Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW1Questions of the Day•What is the Hubble sequence, and how do the properties of galaxies vary along it?•What determines the colors of galaxies?•How can we measure the motions of stars and gas within galaxies?1Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW2The galaxy population can be organized into a sequence:EllipticalsLenticularsBarred SpiralsUnbarred Spirals2Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW3Capital “E” is for ellipticalsCapital “B” indicates the galaxy is barredCapital “S” is for spirals…3Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW4Number indicates how flat the elliptical isLowercase “a”, “b”, “c” indicates how unlike the spiral is to an elliptical4Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UWXAnd once more in French!5Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW56Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW61. “Bulge-to-Disk Ratio”2. Lumpiness of the spiral arms3. How tightly the spiral arms are woundEarly Types Late Types7Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW7There are galaxies beyond the Hubble Sequence that continue this trend.???8Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW8Dwarf Galaxies!9Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW9“Dwarf” or “Irregular” galaxies tend to have more chaotic appearances…• Low mass (107-109 stars, vs 1010 for spirals)• High star formation rates (usually)• No obvious bulge or spiral patterns.• Most numerous type of galaxy in the Universe!10Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW10Dwarf galaxies from the Sloan Digital Sky Survey.11Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW11Dwarf galaxies from the Sloan Digital Sky Survey.12Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW12Dwarf galaxies are the most numerous type of galaxyMap of all the galaxies in the local neighborhood. All but 4 of these are dwarfs!13Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UWXA merger in progress.A very high contrast image showing the faint debris from a disrupting satellite. Solid gray regions are where bright things have been “masked” out, so you can see the faint structure.14Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW13Dwarf galaxies are frequently accreted by massive galaxiesNGC 5907“Minor Merger”: Mass of accreted satellite is a small fraction of the main galaxy. Satellite disrupted, but main galaxy remains similar15Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW14Andromeda galaxyMap of its individual starsGiant galaxies may be made up largely of accreted dwarf galaxies16Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UWXA Good Dwarf Galaxy is Hard to FindThey’re faint and low contrast against the night sky!17Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UWXFind the low surface brightness dwarf!18Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW151. “Bulge-to-Disk Ratio”2. Lumpiness of the spiral arms3. How tightly the spiral arms are woundEarly Types Late TypesDecreasing Mass!19Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW16Varying amounts of bulge & disk components suggests different formation & evolution historyWhich has larger number of HII regions?Pink: Early-Type EllipticalsBlue: Late-Type Spirals20Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW17Which has more recent star formation?Pink: Early-Type EllipticalsBlue: Late-Type Spirals21Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW18Which do you think has more gas?Pink: Early-Type EllipticalsBlue: Late-Type Spirals22Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW19• Older Stars• “Gas Poor”• More Massive• On-going Star Formation• “Gas Rich”• Less MassiveOn average…23Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UWXThe “star formation history” varies along the Hubble sequence:1. Late-type galaxies tend to have a much larger fraction of their normal matter in the form of gas. They’ve used up less of their “fuel” for star formation.24Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW20The variation in “star formation history” along the Hubble sequence leads to variations in colorRedder BluerMust be due to systematically different mixtures of stars25Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW21We see all the stars at the same time.H-R Diagram of Milky Way Stars The collection of stars is called the “stellar population”26Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UWXThe luminosity and color of the galaxy depends on the “stellar population” -- i.e. how many stars are in different locations of the HR diagramHR Diagram of Milky Way Stars 27Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW22The color of the brightest stars pretty much sets the color of the galaxy.These stars tend to be young massive main sequence stars, or red giants.28Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW23Star clusters of different ages have different fractions of these bright red and blue stars29Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW24Galaxies have more complex stellar populations than single clusters.They have a mix of stars formed at different times.YoungOld30Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW25Which cluster would look bluer?(pink): NGC 2362(yellow): M11YoungOld31Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW26Galaxies that are forming stars are BLUER!YoungOldThese are bluer than these!32Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW27Assuming that both clusters had the same mass, which one is more luminous?(pink): NGC 2362(yellow): M11YoungOld33Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW28Galaxies that are forming stars are BRIGHT!YoungOldThese are brighter than these!34Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UWXGalaxies with more recent star formation have a larger fraction of young main sequence stars.Galaxies with no young stars have red giants as their brightest stars.YoungOld35Astronomy 101, Winter 2010Copyright@2010 Julianne
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