The Great Debate Learning Objectives

! What was being debated at the Great Debate? Why is this a difficult problem? How was the debate resolved? ! What are the basic properties (mass, shape, color, luminosity) of elliptical, spiral and irregular galaxies? ! Why are spiral galaxies (their arms) blue? For the same reason, it’s true that irregular galaxies are blue (young) and ellipticals galaxies are red (old) ! How do rotation curves tell us that most of the mass in galaxies (and so in the Universe) is made of dark matter? ! How do galaxies form? What is one observational clue to why galaxies are so different? What is a giant elliptical galaxy? What is a rich cluster? A poor cluster (or group?) “Spiral Nebulae”

! Dim, diffuse “nebulae” with spiral patterns ! Spiral structures noted in the mid-1800s by Lord Rosse (Ireland) Modern M51 picture

“Leviathan” Lord Rosse’s Rosse’s M51 sketch 1.8 m reflecting telescope The Great Debate - How Far Away Are They? ! Much, much disagreement: ! Kant: Our Galaxy is a spiral “island universe” and the other spiral nebulae are the same as our Galaxy and far away ! Herschel and others: Our Milky Way Galaxy is the whole Universe, and the other spiral nebulae are nearby Sun* Galaxy

*Sun wrongly in center; this is before Shapley’s globular cluster studies The Great Debate ! Think about how difficult a problem was being discussed at the Great Debate ! We see these weird things in the sky…but we see lots of weird things...star clusters, novae, giant molecular clouds ! Why should these nebulae be special? ! How might we prove these The Andromeda Galaxy: things are either quite close The largest “spiral nebula” to us (within our Galaxy), or really far away from us (outside of our Galaxy)? Edwin Hubble

! 1923: Hubble resolved M31, the Andromeda “nebula”, into stars ! After resolving stars, Hubble went looking for Cepheid Variables (stars that have a pulsation period- Hubble at Mt. Wilson absolute magnitude relation) Observatory ! Found M31 was ~300,000pc away (modern estimate is ~770,000pc) ! Andromeda is an “island universe” like our own Galaxy The Andromeda “Nebula” The three classes of galaxies Elliptical Galaxies

Irregular Galaxies Spiral Galaxies Classes of Galaxies ! Spirals (S) !Structure: disk, halo, bulge !Medium to large galaxies !The disk has the young blue stars, while the bulge and halo have older red stars ! Ellipticals (E) !Pure bulge, no disk component !Large range in sizes !All older red/yellow stars ! Irregulars (Ir) !Well, erm, irregular !Smaller galaxies !Mostly young blue stars Galaxy Types: Overview

Spirals Ellipticals Irregulars

9 12 5 13 8 10 Mass (MSun) 10 – 10 10 – 10 10 – 10 Luminosity 108 – 1010 105 – 1011 107 – 109 (LSun) Diameter (kpc) 5 - 200 1 - 200 1 - 10 Disk: Blue-to- white Color Reddish-yellow Bluish-white Halo/Bulge: Red-to-yellow Why do we see Spiral Arms?

! They are easily seen as the arms contain numerous bright, blue O and B stars ! These stars also illuminate dust in the arms ! However, stars in general are evenly distributed throughout the disk Effect of Viewing Angle Face-on

Easier to see the spiral arms

Edge-on

Easier to see the dust in the disk Rotation Curves ‣ A rotation curve is a plot of the orbital speed of stars vs. the distance of the stars from the center of their host galaxy ‣ Example: rotation curve of the planets Masses of Galaxies ! As with the Milky Way, we measure the speed that stars orbit in a galaxy’s disk ! We use (Newton’s version of) Kepler’s Laws to calculate the enclosed mass ! Like the Milky Way, all other galaxies have flat rotation curves ! The mass is mostly in the outer parts of galaxies, it is not where the stars exist ! This indicates a halo of dark matter Dark Matter ! Normal matter makes up less than 5% of our Universe ! Dark matter is spherically distributed all around galaxies in huge halos ! What is this stuff that emits no light ! Neutrinos? Black holes? Brown or red dwarfs? Cooled-off white dwarfs? ! MACHOs: Massive Compact Halo Objects ! Something exotic – particles as yet unknown? ! WIMPs: Weakly Interacting Massive Particles ! We don’t know exactly what dark matter is but we categorically, absolutely know it’s there How did our galaxy form?

(a) Proto-galactic clouds are drawn together by gravity; halo stars begin to form (b) Merged cloud collapses (c) Gas & dust settle into a spinning disk; halo star formation stops. As of now there are only old and long-lived objects in the halo (red dwarfs, white dwarfs, black holes etc.) (d) Stars continue to form in the disk. As of now there are still young stars in the disk This model is, of course, a tad simplified for ASTR-1050! But why do some galaxies end up looking so different?

M87 NGC 4414 Observational Clue: Galaxy Clusters ! Galaxies occur in a wide Elliptical galaxies dominate range of sizes of clusters rich clusters of galaxies ! “Rich” clusters have many galaxies, mostly elliptical !At the center of each rich cluster there is usually a giant elliptical galaxy ! “Poor” clusters contain fewer than ~100 galaxies !Our Galaxy is in a “poor” group of ~35 galaxies !For some reason “ending up close” to other galaxies makes galaxies tend to be elliptical Next Time

Objects in the Rear-view Mirror May Appear Closer Than They Are