6: GALAXY FORMATION AND EVOLUTION Flashcards
What are the two methods for finding very high z objects?
The photometric redshift method and the drop out method
What is the photometric redshift method?
By observing several bands (including UV and IR), it’s possible to find the best matching galaxy type and redshift to reproduce all the observed colours.
Most galaxies present between redshift z = 0.5 and z =2. Types of galaxies appear to change above z = 1.5.
Both the optical morphologies and the best-fitting spectrum indicate that at a very high z, there’s an increasing fraction of irregulars with a shortage of real spirals.
DROP-OUTS METHOD:
1. What was it first used for?
2. What happens after the Lyman break at 91.2nm and why?
3. What occurs at z ~ 3?
4. What happens at progressively higher z?
5. At increasing redshift, the Lyman break appears at longer and longer wavelengths. What else does the Lyman alpha forest induce?
6. What does combination of these two effects cause?
7. What do LBG’s numbers and cluster properties indicate?
8. Do narrow band images detect Ly alpha emitters at specific redshifts?
9. How can samples be affected that are selected in this way?
- First used to select Lyman break galaxies
- The spectrum of any source drops away shortly after the Lyman break at 91.2 nm in the rest frame because shorter wavelength photons are removed by photo ionising any H they encounter
- At z ~ 3, the break is redshifted past the U band (360nm), so the observed U-B colour (from the ratio of photons originally emitted at wavelengths either side of the Lyman break) becomes very red or the object is potentially undetected at U.
- At progressively higher z the break moves across longer wavelength bands and objects will ‘drop out’ of samples selected at B, V, R. The accumulated absorption from the Lyman alpha forest lines (due to intervening HI clouds) cuts out the flux shortward of Ly alpha in the source galaxy.
- An increasingly strong break in the rest frame spectrum at 121.6m.
- The combination of these 2 effects mean that R band drop outs occur at z > 4.8 and I band drop outs are used to search for galaxies at z > 5.6
- They’re the direct ancestors of present day giant Es or small galaxies which will alter merge to form a current epoch type galaxy. The galaxies seen at z ~ 4-5 in deep HST data are relatively small compared to present day galaxies.
- Yes, even when the continuum is too faint to see.
- They can be contaminated by sources emitting a different line at lower redshift, while not all galaxies may be strong line emitters.
Some local starburst galaxies have little Ly alpha emission because of dust absorption. What does this suggest?
We should look for high z objects via dust emission redshifted in the sub-mm region.
What does increasing redshift do and how?
It boosts the brightness of galaxies in sub-mm, since the redshift moves the observed band either to the peak of dust emission spectrum at around 100 micro metres, giving a negative k-correction
What are high z sub-mm sources candidates for?
Elliptical galaxies during their main SF phase
When Es have their main SF phase, they are incredibly luminous due to the amounts of star formation. What is the drawback of this?
They are extremely difficult to identify in the optical bands making detailed follow-up and distance determination challenging/
What happens to most of the luminosity from hot young stars?
Absorbed by dust and re-emitted in the sub-mm band
What are the three components of galaxy evolution?
- SF history
- Passive evolution - The fading and reddening of stellar populations
- Increase in mass via mergers
Explain density perturbations in the early universe.
They’re primarily in the dominant dark matter. They grow through gravitational attraction until they’re able to collapse despite the overall expansion of the universe. Baryons in the dark matter halos collapse and interact via shocks & dissipate energy so as to create even denser baryon dominated central regions - protogalaxies.
What happens once a sufficient density is reached in density perturbations?
SF begins
What are metal free population III stars and what is their role?
They’re more massive than normal stars. They have very short lifetimes (no metal-free stars are seen today) and quickly deposit heavy elements in the ISM for the next generation of Population II stars
As most systems are of low mass, what do they do and how is this determined?
They merge to form larger bulge-like systems containing early stars and on a short timescale. It’s reflected in their colours and element abundances since only SNII will have occurred during the SF phase. Additionally, gas in-flow will fuel (and increase) the mass of the BH which will become visible as quasars.
What important role do AGN have?
They feed energy back into the ISM and modulate SF.
How is a spiral galaxy made? What happens if there is further merging?
Bulges can accrete in-falling baryons which form a surrounding disk, because of their angular momentum, making a spiral-like galaxy. Further mergers can create larger Es.
Since we know giant Es contain old stars, what does this tell us about their progenitors?
They must have merged early or have been gas free (‘dry mergers’).
What will future spirals continue to form and what element abundances will they produce?
They form Population I stars from accreted gas over a long time scale, producing element abundances as seen in the sun.
How can we track SF history over the universe?
Via the number of UV (or emission line) emitting galaxies at different z (the Mandau plot)
Define the Butcher-Oemler effect.
The SF per unit volume was constant from high z down to z ~ 1, but declined steeply thereafter due to a decrease in SF in individual galaxies and an increase in the fraction of non-star forming red galaxies, especially in clusters.
Define downsizing.
More massive galaxies ceased SF first.
What might spirals and irregulars be transformed into when they fall into a cluster?
Early types (with or without a final burst of star formation)
How do clusters grow similarly to galaxies?
They grow hierarchically via accretion and mergers.
