1. The Mass of the Galaxy We can use the orbital velocity to deduce the mass of the Galaxy (interior to our orbit): v orb 2 =GM/R. This comes out about 10 11 solar masses. We can also get a mass estimate from the integrated light of the Galaxy (corrected for interstellar absorption). This comes out substantially lower. There must be some “dark matter”.

2. Mapping the Galaxy : Radio Astronomy We can only see our local neighborhood because of interstellar dust. To penetrate this, we can use radio wavelengths (much longer than the size of dust particles). Of course, something has to be producing radio emission…

3. Sources of Radio Emission -1 1)Thermal emission from cold interstellar clouds At a few 10s of K, blackbody emission will be in the radio, or somewhat hotter clouds have a long wavelength tail

4. Sources of Radio Emission -2 2) In a strong magnetic field, spiraling electrons will produce non-thermal “synchrotron” radiation. This can happen near stars or compact objects, or from cosmic rays in the galactic field.

5. Sources of Radio Emission – 21 cm radiation Neutral hydrogen has a very weak radio spectral transition. So the Galaxy is transparent to it. On the other hand, there’s a lot of neutral hydrogen. So we can see it everywhere. There are also molecular lines from CO and other molecules. The transition occurs because electrons and protons have “spin”. Having the spins aligned is a higher energy state. So in about 10 million years it will decay to the ground state (anti-aligned). Or a 21-cm photon can be absorbed and align the spins. Because the Galaxy is transparent, it is hard to tell where the emission is coming from along the line-of-sight. But because we know its precise wavelength, Doppler shifts in this line can tell us how the gas is moving.

6. Optical and Radio Sky

7. Deciphering 21-cm maps With a rotation model of the Galaxy, you can sort of figure out where different parts of the emission are coming from.

8. Radio Data Imaging and velocity maps in CO. Composite image of Perseus region in hydrogen.

9. Finding the Galactic Structure Molecular Clouds 21-cm map

10. Spiral Arms in Galaxies Since inner orbits are faster than outer orbits, you might think that is why one sees spiral arms. But these would rapidly wind tightly; galaxies have had ~100 rotations since they formed. Instead, the spiral arms are “density waves”: apparent patterns where stars are denser due to slowing down from mutual gravity.

11. Density Waves Traffic jams are good examples of density waves. Certain parts of the freeway may have a high density of cars, yet individual cars do not stay with the pattern, but flow through it. They move slowly when at high density, and move quickly when at low density. The site of an accident might produce a stationary density wave (but again, cars are always moving through it). Thus, the spiral arms of a galaxy are just a pattern that may rotate slowly or not at all; individual stars will be passing through it all the time.

12. Spiral Arms and Star Formation When the ISM passes through it, it gets compressed, and star formation is enhanced. This makes bright hot young stars, and the pattern stands out.

13. Tracers of Spiral Arms In addition to radio maps, you can use HII regions or O&B stars to try to locate spiral arms. The Sun is near the Orion-Cygnus arm, but that is a “recent” occurrence. It’s been around about 18 times.

14. Spiral Tracers from Outside O & B stars HII regions 21-cm radiation In other galaxies, the arms are easy to see because their ISM does not hide optical diagnostics from us. There are always only a few arms (often 2), and they are never too tightly wound.

15. The Heart of the Galaxy Infrared X-ray

16. The Galactic Center

17. The Monster Lurking at the Center Recent adaptive optics pictures in the infrared at the Galactic Center show stars orbiting a central invisible mass. Kepler’s Laws yield a mass inside one light year of 2.7 million solar masses! It has to be a black hole (but apparently it is napping at the moment…)

18. The Multi-wavelength Milky Way

19. Stellar Populations Stellar Population Location Star motions Ages of stars Brightest stars Supernovae Star clusters Association with gas and dust? Active star formation? Abundance of heavy elements (mass) Population I Disk and spiral arms Circular, low velocity Some < 100 million years Blue giants Core collapse (Type II) Open (e.g., Pleiades) Yes 2% Population II Bulge and halo Random, high velocity Only > 10 billion years Red giants White dwarf explosions (Type I) Globular (e.g., M3) No 0.1 - 1% Population II stars are old and metal poor, found in large orbits in a random spherical distribution. Population I stars are young and metal rich (including hot stars), all orbiting in the disk in the same direction.

20. Galactic Structure Disk (I) and Bulge (II) (stars, ISM, open clusters) Halo : Pop II (stars, globular clusters) Dark Matter Halo

21. Formation of the Galaxy