1. The Milky Way Physical Astronomy Professor Lee Carkner Lecture 20

2. The Milky Way • We can see the band of the Milky Way on a dark night • Nature of galaxy not known until early 20th century • Basic structure • Central dense bulge • Young disk with spiral arms • Old halo with dark matter

3. Disk • Most visible area of the MW • Sun is ~8 kpc from center • Two components • Height ~ 350 pc • Site of current star formation • Thick disk of older stars • Fainter and has fewer stars (few % of thin disk)

4. Metallicity • We use metal abundance as a proxy for age • Normally use the iron to hydrogen ratio compared to the sun [Fe/H] = log [(NFe/NH)star / (NFe/NH)sun] • Range: • 0 (exactly like the sun) • Not perfectly reliable • Not completely mixed

5. Age of Disk • Thin disk has broad range of metallicities • -0.5 to 0.3 • -0.6 to -0.4 • Formed from episode of star formation between 10 and 11 Gya

6. Spiral Arms • Gas, dust, young stars, bright stars, blue stars all concentrated in arms • Hard to map in our galaxy • From via density waves • As clouds orbit the Milky Way, they get stuck in areas of greater density

7. The Bulge • The central part of the MW is a thickened bar-shaped bulge • Hard to see due to extinction • Due to several waves of star formation • Region within which ½ of the light is emitted

8. Halo • Above and below the disk are the globular clusters About 150 total • Metallicity around -0.8 • May be associated with thick disk • Or else would have broken up over the last ~12 Gyr

9. Rotation Curve • period of sun ~ 230 million years • Rotation speed should fall off with distance • Instead galaxy has flat rotation curve • Rotational velocity constant with increasing distance from center

10. Dark Matter • However, orbits of stars exterior to the sun indicate that there must be a total of about 1012 Msun • Dark matter is about 95% of total galactic mass • Cannot be dust, gas or stars

11. Mass to Light • Ratio of mass in solar masses to light in solar luminosities • For Milky Way ~ 60

12. Dark Matter Candidates • MACHOs • MAssive Compact Halo Objects • White dwarfs, neutron stars, black holes, red dwarfs, brown dwarfs • Should pass in front of other stars, momentarily brightening them • WIMPs • Very low probability of interaction • Should be able to detect in very large isolated detector arrays

13. Galactic Center • Galactic center is 8 kpc from the sun in the constellation of Sagittarius • Can find from distribution of halo globular clusters • Best data from radio, IR and X-ray (not visible) • stars are “isothermal”

14. Radio Observations • A complex series of thermal and non-thermal sources • At the center is a very bright, unresolved source, Sgr A* • Less than ~2 AU in size

15. X-ray Observations • Sgr A* corresponds to a bright X-ray source • Explosions of material must have occurred in the past

16. IR Observations • The K band at 2.2 mm is used to observe stars close to Sgr A* • Can use Kepler’s third law to find mass of Sgr A*

17. The Core • Sgr A* has a mass of 3.7X106 Msun in a space less than 2 AU in size • Destroys near-by stars to provide mass for accretion disk and outflows • Black hole is fairly quiescent

18. Next Time • Read 25.1-25.4 • Homework: 24.2, 24.30, 24.33, 25.2b, 25.8a, 25.8b