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CALTECH AY 21 - Galaxies: Classification

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eaa.iop.orgDOI: 10.1888/0333750888/1666 Galaxies: ClassificationRonald Buta FromEncyclopedia of Astronomy & AstrophysicsP. Murdin © IOP Publishing Ltd 2006 ISBN: 0333750888Downloaded on Thu Mar 02 23:09:55 GMT 2006 [131.215.103.76]Institute of Physics PublishingBristol and PhiladelphiaTerms and ConditionsGalaxies: ClassificationENCYCLOPEDIA OF ASTRONOMY AND ASTROPHYSICSGalaxies: ClassificationGalaxies are majestically beautiful objects, and anyonewho has seen a galaxy through a telescope, or even ourown Milky Way Galaxy with the unaided eye, cannot helpbut wonder at the nature of these remote star systems andwhat they have to say about our universe and ourselves.Much of what is known about galaxies began with asimple classification of their appearance as seen on directphotographs taken with large observatory telescopes.Galaxies present a wide variety of forms, or morphologies,and can be naturally divided into categories in much thesame way as living organisms can be divided into generaand species. However, galaxy morphology presentsproblems for classification that would not be encounteredin biological taxonomy and that make it difficult to classifythem with the precision that is normally associated withliving species. Still, visual galaxy classification continuesto be useful at a time when galaxies have never been betterunderstood. Classification provides order to the dauntingvariety of forms even if we do not yet know how thedifferent forms came about. Classification also providesa framework for further studies and suggests a logicalapproach to studying galaxies.Factors influencing galaxy classification andmorphologyThe main problem with classifying galaxies is that nogalaxy can be brought into a laboratory so that it canbe viewed from any particular direction or distance.Most galaxies have preferred planes of symmetry, andideally for classification we would like to view them allfrom directly above their preferred planes of symmetry.However, these planes are oriented randomly to the lineof sight, making the appearance of many galaxies verydependent on viewing geometry. Also, galaxies are spreadover a wide range of distances, such that the farther theyare away from us, the harder it is to see details of theirmorphology.The appearance of many galaxies also dependson the wavelength of light in which they are viewed.Photography and imaging of galaxies is usually donethrough filters which transmit a relatively small portionof their spectral energy distribution, which refers to thedistribution of their light with wavelength. With someexceptions, spectral energy distributions of galaxies aredominated by starlight, with a small fraction of their lightcoming from glowing interstellar gas. Starlight tendsto be mostly continuous thermal radiation (meaning allwavelengths are emitted) whose color is determined bythe surface temperature of the star. Hot stars (surfacetemperaturesof 20 000 K or more)emit most of their light asultraviolet radiation and appear bluish in color, while coolstars (surface temperatures of 4000 K or less) emit most oftheir light as infrared radiation and appear reddish in color.When imaged in short-wavelength light, such as blue orultraviolet, the appearance of galaxies tends to emphasizehot, blue and relatively young massive stars, but whenviewed in long-wavelength light, such as red or infrared,the appearance of galaxies tends to emphasize cooler andgenerally older less massive stars. Young stars tend tobe less smoothly distributed and much less frequentlypresent than old stars, so many galaxies appear morepatchy and uneven in blue filters than in red filters.The presence of interstellar material known asdust also has a wavelength-dependent effect on theappearance of a galaxy. Dust consists of fine particlesof heavy atoms and light elements that are thought to beproduced by processes connected with the evolution ofstars. In highly flattened galaxies, dust collects withina thin plane at the galaxy’s mid-section; in other, lessflattened galaxies dust may be distributed more randomly.Since interstellar dust scatters short-wavelength lightmore effectively than long-wavelength light (by virtueof the small size of the particles compared with thewavelength of visible light), the effects of dust on galaxymorphology and classification are more serious on bluelight images than on red light images. Dust also impactsthe appearance of galaxies at far-infrared wavelengths,where thermal emission from dust can actually dominateover the contribution of starlight. At longer (radio)wavelengths, starlight from galaxies can be very weak,and the appearance of a galaxy can be determined mainlybySYNCHROTRON RADIATION (nonthermal electromagneticradiation produced by relativisitic electrons spiraling instrong magnetic fields) or by emission from cold neutralatomic hydrogen at 21 cm wavelength.The expanding universe can affect the appearanceof galaxies. The cosmologicalREDSHIFT both shifts andstretches the spectral energy distribution of galaxies whichwould impact their appearance in any set of fixed standardfilters. For example, the blue light appearance of a galaxymay be seen only in a red filter because the redshift hasshifted that part of the spectrum into the red. Thus,galaxies at high redshift can be difficult to classify andcompare with those observed at low redshift.The appearance of a galaxy is not expected to beperfectly fixed with time. Galaxies tend to rotate, newstars may be born and older stars may die, and interstellargas may be consumed or replenished at rates dependenton both internal and external factors. Galaxies tend tobe large compared with their typical separations, so thatinteractions may impact their structure over a long periodof time. Because of the finite speed of light, very distantgalaxies are seen as they were when the universe was muchyounger than it is now, and their appearance may havebeen influenced by the different conditions that existed atthose times.Finally, the morphology of a galaxy is also seriouslyinfluenced by its total mass and its environment. Massivegalaxies tend to be much more structured and wellordered, more luminous and of higher average surfacebrightness than low-mass galaxies, such that catalogsover-represent high-mass galaxies and under-representlow-mass galaxies. Because of this selection effect, thegalaxy classification systems in use today apply mainlyCopyright © Nature Publishing Group 2001Brunel Road, Houndmills, Basingstoke,


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CALTECH AY 21 - Galaxies: Classification

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