1. ASTR 113 – 003 Spring 2006 Lecture 10 April 5, 2006 Review (Ch4-5): the Foundation Galaxy (Ch 25-27) Cosmology (Ch28-29) Introduction To Modern Astronomy II Star (Ch18-24) 1.Sun, Our star (Ch18) 2.Nature of Stars (Ch19) 3.Birth of Stars (Ch20) 4.After Main Sequence (Ch21) 5.Death of Stars (Ch22) 6.Neutron Stars (Ch23) 7.Black Holes (Ch24) Extraterrestrial Life (Ch30) 1.Our Galaxy (Ch25) 2.Galaxies (Ch26) 3.Active Galaxies (Ch27) 1.Evolution of Universe (Ch28) 2.Early Universe (Ch29)

2. Galaxies Chapter Twenty-Six ASTR 113 – 003 Spring 2006 Lecture 10 April 5, 2006

3. Guiding Questions How did astronomers first discover other galaxies? How did astronomers first determine the distances to galaxies? Do all galaxies have spiral arms, like the Milky Way? How do modern astronomers tell how far away galaxies are? How do the spectra of galaxies tell astronomers that the universe is expanding? Are galaxies isolated in space, or are they found near other galaxies? What happens when galaxies collide with each other? Is dark matter found in galaxies beyond the Milky Way? How do astronomers think galaxies formed?

4. First Discovery of Other Galaxies Spiral “nebulae” were thought to be inside the Milky way

5. Hubble proved that the spiral nebulae are beyond the Milky Way Edwin Hubble used Cepheid variables to show that the “nebula” were actually immense star systems far beyond our Galaxy Cepheid variables obey the period- luminosity law

6. Classifying Galaxies Hubble classification: classification is based on appearance only Four major types of galaxies: 1.Spiral galaxies (S) (Type 1+2: 77%) 2.Barred spiral galaxies (BS) 3.Elliptical galaxies (E) (20%) 4.Irregular galaxies (I) (3%)

7. Spiral galaxies (S) Spiral arms are active star-forming region The stars in the spiral arms are mainly metal-rich population I star Sub-classification based on the smoothness of spiral arms and size of bulge: Sa, Sb, Sc –Sa: broad arms, relatively large bulge –Sc: narrow arms, relatively small bulge –Sb: intermediate

8. Barred Spiral galaxies (SB) A bar shaped region running through the nucleus. Spiral arms originate at the end of the bar rather than the nucleus itself Sub-classifications: SBa, SBb, SBc (same as S galaxies)

9. Elliptical galaxies (E) Elliptical shape, have no spiral arms Devoid of gas and dues Consists of old, red and metal-poor population II stars Sub-classifications: E1, E2…E7 (based on flatness) –E1: roundest –E7: flattest

10. Irregular galaxies They are often found associated with other galaxies Irregular galaxies have ill-defined, asymmetrical shapes They are rich in interstellar gas and dust

11. Hubble’s Tuning Fork Diagram E, S, Sb and I types Lenticular galaxies are intermediate between spiral and elliptical galaxies

12. Summary Table of Classification

13. Determine Distances to Galaxies Use Standard Candle, an object that lies within that galaxy and for which we know the luminosity Standard candles include Cepheid variables, supernovae (Type Ia).

14. The Distance Ladder Parallax: 500 pc Spectroscopic parallax: 10 kpc RR Lyrae variable: 100 kpc Cepheid variable (10 4 Ls): 30 Mpc Type 1a Supernovae (10 9 Ls): 1000 Mpc

15. Redshift of Galaxies Hubble found the spectrum of galaxies have redshift z = (λ – λ 0 )/λ 0 z = (Δλ)/λ 0 z: value of redshift λ: wavelength of shifted spectral line λ 0 : wavelength of unshifted spectral line According to Doppler’ law, redshift means the galaxies are receding from us e.g., z=0.20 V (velocity)=0.20 C (speed of light) V = 75000 km/s

16. The Hubble law The Hubble law: the more distance a galaxy, the greater its redshift and the more rapidly it is receding from us. v = H 0 d V: velocity in unit of (km/s) D: distance in init of Mpc H 0, Hubble constant, ~ 71 km/s/Mpc, but not certain The Hubble constant indicates how fast our universe is expanding

