Radio Waves Interaction With Interstellar Matter

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

2. 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

3. 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

4. 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

5. 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

6. 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

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

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