5. Observing Distant Galaxies Flashcards
State what redshift the first halos and stars formed
Halos: z = 30-20
Stars (Pop III): z = 25-15
What are the redshifts of the first “normal” observable galaxies?
Candidates: z = 10-16
Confirmed: z = 8-11
High Z: z>= 5
- Very faint, very red and very small
What is spectroscopy?
When you gather light and then spread it out with a prism/grating/grism
How do we resolve the lines from the spectroscopy
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
What is the relationship between the signal and noise of the photons
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
What is photometry?
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
What can we conclude if the spectral energy distribution (SED) of a galaxy is blue/red?
Blue - young stars
Red - old stars
What is the 4000 Angstrom break?
Occurs when there is metal and hydrogen absorption
- Strengthens with the age of the galaxy
Describe what would be observed in a SED if the galaxy is dusty, gas rich, or redshifted
Dusty - Red light is boosted relative to blue light
Gas rich - Emission lines
Redshift - The whole spectrum is shifted
Describe the characteristics of a young galaxy on a Flux-Wavelength spectrum graph
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
Describe the characteristics of an old galaxy on a Flux-Wavelength spectrum graph
Very little bottom left then steeply heads upwards at 4000 Angstroms and then tails off still sloping upwards
What is the redshift of the distant universe?
z > 5
What happens to optical features, like the 4000 angstom break in the distant universe?
They are shifted into the infrared
Describe what we observe on a spectrum when observing a star in the distant universe
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
What do we know exists between us and the source in a distant universe along our line of sight
Clouds of cool neutral hydrogen gas in the intergalactic medium
What is the Lyman alpha forest?
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
What is Lyman alpha emission
The electron transition in hydrogen
- Occurs from n=2 -> n=1
- At 1216 Angstroms
What happens to the light of the Lyman alpha emission lines at different redshifts
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
Describe why when observing a source in the distant universe, there is a considerable amount of lost light before the Lyman alpha emission lines
- 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
What is the Lyman break?
The combination of the 912 and 1215 Angstrom features
What is the Lyman technique?
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
Describe the properties of a “normal” low z galaxy using the Lyman technique
There are small colours in the adjacent photometric filters
- The ratio of the fluxes is near 1
- Bottom middle of graph
Describe the properties of a high z galaxy using the Lyman technique
Extreme colours in the filters either side fo the break
- Top left of the graph
What are dropout galaxies?
Lyman break galaxies
