4: GAS IN GALAXIES

Question 1

What does the ISM contain?

Answer 1

Neutral hydrogen (HI)
Ionised hydrogen (HII)
Molecular hydrogen (H2)

Question 2

What is the relative importance of the ISM quantified by?

Answer 2

The H mass to the optical light ratio

Question 3

What element is important in radio galaxies?

Answer 3

HI, it’s key to studies of the ISM in our Galaxy and other spirals

Question 4

Define Stromgren sphere

Answer 4

A hot star inside a H cloud of reasonably uniform density will ionise all the gas in the region around the star (Stromgren), making an HII region

Question 5

Why are HII regions sharply ionisation bounded?

Answer 5

Because we ran out of ionising photons rather than atoms to ionise (Density bounded case)

Question 6

How do HII regions produce free-free emission? What is this emission characterised by and what does its intensity depend on?

Answer 6

They produce it in the radio when an electron loses energy by being scattered by a proton, but not captured. The emission is characterised by the mean energy of the electrons and its intensity depends on nenh, integrated along the line of sight

Question 7

What does the variation of extinction with lamba depend on?

Answer 7

The makeup of interstellar grains

Question 8

Why does the Galactic curve have a bump at 220nm?

Answer 8

Due to small particles probably made of graphite

Question 9

What is the UV extinction thought to be produced by?

Answer 9

Small silicate grains

Question 10

What might the visible part of the Galactic curve result from?

Answer 10

Icy grains and polycyclic aromatic hydrocarbons

Question 11

Why are the extinction curves for the LMC and SMC different to the Galactic one?

Answer 11

Because of their lower Z in ISM

Question 12

What do HI size and HI mass correlate with?

Answer 12

HI size correlates with optical size and HI mass correlates with both

Question 13

Where is most of the gas mass?

Answer 13

In the cool component

Question 14

Hot diffuse phase fills most of the volume. Hot plasma forms an extensive halo around the warm clouds. How is cool gas heated and what does it cause?

Answer 14

By SN explosions and rise up out of the plane into the halo before cooling and falling back in

Question 15

How are QSO absorption line systems produced?

Answer 15

Large halos of ionised gas around gaalxies can be detected by their absorption of light from distant quasars

Question 16

What is the optical spectra of quasars?

Answer 16

THe lines of the MGII doublet at 280nm or at higher absorber redshifts, the UV lines of CIV as 155nm

Question 17

What is indicative of the optically thin case?

Answer 17

Total flux or apparent magnitude is unchanged

Question 18

What is indicative of optically thick case?

Answer 18

Apparent magnitude fainter for inclined galaxies

Question 19

When tilting a galaxy by an angle i, what happens to the light absorbed in the optical?

Answer 19

It’s re-emitted in the FIR

Question 20

What are important regions for star formation?

Answer 20

HII regions and bright blue (shortlived stars)

Question 21

What is the process of star formation?

Answer 21

1. Molecular clouds collapse, and their dust content heats up leading to FIR emission.
2. Protostars form, dust near them reaches temperatures of 1000K, thus emitting in the near IR while the surrounding layers are still only at 100k
3. When a star reaches the MS (assuming it’s massive and therefore hot enough_ it will ionise its surroundings, forming a compact HIII region detectable at radio wavelengths
4. As the hot ionised region expands, the dust cools back to FIR emitting temperatures before dissipating, leaving stars visible at optical wavelengths as an OB association or OC

Question 22

What happens once SF has taken place?

Answer 22

Stars start to feed back energy and material into the ISM, allowing the process to repeat.

Question 23

How does feedback occur?

Answer 23

Via the stars’ radiation (heating and ionising of photons) and mechanically from mass loss as the stars age or more spectacularly in SN explosions.

Question 24

Define sequential star formation

Answer 24

Shock waves propagate ahead into the molecular clouds, compressing the gas and instigating gravitational collapse and the formation of further new stars.

Question 25

What are the indicators of global star formation?

Answer 25

H-alpha emission
FIR
Radio
X-Ray

Question 26

Why are SF regions dusty?

Answer 26

They allow for extinction.

Question 27

In the FIR what are there contributions to the emission from?

Answer 27

Dust heated by the general interstellar radiation field

Question 28

What do HII regions and general ionised gas emit?

Answer 28

Thermal continuum radiation

Question 29

What do SF regions give rise to?

Answer 29

SN that accelerate electrons which produce synchrotron emission

Question 30

Why is it preferable to use IR or radio indicators for starburst galaxies?

Answer 30

The dust extinction is large

Question 31

Define Roberts time

Answer 31

Time which existing gas will be used up by SF at the current rate

Question 32

Define Toomre criterion

Answer 32

Threshold value understood from the stability of a gas disk

Question 33

Environment of a galaxy can affect its SF. What has higher SFR?

Answer 33

Strongly interacting galaxies

Question 34

What do HI deficient galaxies suggest?

Answer 34

That gas consumption was faster in the cluster environment, possible as a result of spirals falling in to the cluster.

Question 35

When does SFR decrease in regards to local environment?

Answer 35

Increasing density of local environment