1. The Milky Way PHYS390 Astrophysics Professor Lee Carkner Lecture 19

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

3. Disk  Most visible area of the MW  Diameter  Sun is ~8 kpc from center   Two components  Thin disk of younger stars   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 [(N Fe /N H ) star / (N Fe /N H ) sun ]  Range: --  0 (exactly like the sun)   Not perfectly reliable  Iron comes from Type Ia supernovae and may vary with region  Not completely mixed

5. Age of Disk  Thin disk has broad range of metallicities   Started forming stars 8 Gya and still going on today   -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  Form 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 for us 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  Luminous mass looks concentrated at the core   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 10 12 M sun   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 total Milky Way ~ 60

12. Dark Matter Candidates  MACHOs   White dwarfs, neutron stars, black holes, red dwarfs, brown dwarfs  Not enough detected in microlensing surveys   WIMPs  Weakly Interacting Massive Particles   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  m 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 3.7X10 6 M sun 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