I Hubble deep field & ancient galaxies

[seabass101]

TL;DR Summary

Observed differences between ancient and modern galaxies

Hubble deep field allowed us to study galaxy evolution from 500 million years onward. Based on my (limited) understanding, I would expect ancient galaxies to contain fewer heavy elements and to have a more "juvenile" appearance, as compared to modern galaxies. Have we actually observed these things? What other differences have we noticed between ancient and modern galaxies?

 

[lomidrevo]

Regarding metallicity (abundance of heavier elements than hydrogen and helium), yes generally speaking the early galaxies should be metal-poor. But obviously that is not a complete truth, the observation are more complex. See here:

High redshift QSO absorption line systems show a wide range of metallicities, from one thousandth solar up to 1/3 solar. Thus, while the average metallicity of the Universe certainly must have increased since the early epochs, the situation is more complex than a simple picture where high redshift means metal-poor, and low redshift metal-rich. Objects with high and low metallicities are found at all redshifts. Surely we expect objects that in the local Universe appear as metal deficient to be even more deficient at high redshift, if we could observe their precursors. Also the ancestors of the local metal rich galaxy population, i.e. the giant spirals and ellipticals, should have started out with very low abundances unless they were gradually built up by merging smaller galaxies. Currently, both the theoretical and observational pictures, tell that the latter is an important mechanism. Dwarf galaxies, the survivors who form the local metal-poor galaxy population, may thus be the principal building blocks of the Universe on large scales.

Regarding the morphology of the more distant (older) galaxies, the elliptical galaxies tend to be less abundant comparing to spiral galaxies.

 

[lomidrevo]

Another noticeable difference is that older galaxies are bluer (after compensating the redshift, of course) than today's nearby galaxies. See the Butcher–Oemler Effect:
https://astronomy.swin.edu.au/cosmos/b/Butcher-Oemler+Effect

That indicates an increased level of star formation, which makes sense as there was more gas available to allow it.

 

[seabass101]

Thanks for the detailed answer!

One thing I'm still confused about is the metallicity result. The excerpt you shared says "the average metallicity of the Universe certainly must have increased since the early epochs." Thus, shouldn't we see a statistically significant difference in metallicity when we compute the mean for old and new galaxies? Is the problem that it's hard to tell the difference based on the data available?

 

[Tom.G]

Turns out it depends on the shape of the galaxy!

From: https://academic.oup.com/mnras/article/350/1/351/987015

"We find the main metal production in spheroids (ellipticals and bulges) to occur at very early times, implying an early peak in the metal production and a subsequent decrease. On the other hand, the metal production in spirals and irregulars is always increasing with time. We find that the average [O/Fe]*,E ratio in stars in spheroids should be +0.4 dex, whereas the average [O/Fe]gas,E ratio in the gas should be −0.33 dex, as a result of the large amount of Fe produced in these systems by Type Ia supernovae after star formation has stopped."

(above found with: https://www.google.com/search?&q=average+metallicity+of+the+universe+versus+time+graph
Tip: start typing that search string into Google for similar searches)

Cheers,
Tom

 

[lomidrevo]

And the shape (morphology) of galaxies is related to the environment in which they are located, namely the density of galaxies found in the neighborhood. According to morphology-density relation, spiral galaxies are common when isolated, but are rare in highly dense regions.

The morphology density relation is believed to indicate that galaxy evolution is affected by the environment in which the galaxy finds itself. Specifically, there is strong evidence to suggest that star formation is suppressed when galaxies enter high density environments such a clusters. This suppression of star formation is not well understood, and a number of processes (including ram pressure stripping, galaxy strangulation and galaxy harassment) have been proposed to account for it.

So maybe the ancestors of todays galaxies located in dense clusters might have allowed quite extensive metal production in their early times, just before the interactions between galaxies made the rate of star production slower?

 

[lomidrevo]

Thus, shouldn't we see a statistically significant difference in metallicity when we compute the mean for old and new galaxies? Is the problem that it's hard to tell the difference based on the data available?

I cannot tell, I haven't seen such data. Maybe someone else knows better.