2.6 Hydrogen in the ISM

Question 1

What is a hyperfine transition?

Answer 1

The GS of H is split into 2 levels separated by 6 micro eV

- The proton and electron can have parallel or anti parallel spins

Question 2

Which energy level has a higher energy? Parallel or anti parallel spins?

Answer 2

Parallel

Question 3

What is the 21cm line?

Answer 3

An EM spectral line caused by the change in the ground state energy of neutral hydrogen
- Has a frequency in the radio range of 1.4GHz and is the most prominent line in the ISM due to Hydrogen being 73% of the ISM despite the low transition rate

Question 4

What three factors affect the balance between neutral and ionised hydrogen?

Answer 4

Product of temperature, density and radiation field

- All these effects play a role in SFR

Question 5

How does temperature affect the balance between neutral and ionised hydrogen?

Answer 5

Higher temperature means higher energy levels are populated

- Closer to the continuum so easier to ionise

Question 6

How does density affect the balance between neutral and ionised hydrogen?

Answer 6

Density drives the collisions between atoms which transfers energy and this energy can ionise the electron itself

Question 7

How does the radiation field affect the balance between neutral and ionised hydrogen?

Answer 7

Photons carry energy and can promote electrons to higher energy levels or even ionise themselves

Question 8

Describe the change between ionised and neutral hydrogen in terms of a diagram

Answer 8

• Stellar cores make an ionised region (H II). T = 10^4 K
• Dense region of neutral H (H I) T = 10^2 K
• Low dense cold region (H I H_2) T = 10-20K

Question 9

What is a Lyman (Ly) series?

Answer 9

Any transitions to/from n = 1 (UV)

Question 10

What is a Balmer (Ba/H) series?

Answer 10

Any transitions to/from n = 2 (Vis.)

Question 11

What is a Paschen (Pa) series?

Answer 11

Any transitions to/from n = 3 (NIR)

Question 12

What is a Brackett (Br) series?

Answer 12

Any transitions to/from n = 4 (IR)

Question 13

What is a Pfund (Pf) series?

Answer 13

Any transitions to/from n = 5 (IR)

Question 14

What is the Rydberg formula for hydrogen?

Answer 14

1/λ_vac = R_H [ (1/n1)^2 - (1/n2)^2 ]
R_H = 1.1E7

Question 15

What is a bound free transition?

Answer 15

Transitions from any energy level to the continuum

Question 16

What energy does a photon need to cause a bound free transition?

Answer 16

Energy above the series limit

Question 17

What are Lyman/Balmer breaks?

Answer 17

Distinctive drops in the flux corresonding to specific wavelengths
- Higher energy photons can ionise electrons from n=2 and n=1 and during ionisation, photon is absorbed/scattered leading to lower flux

Question 18

How can we measure the red shift of a galaxy using the hydrogen line?

Answer 18

z = λ’ - λ / λ where λ’ is the measured hydrogen line, and λ is the rest frame wavelength

Question 19

What is the relationship between the measured z of a galaxy and the wavelength in the rest frame?

Answer 19

A galaxy with z has observed λ’ (z+1) bigger than the rest frame λ
- Wavelength observed is (z+1)x bigger than the wavelength in the rest frame

Question 20

How can the red shift of a galaxy be observed in terms of the Lyman and Balmer series?

Answer 20

The Ly/Ba breaks are shifted to the visible/IR which are observable on the ground
- Redshift of the galaxy can be inferred from the position of these breaks