Galaxies

1. 0 Galaxies What do galaxies look like? How do we find the distances to them? How do they differ in size, luminosity, & mass? Do others contain supermassive black holes & dark matter? Why are there different kinds of galaxies?

2. The Family of Galaxies: Even seemingly empty regions of the sky contain thousands of very faint, very distant galaxies 0 Large variety of galaxy morphologies: Spirals Ellipticals Irregular

3. Galaxy Classification: • Gas & dust influence the appearance • Gas & dust  star formation  hot, bright stars  bluer & emission nebula • Little gas & dust  less star formation  less hot, bright stars  redder, more uniform look

4. Galaxy Classification: • Basically three types; • Elliptical, “E” •  Little gas and dust • Spiral, “S” •  More gas and dust than type E • Irregular, “Irr” • Chaotic mix of stars, gas, and dust • No bulge or arms • SMC and LMC are Irr Galaxies with disk and bulge, but no dust are termed “S0”

5. 0 Galaxy Classification: Elliptical Galaxies Spiral Galaxies E0, …, E7 Sa Large nucleus; tightly wound arms E0 = Spherical E1 Sb Sc Small nucleus; loosely wound arms E7 = Highly elliptical E6

6. 0 Barred Spirals: Some spirals show a pronounced bar structure in the center. Sequence: SBa, SBb, & SBc, analogous to regular spirals.

7. 0 Irregular Galaxies: Often: result of galaxy collisions / mergers Often: Very active star formation (“Starburst galaxies”) Some: Small (“Dwarf galaxies”) satellites of larger galaxies (e.g., Magellanic Clouds) The Cocoon Galaxy Large Magellanic Cloud NGC 4038/4039

9. Properties of Galaxies: • We would like to know; • Diameter • Luminosity • Mass • In order to determine these… • We need the distance!

10. Distances to Galaxies: • Distances are so large, it is convenient to express them not in pc, but rather Mpc (million parsecs), or Gpc (billion parsecs) • Methods: • Look for objects of known L  • Cepheids: measure P  M • Glob. clusters: the brightest  M = -10, assume the brightest in other galaxies have similar M • PN: The post-AGB star peaks in the UV  the PN absorbs and reradiates  find M of the brightest PN  PN in other galaxies should have similar M • Type Ia SN: All reach the same M at max. • Total L of galaxies: Classify galaxy  similar galaxies should have similar M • Hubble Law…

11. 0 Distance Measurements to Other Galaxies (II): Hubble’s Law E. Hubble (1913): Distant galaxies are moving away from our Milky Way, with a recession velocity, vr, proportional to their distance d: H0≈ 70 km/s/Mpc is the Hubble constant. In km/s In Mpc => Measure vr through the Doppler effect  infer the distance. Consequence… The universe is expanding!

13. Properties of Galaxies: diameter & luminosity • Diameter; • Find d, measure angular diameter (α) • small angle approx  diameter Luminosity; Find d, measure m  

14. 0 Mass: Rotation Curves of Galaxies From blue / red shift of spectral lines across the galaxy  infer rotational velocity Plot of rotational velocity vs. distance from the center of the galaxy: rotation curve Observe frequency of spectral lines across a galaxy. Using Kepler’s 3rd law  galactic mass

15. Mass: other methods Velocity dispersion method; Measure motions of matter in a galaxy Assume that matter is gravitationally bound to the galaxy Apply same reasoning as in the cluster method • Cluster method; • Measure • Velocities of galaxies in a galaxy cluster • Size of the cluster • Ask; how much mass is required to hold the cluster together with the observed velocities? (Total energy must be negative for a bound system…) •  Divide your result by the number of galaxies  average galactic mass of the cluster

16. Supermassive black holes: 0 From the measurement of stellar velocities near the center of a galaxy: Infer mass in the very center  Central black holes! Several million, up to more than a billion solar masses! Supermassive black holes

17. Clusters of galaxies: 0 Galaxies do not generally exist in isolation, but form larger clusters of galaxies. Rich clusters: 1,000 or more galaxies, diameter of ~ 3 Mpc, condensed around a large, central galaxy Poor clusters: Less than 1,000 galaxies (often just a few), diameter of a few Mpc, generally not condensed towards the center

18. Hot Gas in Clusters of Galaxies : 0 Space between galaxies is not empty, but filled with hot gas (observable in X rays) That this gas remains gravitationally bound, provides further evidence for dark matter. Visible light X rays Coma Cluster of Galaxies

19. Gravitational Lensing: • Einstein’s General Relativity (1916) • Gravity warps space-time • Light alters its path near massive objects

20. Gravitational Lensing:

21. 0 Gravitational Lensing: The huge mass of gas in a cluster of galaxies can bend the light from a more distant galaxy. Image of the galaxy is strongly distorted into arcs.

