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JWST’s first image stunned everyone who appreciates astronomical photography. Professional astronomers noticed something more, however, and have already put the preprint of a paper on ArXiv.org – a remarkably fast turnaround in a field where analysis usually takes years.
The only elements in the early universe were hydrogen, helium, and a little lithium. All the others, which astronomers collectively refer to as metals, were formed in stars – particularly in the death throes of giants.
Consequently, when we look back at very old galaxies we see low metal concentrations – there just hadn’t been the time for many stars to form, live, and spread their metals across nearby regions. JWST offered Dr Mirko Curti of the University of Cambridge and co-authors an opportunity to examine galaxies even older than those we have previously been able to see clearly.
The authors picked three high redshift galaxies from the first photo – this indicates they are a very long way away and we are thus seeing them close to the dawn of time. The trios’ spectra were examined to determine the average composition of oxygen in each galaxy, as a proxy for other elements heavier than lithium.
In two cases results matched expectations – each galaxy was low in metals, but only about as low as we would expect, given what we have seen of somewhat older galaxies.
ID4590, the least massive of the three, proved to be a surprise, with a metal content even lower than expected. In other words, it consists almost entirely of hydrogen and helium, creating a puzzle as to why other elements are yet to appear.
The fact that two of the three galaxies fitted existing models suggests we have not fundamentally misunderstood the early universe, but ID4590 indicates our models probably need some tweaking. Only a larger sample size will tell us how big the required modifications are.
More massive galaxies in our part of the universe have higher metal contents than smaller ones, but this is taken into account in the model Curti and co-authors used, so the low mass of the anomaly is unlikely to be an explanation.
The models of metal evolution are based on galaxies with redshifts up to about 3.3. They indicate metal formation (and by implication star-birth) happens fairly smoothly, rather than in sharp bursts, in evolved galaxies. ID4590 has a redshift of 8.5, placing it more than 30 billion light years away and indicating just how much further back in time JWST can see.
Gravitational lensing provided by the closer SMACS J0723.3–7327 cluster contributed to the capacity to get enough light from the trio to analyze the spectra.
The work is still to undergo peer review for acceptance in a journal, but it’s a taste of the real science to come from JWST, along with all the pretty pictures.
[H/T New Scientist]
