5. Observing Distant Galaxies

Question 1

State what redshift the first halos and stars formed

Answer 1

Halos: z = 30-20
Stars (Pop III): z = 25-15

Question 2

What are the redshifts of the first “normal” observable galaxies?

Answer 2

Candidates: z = 10-16
Confirmed: z = 8-11
High Z: z>= 5
- Very faint, very red and very small

Question 3

What is spectroscopy?

Answer 3

When you gather light and then spread it out with a prism/grating/grism

Question 4

How do we resolve the lines from the spectroscopy

Answer 4

Need a resolving factor of wavelength / change in wavelength of ~ 2000
- E.g. for optical light, wavelength is ~ 500nm so we need the change in wavelength per pixel of 2.5
- Very low signal to noise ratio

Question 5

What is the relationship between the signal and noise of the photons

Answer 5

N photons for the signal
root N for the noise
- For spectroscopy, we can only observe a few targets at a time which makes it expensive

Question 6

What is photometry?

Answer 6

When you make a series of images each through a filter collecting through a change of wavelength of 200nm
- Get colour information on thousands of galaxies
- High signal to noise ratio

Question 7

What can we conclude if the spectral energy distribution (SED) of a galaxy is blue/red?

Answer 7

Blue - young stars
Red - old stars

Question 8

What is the 4000 Angstrom break?

Answer 8

Occurs when there is metal and hydrogen absorption
- Strengthens with the age of the galaxy

Question 9

Describe what would be observed in a SED if the galaxy is dusty, gas rich, or redshifted

Answer 9

Dusty - Red light is boosted relative to blue light
Gas rich - Emission lines
Redshift - The whole spectrum is shifted

Question 10

Describe the characteristics of a young galaxy on a Flux-Wavelength spectrum graph

Answer 10

Starts top left and slopes downwards with a small break at 4000 Angstroms
- Has emission line spikes as gas rich
- More flux at top left in UV spectrum than bottom right

Question 11

Describe the characteristics of an old galaxy on a Flux-Wavelength spectrum graph

Answer 11

Very little bottom left then steeply heads upwards at 4000 Angstroms and then tails off still sloping upwards

Question 12

What is the redshift of the distant universe?

Answer 12

z > 5

Question 13

What happens to optical features, like the 4000 angstom break in the distant universe?

Answer 13

They are shifted into the infrared

Question 14

Describe what we observe on a spectrum when observing a star in the distant universe

Answer 14

Observe a big spike in flux at the Lyman alpha emission line
- Redshift of 1216(1+z_star) angstroms
- Then see absorption lines where there are clouds of hydrogen and other galaxies
- These occur at 1216(1+z_cloud) angstroms

Question 15

What do we know exists between us and the source in a distant universe along our line of sight

Answer 15

Clouds of cool neutral hydrogen gas in the intergalactic medium

Question 16

What is the Lyman alpha forest?

Answer 16

As light passes through each hydrogen cloud, the cloud will absorb light at its own redshift for each electron
- Strongest is at the Lyman alpha emission (n = 2 -> n=1) at 1216 Angstroms

Question 17

What is Lyman alpha emission

Answer 17

The electron transition in hydrogen
- Occurs from n=2 -> n=1
- At 1216 Angstroms

Question 18

What happens to the light of the Lyman alpha emission lines at different redshifts

Answer 18

At z=3: About half of the light blueward of the Lyman alpha line is lost
At z>5: This exceeds 90%
- Universe is denser and more gas rich

Question 19

Describe why when observing a source in the distant universe, there is a considerable amount of lost light before the Lyman alpha emission lines

Answer 19

• Photons blueward of 912 Angstroms have E > 13.6eV
• They can ionise Hydrogen and scaller very efficiently so they are lost from the line of sight

Question 20

What is the Lyman break?

Answer 20

The combination of the 912 and 1215 Angstrom features

Question 21

What is the Lyman technique?

Answer 21

Using at least 3 filters which exist before the break, straddling the start of the break and redwards of the break
- Plot a graph of M1 - M2 against M2 - M3
- Normal low z galaxies are bottom right, Lyman break galaxies are top left

Question 22

Describe the properties of a “normal” low z galaxy using the Lyman technique

Answer 22

There are small colours in the adjacent photometric filters
- The ratio of the fluxes is near 1
- Bottom middle of graph

Question 23

Describe the properties of a high z galaxy using the Lyman technique

Answer 23

Extreme colours in the filters either side fo the break
- Top left of the graph

Question 24

What are dropout galaxies?

Answer 24

Lyman break galaxies

Question 25

What is the Schecter function, and the key result from this?

Answer 25

It is the galaxy mass function
- You can count the abundance of galaxies as a function of luminosity

Question 26

When looking at the results of the Schecter function, what do we need follow up spectroscopy for?

Answer 26

Precise redshifts, ionizing spectrum, evaluation of contamination, ages, exact masses and metal enrichment
- Requires 10s of hours on 10m telescopes

Question 27

Describe the key properties of the JWST

Answer 27

Launched 2021 with 6.5m diameter
- IR, L2 solar orbit, imaging and high multi slit spectrograph

Question 27

Describe the key properties of the Hubble telescope

Answer 27

Launched 1990 with 2.4m diameter
- Optical spectrum, LEO, imaging and 1 long slit spectograph