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eaa.iop.orgDOI: 10.1888/0333750888/1581 Active Galaxies: ObservationsBelinda J Wilkes, Bradley M Peterson FromEncyclopedia of Astronomy & AstrophysicsP. Murdin © IOP Publishing Ltd 2006 ISBN: 0333750888Downloaded on Thu Mar 02 22:46:21 GMT 2006 [131.215.103.76]Institute of Physics PublishingBristol and PhiladelphiaTerms and ConditionsActive Galaxies: ObservationsENCYCLOPEDIA OF ASTRONOMY AND ASTROPHYSICSActive Galaxies: Observations‘Active galaxy’ is a general term which refers to anygalaxy that produces significant emission in additionto that from its constituent stars, stellar remnants andinterstellar medium. The characterization of such galaxiesas ‘active’ is because the unusual emission characteristicsare generally assumed to be associated with high-energy,eruptive phenomena. The earliest consistent user of theword ‘activity’ as applied to the nuclei of galaxies seemsto have been V A Ambartsumian.In most cases, this non-stellar emission appears tooriginate in the very center, or nucleus, of the galaxy,and these sources are known generically as ‘active galacticnuclei’ (AGN). A galaxy that harbors such a source isknown as the AGN host. Historically, some other typesof sources, such asSTARBURST GALAXIES, that have somephenomenological similarities to AGN (e.g. strong near-UV emission) have also been known as active galaxies,though in these cases the activity might not be confined tothe nuclear regions.AGN are the most luminous long-lived sources inthe universe. They emit strong radiation over the entireobservable wavelength range, from x-rays and γ -raysthrough long-wavelength radio. Acomplete picture of theemission of an AGN can be obtained only by observingit at many wavelengths, preferably simultaneously overthe entire spectrum because they are variable sources.There are a number of different classes of AGN (seeACTIVE GALAXIES: OVERVIEW); SEYFERT GALAXIES constitute thelow-luminosity end of the AGN phenomenon, i.e. theirnuclear luminosities are roughlycomparable with the totalstarlight from the host galaxy. Higher-luminosity AGNare known as ‘quasars’ or ‘quasi-stellar objects’ (QSOs)(seeQUASISTELLAR OBJECTS: OVERVIEW) with the former beingreserved (originally) for the stronger radio sources, andthe latter weaker; in these cases the optical light from theAGN can exceed, sometimes by factors of more than 100,the stellar light from the host galaxy at all wavelengths.There are a wide variety of subclasses within these groups,based on the relative prominence of UV–optical emissionlines, radio properties and polarization (seePOLARIZATIONIN ACTIVE GALAXIES). These will be mentioned below asnecessary.This section will discuss the current status ofobservations over the electromagnetic spectrum. We firstdiscuss the continuous emission of AGN in terms oftheir ‘spectral energy distributions’ (SEDs), i.e. amountof energy emitted in various wavebands. We then focuson the prominent emission lines that are detected in theUV–optical (and, more recently, x-ray) spectra of mostAGN and follow this with a brief description of absorptionfeatures in AGN spectra. Spatially resolved structures inAGN are also discussed.Spectral energy distributionsThe SEDs of normal stars (and galaxies) are wellapproximated as blackbodies in the temperature rangeFigure 1. The median radio–soft x-ray spectral energydistribution for radio-loud (dashed) and radio-quiet (solid)low-redshift quasars. Data from Elvis et al 1994 Astrophys. J.Suppl. 95 1.2000–50 000 K, and thus their emission is stronglyconcentrated in the ultraviolet (UV) through near-infrared(IR) parts of the spectrum. In contrast, AGN emitcomparable energy (per unit logarithmic bandwidth) overmost of the observable spectrum, as can be seen in figure 1.The one exception is the radio region, in which ‘radio-loud’ (RL) AGN are some 3 orders of magnitude brighterthan their ‘radio-quiet’ (RQ) counterparts. These RL AGNconstitute ∼10% of the population. Figure 1 shows thelow-redshift median SED for RL and RQ objects from asample of more than 40 AGN. It is plotted as log(νLν)versus log ν, which shows the energy output in eachwaveband and also highlights the important structure inthe SED.Our knowledge of these SEDs is limited by currentobservational technology to fairly low-redshift, brightsourcesand includes (as is clear in figure 1) important gapsin various wavebands.1. The absence of observations at high energies (γ -rays)is due to current technical limitations; higher-energydetectors are not sensitive enough to detect mostAGN. Those that have been detected so far aregenerally core-dominated (CD) RL AGN, i.e. thosewhose emission is boosted by a relativistic flow alongour line of sight (seeACTIVE GALAXIES: UNIFIED MODEL formore detail).2. The EUV gap, between the far-UV and soft x-rayregions of the spectrum (i.e. between 912 Åand about0.1 keV, spanning a factor of about 10 in photonenergy), is due to the large opacity of the interstellarCopyright © Nature Publishing Group 2001Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No. 785998and Institute of Physics Publishing 2001Dirac House, Temple Back, Bristol, BS1 6BE, UK1Active Galaxies: ObservationsENCYCLOPEDIA OF ASTRONOMY AND ASTROPHYSICSmedium in our own Galaxy to hydrogen-ionizingphotons. Photons in this energy range are especiallyimportant because these drive the strong emissionlines seen throughout the UV and optical spectra ofAGN.3. The gap between the submillimeter and short-wavelength radio is again due to technological limits.New detectors and satellites have recently reducedtheselimitations considerablyand revealedsharp far-IR spectral breaks for all but the CD RL AGNs.The most prominent feature of AGN SEDs is thestrong peak in the UV spectrum. The feature, whichis often known as the ‘big blue bump’ (BBB), begins inthe near-IR and may extend all the way to soft x-rayenergies. It is observed with a combination of low-resolutionopticaland UVspectroscopy and/ormulticolor,optical photometry. It can beplausibly identifiedas arisingin an accretion disk (AD) containing material with a widerangeof temperaturesand orbitingaround asupermassiveblack hole. The BBB peaks in the extreme UV, which isconsistent with the expected emission from an AD arounda ∼108Mblack hole that is accreting material at theEddington rate.The IR emission from RQ AGN is mainly observedvia multicolor photometry, although IR spectroscopy isa rapidly


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CALTECH AY 21 - Active Galaxies: Observations

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