S3 - Radiation and the ISM 2 Flashcards
Why do molecular clouds emit radio waves?
The most abundant element in the universe, hydrogen, emits radio waves even in cold molecular clouds. When excited hydrogen atoms de-excite, they release their excess energy at about 1420MHz (λ = 21cm) (very low energy) radio waves. This emission from a hydrogen atom occurs when the spin of the electron within it changes its orientation from the higher energy state configuration to the lower one (spin-flip transition) at the ground state. This spin-flip transition in hydrogen requires very little energy so even cold molecular clouds with little thermal energy can induce the excitation of an electron’s spin in a hydrogen atom into a higher energy state spin configuration.
Why can radio waves be used to map the distribution of molecular clouds in the Milky Way?
Radio waves from molecular clouds are subjected to slightly different Doppler shifts (red shift/blue shift) relevant to the observer as the clouds are moving at different velocities due to their different distances to the galactic centre.
What happens to the motion of an object the further it is from the galactic centre?
The further an object is from the galactic centre, the slower it will move.
Why is the first excited state of CO very commonly found to populate cold molecular clouds?
CO is a polar molecule meaning that when it rotates, it changes energy between rotational states. The J = 1 rotational state lies above the ground state by only 0.48meV or 5.5K. As only 5.5K is required to excite CO to the J = 1 state, it is commonly found in molecular clouds. Excitations occur primarily through collisions with ambient H2.
What method can be used to determine the composition of and the temperature of molecular gas?
Rotational spectroscopy. The spectrum of rotational transitions that occur are different at different temperatures and for different molecules.
Where is most molecular gas in the Milky Way located?
Most molecular gas is in the Giant Molecular Clouds (GMCs) confined in the spiral arms (>10^5 solar masses). Throughout the galactic disc are small clouds and complexes (about 10^4 solar masses).
What are the sites of star formation in galaxies?
Dense molecular clouds
How is the atomic gas throughout the galaxies detected?
Via the spin-flip transition of atomic hydrogen (HI) at a wavelength of 21cm.
How is the molecular gas throughout the galaxies detected?
Via the low-lying rotational energy level transitions of carbon monoxide that are easily excited in cold gas.
