Exploring Galaxies: Types, Sizes, and Evolution

1. Galaxies What is a galaxy? How many stars are there in an average galaxy? About how many galaxies are there in the universe? What is the name of our galaxy?

2. Galaxy • Gravitationally bound stars, gas, dust, neutron stars, black holes, etc. • Billions of stars in a galaxy and billions of galaxies in the universe • Milky Way: 1,000 LY thick and 100,000 LY across!

3. Types of Galaxies • Elliptical: • Spiral: • Irregular:

5. Elliptical Galaxies

6. Spiral Galaxies

8. Note: There is no direct or simple evolutionary connection between the types of Galaxies. However, there is strong evidence that interactions BETWEEN Galaxies may drive galaxy evolution. X

9. Our, nearly Normal, Galaxy VLA image at l=90 cm ~45” resolution inner few degrees of the Galaxy

10. Active Galactic Nuclei (AGN)‏ • Normal Galaxies – one million to one trillion solar luminosities • AGN - several hundreds of trillions of solar luminosities! • Spectrum is not black body! What does this mean?

11. An Active Galaxy M87 HST Image M87 Distance = 15 Mpc Jet length = 2 kpc

12. Cygnus A – prototypical radio galaxy -- FR II Carilli et al.

13. Components of an AGN Lobe Jet Core Counter-Jet 30 kpc Hot spot “core” contains the central engine

14. AGN What is the power source that generates such enormous luminosities in AGNs?

15. AGN Energy Source • Rapidly varying intensity => small size • High luminosity and small size => supermassive black hole • Billions of solar masses! • Consumes entire stars! (ANIMATION)‏

16. Cosmic Distance Scale • Closest galaxy: Andromeda – 2.5 million light years away! • “Standard Candles” • Extremely bright events with well-known luminosities • Can you think of an example of such an event?

17. Cosmic Distance Scale • “Standard Candles” • Extremely bright events with well-known luminosities • Ex. Type Ia (carbon-detonation) supernovae • Works out to ~ 3 billion light years! Recall: Apparent Brightness = Luminosity/Distance^2

18. On intermediate scales, galaxies form identifiable structures. Structures of Galaxies Groups A few to a few dozen galaxies bound together by their combined gravity. Local Group

19. Clusters Largest (known) gravitationally bound structures - typically containing thousands of galaxies. The center of the Hercules Cluster The Virgo Cluster of about 2500 galaxies (central part shown).

20. Superclusters Larger structures made up of clusters and groups. 10,000's of galaxies. The Local Supercluster consists of the Virgo Cluster, the Local Group and several other groups.

21. Rotation of the Disk Sun moves at 225 km/sec around center. An orbit takes 240 million years. Stars closer to center take less time to orbit. Stars further from center take longer. => rotation not rigid like a phonograph record or a merry-go-round. Rather, "differential rotation". Over most of disk, rotation velocity is roughly constant. The "rotation curve" of the Milky Way

22. Cosmological Structure? We have seen that galaxies are organized into clusters and superclusters on intermediate scales in the universe. How is matter distributed in the universe on the very largest scales?

23. No Structure on Largest Scales (Galaxies distributed fairly uniformly)‏ • Surprising given observations of Groups, clusters, and superclusters on smaller scales • Cosmological Principle • Universe is homogeneous and isotropic • Homogeneity => ? • Isotropy => ?

24. No Structure on Largest Scales (Galaxies distributed fairly uniformly)‏ Surprising given structure on smaller scales Cosmological Principle: Universe is homogeneous and isotropic Homogeneity => universe has no edge Isotropy => universe has no center

25. Olbers` Paradox If the universe is homogeneous, isotropic, infinite, and unchanging, thenthe night sky should be as bright as the sun. Why isn't it?

26. The universe is expanding! => Assumption that universe is unchanging is incorrect. => Doesn’t matter whether universe is finite or infinite, we only see a finite part. => Light from objects greater than 14 billion light years away has not had time to reach us!

27. Cosmological Scales How do we know that the universe is expanding?

28. Almost all galaxies exhibit show redshifted spectral lines => they are receding from us! Size of universe is not static, nor shrinking due to influence of gravity. Universe is expanding.

29. 1 pc ~ 3 Light Years Redshiftor recessional velocity is proportional to distance: the farther away a galaxy is, the faster it is receding. V = H0 x D (Hubble's Law)‏ velocity (km / sec) Distance (Mpc)‏ Hubble's Constant (km / sec / Mpc)‏ Or graphically. . . Current estimate: H0 = 60 - 75 km/sec/Mpc

30. Hubble Expansion Analogy 1 Mpc 6 Mpc 12 Mpc 1 pc ~ 3 Light Years 2 Mpc 12 Mpc 24 Mpc • Accuracy increases with distance. • The farther out , the faster the expansion. • Works for any chosen point of reference. • As the ruler cools, all points converge.

31. The Fate of the Universe What property determines the ultimate fate of the universe?

32. Affect of Density on the Universe

33. Density Determines the Eventual Fate of the Universe How is the expansion rate changing with time? Will the universe expand forever? What is it that is causing the expansion of the universe to accelerate?

34. Question The spectrum of an AGN is _____. A) mainly due to starlight B) very dim C) non-blackbody D) highly stable

35. Question Hubble's law tells us that the ______. A) closer a galaxy is, the faster it moves away B) farther a galaxy is, the faster it moves away C) farther a galaxy is, the slower it moves away D) farther a galaxy is, the larger it is

36. Question Isotropy implies that the universe has no _____. A) center B) curvature C) horizon D) edge