3.3 - 3.4 Dust Emission and SFRI

Question 1

What is the energy input for dust emission?

Answer 1

A nearby star with luminosity L

Question 2

What are the components for the energy input into a dust grain?

Answer 2

The flux from the star, multiplied by the geometric cross section of the dust grain and an absorption coefficient

Question 3

What is the typical temperature of dust grains for galaxies hosting blue stars, and what is the peak emission wavelength?

Answer 3

15-30K

Using Wein’s law, you get 100-200 μm (FIR)

Question 4

How can we describe the radiation of a dust particle, and describe it

Answer 4

A Grey body not a BB

- Emits preferentially at specific wavelengths depending on composition

Question 5

How does dust assist in the formation of stars forming from collapses H_2 clouds?

Answer 5

H_2 is catalysed by dust, assisting the formation

Question 6

How does dust assist in the collapsing of gas clouds?

Answer 6

It acts as a coolant

- It reprocesses UV into IR, lowering the temperature of the gas allowing the collapse to continue

Question 7

What is a star formation indicator, and its relation to dust?

Answer 7

Dust is a star formation indicator

- An observable property of a galaxy that relates to SFR

Question 8

What are the three types of dusty galaxies, and what is their peak emission?

Answer 8

Dusty Star forming galaxies (DSFGs) - peak at 10-30μm
Ultra luminous IR galaxies (ULIRGs) - peak at 100-200μm
Sub mm galaxies (SMGs) - peak at 0.85mm

Question 9

What is a continuum?

Answer 9

The region of a spectrum fat from absorption or emission lines (e.g. BB envelope)

Question 10

Describe the properties of the UV continuum

Answer 10

Measured between 150-250mm and dominated by massive, short lived stars
- A lower limit on SFRI as UV is absorbed by dust

Question 11

Describe the properties of the dust continuum

Answer 11

Measured between 10μm and 1mm

- Compensates for radiation lost by the UV continuum

Question 12

How can we calculate the total SFRI?

Answer 12

Sum the SFRIs from the UV and the dust continuum

Question 13

How can the SFR be measured for nearby galaxies?

Answer 13

Can count stars

- Reconstruct the SFR vs time by counting stars and measuring their age

Question 14

How can we measure SFR for further away galaxies, and describe this method

Answer 14

SED fitting (Spectral energy distribution)
- Model the whole spectrum vs time from an episode of star formation and compare to observed spectra

Question 15

When did massive galaxies form most of their stars?

Answer 15

Earlier in history and very rapidly

Question 16

When did galaxies in low density environments form most of their stars?

Answer 16

More recently over a broader range of time compared to massive galaxies

Question 17

Describe how emission lines can be used as a SFRI

Answer 17

• H alpha and Ly-alpha lines form from recombination of HII in star forming regions
• Powered by high mass, short lifetime stars

Question 18

Describe how the radio continuum from SN remnants can be used as a SFRI

Answer 18

Remnants have strong magnetic fields which accelerate electrons
- These electrons emit radiation in radio spectrum, but it is a delayed indicator as you need to wait for the SN