"!0!0!obs= (1 + z)!0v ! zcA photon propagating through a vacuum can redshift inone of several ways, each of which produce a freq.shift that is independent of the photon frequencyIn the classical Doppler effect calculation the velocity ofthe emitter causes an apparent shift of frequencyRedshiftWhile Hubble first interpreted galaxy velocities asmeaning galaxies were moving through space, this wasincorrect, galaxy redshifts are cosmological, whichmeans they are due to the expansion of space itselfPhotons from galaxies are travelling through expandingspace and suffer cosmological redshift, and not thesimple Doppler effectαnow and αthen are scale factors for the UniverseConceptual analogy• Classical Dopplereffect• Roll balls acrossrubber sheet• CosmologicalRedshift effect• Stick balls to rubbersheet and stretchthe sheetConsider two galaxies in the universe represented byball bearings and space is an infinite rubber sheet thatcan be imagined with a coordinate grid drawn on itVariety of Active GalacticNucleiQuasars most luminousLess luminous nearby AGN areSeyfert galaxiesLow Ionization Low luminosityones are LINERSFeatureless optical spectra forblazars• Can onefundamentalphysicalexplanation coverall of thesevariants?So, what are AGN? Most/all nucleated galaxies harbor a black hole at their center -we knowthis from studying the velocities of stars around galaxy nuclei We see that BH mass scales with galaxy bulge mass so theformation of the two must be linked, although unclear how. Manyof these BHs “switched off” (dormant)‘Lifetime’ of an AGN• Extent of some radio source structures is > 1 Mpc• Light travel time over that distance is ~ 107 years• AGN must live longer than that for the structures(associated with the activity) to have formed•


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UMBC CMPE 315 - Active Galactic Nuclei

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