HarlowShapley'sPosition

Allthedistantnebulawecouldseewherejustsmallthingslocatedinthemilkyway.Essentially,theMilkywaywastheextentoftheknownuniverse.

HeberCurtis'Position

Allthedistantnebularwereinfactothergalaxies,likethemilkyway."IslandUniverses"asKantcalledthem.

Galaxies

1.EarlyAttemptstocatalogandclassify1.MessierCatalog2."TheGreatDebate"3.Spiral:Andromeda4.Ellipticals

2.Updatestothescheme1.NGC13002.GrandDesignSpiral3.GrandDesignSpiral4.Flocculentspiralgalaxy

3.Starformation4.SpiralOrigins

1.DensityWaveTheory5.Ellipticals6.What'soutthere?

Early Attempts to catalog and classify

Messier Catalog

Acatalogof110astronomicalobjects-startedin1771HSTcollectionofimagesfromthecatalog

"The Great Debate"

26April1920SmithsonianMuseumofNaturalHistory

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Fig.1The100inchreflectingtelescopeatMt.Wilson,nearLA.CourtesyofTheObservatoriesoftheCarnegieInstitutionforScienceCollectionattheHuntingtonLibrary,SanMarino,Calif. Fig.2Hubbleatthetelescope

EdwinHubblePapers/CourtesyofHuntingtonLibrary,SanMarino,Calif.

Fig.3TheSequenceofNebulaTypesEdwinHubbleTheRealmoftheNebulaeDoverPublicationsInc.1958

Hubble'sfirstclassificationschemewaslaidoutaround1926.Theyweregroupedinthreemaincategories:ellipticals(E),spirals,andirregulars(Irr).Spiralscouldbenormal:(S)orbarred:(SB).Lenticularswereinbetweenshapes.Hubblesurmisedthatthisarrangementalsorepresentedanevolutionintime:ellipticalgalaxieswouldeventuallybecomespirals...(wenowknowthisisnottrue).

Ellipticity:

where and aretheapparentmajorandminoraxesofthegalaxy.EllipticalsthatarenearlysphericalarecalledE0Ellipticalsmighthaveellipticityvaluesof0.7:E7Largestbulge/diskratio,i.e.mostprominentbulge:Sa

SpiralArmstightlywound

ϵ ≡ 1 −β

α(1)

α β

/ ∼ 0.3Lbulge Ldisk

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Fig.4

Fig.5M87VirgoaFig.6M84andM86

Spiral: Andromeda

M31 - Andromeda

Closestmajorgalaxy2.5millionlight-yearsWehaveameetinginabout4.5billionyears

Ellipticals

Updates to the scheme

SmoothdistributionofstarsLessobviousbulges:Sc

LooselywoundarmsClumpsofstars

/ ∼ 0.05Lbulge Ldisk

Messier87(alsoknownasVirgoAorNGC4486,andgenerallyabbreviatedtoM87)isasupergiantellipticalgalaxyintheconstellationVirgo.Oneofthemostmassivegalaxiesinthelocaluniverse,itisnotableforitslargepopulationofglobularclusters—M87containsabout12,000comparedtothe150–200orbitingtheMilkyWay

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Fig.7TheHubble-deVaucouleurssystemByAntonioCiccolella/M.DeLeo-https://en.wikipedia.org/wiki/File:Hubble-Vaucouleurs.png,CCBY3.0,https://commons.wikimedia.org/w/index.php?curid=50260841

NGC1300

Fig.8NGC1300isanexampleofabarredgalaxy.

Hubble'ssystemhasbeenrefinedovertheyears.

1.Spiralswithnobars:SA2.Spiralswithsomehintsofabar:SAB3.StronglyBarredspirals:SB

Thefinalschemefornon-barredspiralgalaxieslookslike:E0,E1,...E7,S01,S02,S03,Sa,Sab,Sb,Sbc,Sc,Scd,Sd,Sm,Im,Ir

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Fig.9Messier100isagranddesignspiral

Fig.10

Grand Design Spiral

A'granddesign'spiralhaswelldefinedarms.

