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CALTECH AY 21 - Seyfert Galaxies

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eaa.iop.orgDOI: 10.1888/0333750888/1584 Seyfert GalaxiesMark Whittle FromEncyclopedia of Astronomy & AstrophysicsP. Murdin © IOP Publishing Ltd 2006 ISBN: 0333750888Downloaded on Thu Mar 02 23:38:01 GMT 2006 [131.215.103.76]Institute of Physics PublishingBristol and PhiladelphiaTerms and ConditionsSeyfert GalaxiesENCYCLOPEDIA OF ASTRONOMY AND ASTROPHYSICSSeyfert GalaxiesIn many respects Seyfert galaxies are similar to anyother galaxy, except their nuclear regions show unusualproperties: they can be much brighter and their spectrashow strong emission lines of high excitation. Such nucleiare called ‘active’, and so Seyfert galaxies belong to thewider category of ‘ACTIVE GALAXIES’ or, more specifically,they have ‘ACTIVE GALACTIC NUCLEI’ (AGN). Seyferts were,in fact, the first type of active galaxy to be discovered.In the zoo of AGNs the power of the nuclear activityspans a wide range. Within this zoo, Seyferts showintermediate levels of activity, being more powerful thanLINER galaxies but less powerful thanQUASARS. Seyfertsalso join the majority of active galaxies in being ‘radio-quiet’, with radio luminosities 103–104times weaker thanthe ‘radio loud’ category, which includesRADIO GALAXIESand radio quasars.Over the past 20 years much effort has gone intofinding Seyfert galaxies amongst the more normal galaxypopulation. Efficient ways to detect Seyferts includelooking specifically for galaxies with bright UV emission,bright x-ray emission, or unusual far-infrared colors, sincethese are common characteristics of Seyferts. The fractionof galaxies with active nuclei depends on the level ofactivity: moderate luminosity Seyferts make up ∼ 1–2% ofnearby bright galaxies, lower luminosity Seyferts make up∼ 10%, while LINERs make up ∼ 40%. Currently, about800 Seyferts are known, though the number continues torise.Froma widerange of studies, apicture hasemerged ofthe structure of the central regions of Seyferts. Briefly, thegravitational field of a supermassive (106–109M) blackhole provides the ultimate energy source, as gas fallsinwards via a luminous accretion disk (seeSUPERMASSIVEBLACK HOLES IN AGN). A small region of fast moving cloudssurrounds this central engine, spanning a few light daysor weeks, while a larger region of slower moving cloudsextends out a few hundred to a few thousand light years.In some cases, oppositely directed collimated jets of lowdensity gas plow into the surrounding galaxy, acceleratingthe interstellar medium and creating jet-like or linearradio structures. In at least some Seyferts, dense gas anddust shroud the innermost regions, hiding them from ourdirect view, though the nuclear radiation escapes in otherdirections to light up gas and dust residing much furtherout in the galaxy.The conditions that give rise to Seyfert activityare still under debate. While Seyferts are found in awide range of galaxy types, they are more commonlyfound in reasonably luminous early-type spirals (e.g.types S0/a, Sa, Sb in the Hubble classification scheme).It seems, therefore, that prerequisites favoring Seyfertactivity include a massive galaxy bulge and the presenceof an interstellar medium. There is also evidence thatSeyferts prefer galaxies with an asymmetric gravitationalfield arising from a bar or other distortion, possiblyinduced by a nearby companion galaxy. Such distortionsare thought to be instrumental in funneling gas down tothe nuclear regions where it can then fuel the central blackhole.Discovery, classification and detectionThe early identification of nearby Seyfert galaxies restedon two observational properties—their unusual nuclearspectra and their unusual nuclear brightness. Whilestudying galaxy spectra for his PhD work in 1908, Fathnoted strong emission lines in the spectrum of the nucleusof NGC 1068. These observations were confirmed in 1917by Slipher and by 1926 Hubble had added two moregalaxies with similar spectra: NGC 4051 and NGC 4151. In1932 Humason noted that NGC 1275 had a bright starlikecenter, but it wasn’t until 1943 thatCARL SEYFERT recognizeda distinct class of galaxies with unusually bright andconcentrated nuclei, and studied in detail six of the 12then known cases (NGC 1068, 1275, 3516, 4051, 4151,7469), leading to the group’s name: Seyfert galaxies (seefigure 1). Seyfert’s spectra showed emission lines whichwere not only unusually strong, but also unusually wide.Interpreting the width of the spectral lines as Dopplershifts caused by motion of the ionized (line emitting) gasimplied gas velocities in some objects of several thousandkm s−1, far higher than velocities found in normal galaxies.A further subtlety Seyfert noticed was that in some objectsthe hydrogen lines were broader than the other lines.Little work was done until the 1960s, when it becameclear that Seyfert galaxies shared a number of propertieswith the then recently discovered quasars. At this time,quasars posed a serious theoretical puzzle. While theirhighREDSHIFTS implied great distance and therefore highluminosity, their brightness had been found to vary frommonth to month, indicating that the region producingall this energy was only a few light months across—tinycompared to the rest of the galaxy. The puzzle of how somuch energy could emerge from so small a volume wassufficiently acute that some astronomers even questionedwhether the quasar redshifts were true indicators ofdistance, leading to the so-called ‘redshift controversy’.Although this controversy has since been resolved infavor of large distances it might never have been asproblematic if Seyfert galaxies had been better studiedin the early 1960s, since they are clearly low redshiftcounterparts to the quasars. Not only are their spectrabasically similar, but in 1968NGC 4151 was found to vary inbrightness, confirming a highly compact nuclear energysource. As more Seyferts and quasars were discovered,their separation in luminosity narrowed and finally evenoverlapped. Continuity had been established and, formost astronomers, the reality of the quasar distances andluminosities was no longer in doubt.In 1974, Khachikian and Weedman identified twotypes of Seyfert galaxy on the basis of the widths of thenuclear emission lines. Figure 2 shows example spectraof each Seyfert type. While spectra of type 2 Seyfertshave a single set of relatively narrow emission lines, inthe spectra of type 1 Seyferts the hydrogen and heliumCopyright © Nature Publishing Group 2001Brunel Road,


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