6.4-5

Question 1

HI 21 cm or 1420 MHz line

what is its significance?

Answer 1

Significance in Astrophysics: Traces most important gas.

Question 2

Hyperfine-structure line

Answer 2

Spin change between proton and electron.

It is a spectral line that results from the above. (in hydrogen)

(i think at least)

Question 3

Lower level spins

Answer 3

• Antiparallel
• g_l = 1

Question 4

Upper level spins

in the context of HI 21 cm measurements

Answer 4

• Parallel
• g_u = 3

not sure what g stands for

Question 5

h𝛎₀

Context: HI 21 cm measurements

numerical value asked

Answer 5

• ≈ 10^-5 eV ≪ k_B T
• Even for T = 10 K

Question 6

A constant

high density limit

Answer 6

• ≈ 2.85 · 10^-15 s^-1

Question 7

n_u/n_l ratio

in HI 21 cm measurements

Answer 7

• = 3/1
• or: n_u = 3/4 n_H

I believe that what this is saying is that there are three times more hydrogen atoms in the upper state compared to the lower state?

Question 8

Specific intensity I(21 cm)

what is it proportional to?

Answer 8

1/(4π) · 3/4 · h · ν₀ · A_ul · ∫ n_H ds

once again, Im not sure we need to know this, idk. maybe its good to know that: intensity is proportional to the integral of the density which makes sense

Question 9

Mapping galactic disk with HI

what does it yield and what indicated the rotation?

Answer 9

• I(21 cm) yields ∫ n_H ds along line of sight
• Doppler shift indicates galactic rotation

Question 10

Radial velocity v_r(ℓ) formula

Answer 10

Derived using the law of sines:

v_r(ℓ) = Θ · cos α - Θ₀ · sin ℓ

Question 11

Cloud radial velocity observations

Answer 11

• v_r(ℓ) > 0 for R < R₀
• v_r(ℓ) = 0 for R = R₀
• v_r(ℓ) < 0 for R > R₀

Question 12

Maximum v_max(ℓ) for galactic rotation

Answer 12

v_max(ℓ) = R₀(Ω(R_min(ℓ)) - Ω₀) sin ℓ

yields Ω(R) for R

Question 13

Galactic ℓ-RV emission line maps

Answer 13

HI and CO maps show similar properties:

• outer Milky Way emission
• central region

i feel like this card doesnt really say much if you know what i mean

Question 14

Galactic rotation curve

Answer 14

v_rot(R) = √(G * M_R(R) / R)

• Measures Milky Way mass inside radius R
• Dark matter exists in galactic halo

derive by equating Centripetal to gravitational force

second point refers to M_R

Question 15

Photoionized gas and HII regions

how are they created?

Answer 15

1. Hot stars emit photons ionizing neutral hydrogen
2. Creating HII regions

Question 16

Ionization degree equilibrium

Answer 16

n_H⁰ ∫ (4πIν / hν) aν(H) dν = n_e n_p α(H, T)

I dont think we need to know this, delete if you agree, if you dont, let me know and ill do the formatting

post exam: yeah, dont listen to past Michal, learn it!

Question 17

Strömgren sphere

Answer 17

• Describes global photo-ionization equilibrium
• Defines Strömgren-radius

Question 18

Temperature equilibrium in ionized regions

Photoionised Gas and HII Regions

Answer 18

• Heated by photoionization
• Cooled by collisionally excited lines
• T ≈ 10,000 K