(updated for 2013)
- HII scale height: 1 kpc
- CO scale height: 50-75 pc
- HI scale height: 130-400 pc
- Stellar scale height: 100 pc in spiral arm, 500 pc in disk
- Stellar mass:
- Dark matter mass:
- HI mass:
- H2 mass (inferred from CO):
- HII mass:
- -> total gas mass (including He).
- Total MW mass within 15 kpc: (using the Galaxy’s rotation curve). About 50% dark matter.
So the ISM is a relatively small constituent of the Galaxy (by mass).
The Milky Way is a very thin disk (think a CD with a ping-pong ball in the middle) In class (2011), we drew the following schematic of the “topology” of phases in the ISM.