Grand Design Spiral

Flocculent spiral galaxy

Flocculent("havingorresemblingtuftsofwool.")spiralsdon'thavewelldefinedarmsAbout30%ofspiralsarelikethis.

Disks:generallymetalrichstarsandISM,nearlycircularorbitswithlittlerandommotion,spiralpatterns

Thindisks:younger,starforming,dynamicallyverycoldThickdisks:older,passive,slowerrotationandmorerandommotions

Bulge:metalpoortosuper-metal-richstars,highstellardensities,mostlyrandommotion–similartoellipticalsBar:presentin~50%ofdiskgalaxies,mostlyolderstars,somerandommotionsanda~solidbodyrotation?Nucleus:central(<10pc)regionofveryhighmassdensity,massiveblackholeorstarburstornuclearstarclusterStellarhalo:verylowdensity(few%ofthetotallight),metalpoorstars,globularclusters,lowdensityhotgas,littleornorotationDarkhalo:dominatesmass(andgravitationalpotential)outsideafewkpc,probablytriaxialellipsoids,radialprofile~singularisothermalsphere,DMnatureunknown

A largely successful classification scheme.

Althoughitwasstillratherqualitativebasedonjust'howtheylooked'.

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Fig.11Someparametersdon'tcorrelatemuchwithgalaxytype.

Fig.12MessierObject81indifferentwavelengthsNASA/JPL-Caltech/K.Gordon(UniversityofArizona)&S.Willner(Harvard-SmithsonianCenterforAstrophysics),N.A.Sharp(NOAO/AURA/NSF)

M-81 in different wavelengths

Star formation

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Fig.13

Fig.14Starsclosertothecenterwilltakelesstimetogoaround.Overtime,thearmswillbecome'tightlywound'andnotobservablespirals.

Fig.15

Ellipticalsandearlytypespiralsformedmostoftheirstarsearlyon(useduptheirgas,haveolder/redderstars)Latetypespiralshavesubstantialon-goingstar-formation,didn’tformasmanystarsearly-on(andthuslotsofgasleft)

Spiral Origins

Density Wave Theory

Theorbitsinthegalaxyareelliptical,butslightlyrotated.Thiscausesregionsofdifferingdensities.Higherdensitymeanshighergravitationalforce.Objects(suchasgasclouds)willbeattractedtotheseregionsandwilldrifttowardsthem.

Spirals

Spiralarmsarewavesofcompressionthatmovearoundthegalaxyandtriggerstarformation

Thiswilltakenotalotoftime~afewbillionyears.So,ifthiswerethemechanismforgettingspirals,thenitweshouldn'tseespiralsinthecurrentuniverse.Butwedo.

Thespiralappearanceisthoughttobearesultofdensitywavesrotatingthroughthegalaxy.

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Fig.16Hoag'sobject:aringgalaxyByNASAandTheHubbleHeritageTeam(STScI/AURA);Acknowledgment:RayA.Lucas(STScI/AURA)-http://hubblesite.org/newscenter/archive/releases/2002/21/image/a/http://antwrp.gsfc.nasa.gov/apod/ap020909.html,PublicDomain,https://commons.wikimedia.org/w/index.php?curid=610534

StarformationwilloccurwherethegascloudsarecompressedSimulationsHere.

Ellipticals

Hubbleoriginallythoughttherewasnotmuchgoingonellipticalgalaxies.Theyjustseemslesscomplex.Turnsoutthat'snotreallythecase.Theyarefullofinterestingfeatures.

What's out there?

Othertypes:RingsLenticularIrregularsDwarfGalaxiesabout10^11galaxiesinthevisibleuniverse-maybemore.Overbillionsofyears,galaxiesinteract.

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