Radio Waves Interaction With Interstellar Matter A cosmic journey brought to you by McGourty-Rideout

Interstellar Medium Gaseous - almost all hydrogen H I- neutral hydrogen one proton and one electron( most) H II ionized hydrogen- - no electron just a proton - near hot stars Dust – C, N, and silicates

21-cm line Radiation Radio emission from hydrogen when an electron “flips its spin” H I gas emits a particular wavelength of radio energy from an energy change in the hydrogen atoms. The wavelength is 21.1cm (frequency = 1420.4 MHz) called 21-cm line radiation Temperatures between 100 K to about 3000 K

21-cm line Radiation An electron moving about a proton can have a spin in the same direction as the proton or in the opposite direction The energy state of an electron spinning opposite is slightly lower than the energy state when both are in the same direction

21-cm line Radiation Atoms always want to be in the lowest energy state If the spins are in the same direction, the electron will eventually flip to the opposite spin direction The energy difference is very small, so a hydrogen atom may wait a few million years before it undergoes this transition

Uses of 21cm Line The Milky Way alone has about 3 billion solar masses of H I gas so enough hydrogen atoms are emitting the 21-cm line radiation to be easily detected with radio telescopes Not blocked by dust Best way to map the structure of the galaxy Radio emission leads to penetration

Waves Transformed in Space Slowed down & refracted by interstellar gas and plasma Spectral lines absorbed by interstellar molecules & atoms Doppler shifted by moving sources

Waves Transformed in Space Slowed down & refracted by strong gravitational fields Red shifted by gravitational sources Polarization rotated by magnetic fields