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Astronomy 404/CSI 769

Astronomy 404/CSI 769. Extragalactic Astronomy. http://physics.gmu.edu/~satyapal/ASTR404spring05. Course Details. ASTR 404, Galactic Astrophysics (3) Lecture Day/Time: Thursday 4:30-7:10 PM Lecture Place: Room 310, S&T I Prerequisites ASTR 328, Math 214, PHYS 308. What are Galaxies?.

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Astronomy 404/CSI 769

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  1. Astronomy 404/CSI 769 Extragalactic Astronomy http://physics.gmu.edu/~satyapal/ASTR404spring05

  2. Course Details • ASTR 404, Galactic Astrophysics (3)Lecture Day/Time: Thursday 4:30-7:10 PMLecture Place: Room 310, S&T I • Prerequisites ASTR 328, Math 214, PHYS 308 ASTR 404/CSI 769 1/27/05

  3. What are Galaxies? • Just 80 years ago, we did not even know there were galaxies in our Universe …. • Most distant objects we see in the Universe (~10,000 galaxies). • 16 billion times fainter than what the human eye can see. ASTR 404/CSI 769 1/27/05

  4. The Menagerie of galaxies ASTR 404/CSI 769 1/27/05

  5. Tentative Lecture Schedule ASTR 404/CSI 769 1/27/05

  6. Review of a few astronomical units of measurement Brief review of stars History of the Milky Way Star counts Morphology and major components of our Galaxy Today’s Lecture ASTR 404/CSI 769 1/27/05

  7. Late 1700s - Herschels counted stars in 683 regions of sky, assumed all are equally luminous. Concluded that Sun at center of a flattened system. Brief History What is the Shape of the Milky Way? ASTR 404/CSI 769 1/27/05

  8. 1920 - Kapteyn used a greater number of star counts and came to roughly the same conclusion Star Counts: If stars are distributed uniformly in space, then in any patch of sky, the total number of stars with flux less than a limiting flux, f is: Log(N(m’<m)) = C + 0.6m; where C = constant (Note: These formulae were derived on the board in class) ASTR 404/CSI 769 1/27/05

  9. logN m Actual star counts are less than would be predicted by this relationship at fainter flux levels (or larger magnitudes) ASTR 404/CSI 769 1/27/05

  10. Conclusion: stellar density not uniform but decreases with distance from Sun; faster in direction perpendicular to Milky Way and slower in the direction of the Milky Way Milky Way is a highly flattened disk ASTR 404/CSI 769 1/27/05

  11. 1919 - Shapley studied globular clusters; used distance derived from pulsating stars to determine that Sun is not at center of Milky Way. These were found at great distances above and below the plane of the Galaxy, where extinction effects are much less than that found along the Milky Way ASTR 404/CSI 769 1/27/05

  12. Globular clusters • Definitely bound by gravity • Contain large numbers of stars in a very small volume: 20,000-1,000,000 stars in a volume 20 pc in diameter • very round and symmetrical in shape - very old -- among the first stellar complexes formed in the galaxy ASTR 404/CSI 769 1/27/05

  13. Distances from Variable Stars ASTR 404/CSI 769 1/27/05

  14. Morphology of Galaxy ASTR 404/CSI 769 1/27/05

  15. ASTR 404/CSI 769 1/27/05

  16. Disk • Young thin disk • Old thin disk • Thick disk ASTR 404/CSI 769 1/27/05

  17. Thin disk • Diameter ~ 50 kpc • Young thin disk scale height = 50 pc • Old thin disk scale height = 325 pc • Contains youngest stars, dust, and gas • Contains Sun, which is 30 pc above midplane • M* = 6  1010 Msun • Mdust+gas = 0.5  106 Msun (scale height 0.16) • Average stellar mass ~ 0.7 Msun • LB ~ 1.8  1010 Lsun • Population I stars in the Galactic plane • Contains ~ 95% of the disk stars • [Fe/H] ~ -0.5 - +0.3 • Age ~ < 12 Gyr • Spiral structure seen in neutral H, HII regions, young O and B stars ASTR 404/CSI 769 1/27/05

  18. Thick disk • Diameter ~ 50 kpc • Scale height = 1.4 kpc • M* = 2-4  109 Msun • LB ~ 2  108 Lsun • [Fe/H] ~ -1.6 - -0.4 (less metal rich than thin disk) • Age ~14-17 Gyrs ASTR 404/CSI 769 1/27/05

  19. Gas and Dust ASTR 404/CSI 769 1/27/05

  20. Spheroidal Components • Central bulge • Stellar Halo • Dark Matter Halo ASTR 404/CSI 769 1/27/05

  21. Central Bulge • Diameter ~ 2 kpc • Scale height = 0.4 kpc • M* = 1  1010 Msun • LB ~ 0.3  1010 Lsun • [Fe/H] ~ -1.0 - +1.0 (less metal rich than thick and thin disk) • Age ~10-17 Gyrs ASTR 404/CSI 769 1/27/05

  22. Stellar Halo • Diameter ~ 100 kpc • Scale height = 3 kpc • number density distribution ~  r-3.5 • M* = 0.1  1010 Msun • LB ~ 0.1  1010 Lsun • [Fe/H] ~ -4.5 - -0.5 (metal poor) • Age ~14-17 Gyrs ASTR 404/CSI 769 1/27/05

  23. Dark Matter Halo? Rotation of Galaxy implies that there is a lot of mass in our Galaxy that we don’t see (ie, if we count up the mass from the stars that emit visible light, it’s much less than that implied by observing the motion of stars as a function of radius from the center of the Galaxy. How do we know that the stars in the disk rotate around the center of the Galaxy? How do we know the rotational velocity of the Sun? How do we know the rotation curve? (rotational velocity as a function of radius from the Galactic center?) ASTR 404/CSI 769 1/27/05

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