23. Dark matter: 0 Adding all the “visible” mass in stars, interstellar gas, dust, etc., we find that most of the mass is “invisible”! We still don’t know what this stuff is… Only that it exists and that it has a gravitational effect on other mass…. Speculations: Brown dwarfs, low-L WDs, small black holes, exotic elementary particles, neutrinos, MACHOs and WIMPs.

24. Stars ~ 2% Gas ~ 5 - 15% Bullet Cluster

25. Evolution of galaxies: Why do some galaxies become spirals, others elliptical, yet others irregular? Galaxies tend to cluster; rich  > 1000 galaxies poor  < 1000 galaxies MW is in a poor cluster ~ few dozen galaxies ~ 1Mpc dia. A galaxy’s environment seems to determine its structure… Perhaps galaxy collisions impact a galaxy’s evolution…

26. Collisions: Average separation between galaxies ~ 20 times the galactic diameter (stars however are separated on average by 107 times their dia.)

27. Devil’s Mask Hoag’s Object

28. NGC 2207 & IC 2163 encounter

29. NGC 4038 - NGC 4039 antennae collision

31. 0 Interacting Galaxies: Cartwheel Galaxy Particularly in rich clusters, galaxies can collide and interact Galaxy collisions can produce ring galaxies and tidal tails NGC 4038/4039 Often triggering active star formation: Starburst galaxies (galaxies with lots of gas & dust, but little star formation, have few neighbors…)

32. M51-Whirlpool 0 Interacting Galaxies: Interactions could induce the formation of spiral arms; i.e., induce a density wave

33. Warped galaxy

34. 0 Tidal Tails: Example for galaxy interaction with tidal tails: The Mice Computer simulations produce similar structures.

36. 0 Simulations of Galaxy Interactions Numerical simulations of galaxy interactions have been very successful in reproducing tidal interactions like bridges, tidal tails, and rings.

37. 0 Mergers of Galaxies: Radio image of M64: Central regions rotating backwards! NGC 7252: Probably result of merger of two galaxies, ~ a billion years ago: Small galaxy remnant in the center is rotating backwards! Multiple nuclei in giant elliptical galaxies

38. Origin and evolution: Put all the observations and theory together and see if we can explain the history of the objects studied… Eliminate certain scenarios; Ellipticals can’t become spirals contain too little gas and dust Spirals and irregulars can’t become ellipticals  they contain both young and old stars => S & Irr must be old like the ellipticals => A single galaxy doesn’t seem to change from one class to another except via mergers & interactions with other galaxies A few collisions and mergers can leave a galaxy devoid of gas and dust Ellipticals - seem to be the product of mergers which triggered star formation and used up all the gas and dust Starburst galaxies – luminous in IR  collision triggered star formation which heats the dust Dwarfs too small to be the product of mergers  may be the leftovers Spirals – seem to never have suffered a major collision  disks too delicate & would be destroyed  contain lots of gas & dust which would have been used up in a large merger

39. Origin and evolution: Barred spirals are common, but models show that the bars shouldn’t last long for an isolated galaxy  tidal interactions with other galaxies may regenerate the bars S0’s may have lost their gas and dust in bursts of star formation yet still remain disk-shaped

40. Interactions of Galaxies with Intergalactic Matter: 0 Galaxies may not only interact with each other directly, but also with the gas between them • Gas within a galaxy is stripped off the galaxy by such an interaction • Could explain dwarf ellipticals… (too small to have formed from merging spirals) • In contrast, Irr may be fragments of galaxies ripped apart by collisions

42. 0 The Furthest Galaxies: The most distant galaxies visible by HST are seen at a time when the universe was only ~ 1 billion years old.

43. 0 The Furthest Galaxies: • The farther we look in distance, the farther back in time we see… • This is when galaxies were first forming… •  More spirals and fewer ellipticals • More compact • More irregular • Closer together • Tend to be in pairs (33% compared to the present value of 7%) Blue dwarfs – small Irr galaxies, blue  rapidly forming stars We see tremendous amounts of these at long look-back times, but none at short look-back times => no longer exist in the present universe

44. 0 Galaxies on the whole: • ~100 – 400 billion galaxies, possibly more… • Most distant visible galaxies; ~ 13 billion light years (recent observation) • If the MW was a semi-truck, • the smallest dwarf galaxies  Hotwheels car • the largest giant ellipticals  747 jumbo jet • Typical masses; • Smallest  10-6 MMW • Largest  50 MMW • 90 – 95 % of a galaxy’s mass appears to be dark matter