17. ASTR 113 – 003 Spring 2006 Lecture 11 April 12, 2006 Review (Ch4-5): the Foundation Galaxy (Ch 25-27) Cosmology (Ch28-29) Introduction To Modern Astronomy II Star (Ch18-24) 1.Sun, Our star (Ch18) 2.Nature of Stars (Ch19) 3.Birth of Stars (Ch20) 4.After Main Sequence (Ch21) 5.Death of Stars (Ch22) 6.Neutron Stars (Ch23) 7.Black Holes (Ch24) Extraterrestrial Life (Ch30) 1.Our Galaxy (Ch25) 2.Galaxies (Ch26) 3.Active Galaxies (Ch27) 1.Evolution of Universe (Ch28) 2.Early Universe (Ch29)

18. Galaxies are grouped into clusters and superclusters Galaxies do not evenly or randomly distributed throughout the Universe Galaxies tend to be grouped into clusters Rich cluster has far more number of galaxies than poor cluster A poor cluster is often called group Local Group is the galaxy cluster containing the Milky Way Galaxy; Local Group is a poor cluster of about 40 galaxies

19. Local Group A poor, irregular cluster of about 40 galaxies The diameter is about 1 Mpc (mega parsec) The largest is M31, the Andromeda Galaxy The Milky Way is in the second place Both Milky Way and M31 are surrounded by a number of small satellite galaxies

20. A rich cluster contains hundreds or even thousands of galaxies The Coma cluster, a rich and regular cluster is about 90 Mpc (300 million light year) from the Earth It has as many as 10000 galaxies An example of Rich Cluster of Galaxies Coma Cluster of Galaxies

21. Supercluster of Galaxies A supercluster of galaxies is a huge association of clusters of galaxies A typical supercluster contains a dozen of individual clusters It spans up to 50 Mpc

22. Distribution of Galaxies in the Universe This map shows 1.6 million galaxies from the 2MASS (Two-Micron All-Sky Survey) survey Supercluster of Galaxies lie along filaments There are large dark voids that contain few galaxies

23. Distribution of Galaxies in the Universe This map shows 60000 galaxies in two wedges extending up to redshift z=0.25 from 2dfGRS (Two Degree Field Galactic Redshift Survey) It also show filements and voids The voids are roughly spherical, 30 to 120 Mpc in diameter

24. Galaxy Collision The gravitational tidal force deforms the galaxies: stars are hurled into intergalactic space along arching streams

25. Galaxy Collision Two galaxies can merge into a bigger galaxy Galactic cannibalism Interstellar gas can be compressed, triggering star formation

26. Dark Matter inferred from Rotation Curve If mass distribution follows the luminosity distribution, the rotation curve would fall off according to Neuton’s and/or Kepler’s Law The flat rotation curve at large distance indicates the presence of extended halo of no-luminous matter, or dark matter

27. Dark Matter inferred from Gravitational Lensing Gravitational Lensing: a massive galaxy deflects light rays like a lens so that an observer sees multiple distorted images of a more distant galaxy Gravitational lensing is predicted by Einstein’s general theory of relativity: space is curved due to gravity

28. Dark Matter inferred from Gravitational Lensing Examples of Gravitational lensing The mass of galaxies calculated from gravitational lensing is much larger than the visible mass; again, 90% dark matter

29. One More Example of Gravitational Lensing

30. Dark Matter Candidates Massive neutrons, called WIMPs (weakly interacting massive particles) MACHOS (massive compact halo objects), e.g., small black holes or brown dwarfs

31. Galaxies formation A full-size galaxy is formed by the merger of smaller objects (or sub- galactic unit) These small objects (less than 1 kpc) (numbered in the figure) are seen when the Universe is young (3400 Mpc away, or 11 billion lys ago)

32. Formation of Spiral or Elliptical Galaxies It depends how fast the gas is used up to form galaxies If star formation is fast, no gas is left  elliptical galaxy If star formation is slow, gas forms disk  spiral galaxy

33. Key Words anisotropic barred spiral galaxy clusters (of galaxies) dark-matter problem distance ladder dwarf elliptical galaxy elliptical galaxy fundamental plane galactic cannibalism giant elliptical galaxy gravitational lens groups (of galaxies) Hubble classification Hubble constant Hubble flow Hubble law intracluster gas irregular cluster irregular galaxy isotropic lenticular galaxy Local Group maser poor cluster redshift regular cluster rich cluster spiral galaxy standard candle starburst galaxy supercluster Tully-Fisher relation tuning fork diagram void