The 21 cm Line

• physics: hyperfine splitting
• awesome examples:
  • the sky in 21 cm; disk map in velocity space
  • galaxy M33 in optical and in 21 cm
  • galaxy rotation curves at 21 cm
  • cosmic 21 cm radiation; LOFAR, SKA

Atomic Collisions

• cross sections
• critical densities

Nebular Diagnostics