Science1 publisher2 min readPublished
Six MEGATRON papers predict how the young Milky Way should appear to Webb and Hubble
Harley Katz's team at the University of Chicago ran three years of simulations showing the Milky Way assembling from thousands of smaller galaxies. Published across six papers, the models can now be checked against what Webb actually records.
The Scientist · Science desk
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What happened
- The model contains "galaxies" with no stars that still shine, some possibly emptied by stars that exploded or collapsed into black holes, others only ever made of gas.
- Smaller, fainter galaxies usually hold less iron, but in extremely faint systems the iron level stays constant regardless of mass.
- MEGATRON attributes that constant iron level to explosions of Population III stars, made only of hydrogen and helium, which yield more iron than other supernovae.
- Another of the papers is the first to show how Population III stars form in a Milky Way-like setting and predicts where any survivors would most likely be found.
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Why it matters
- capability Webb and Hubble observations of early galaxies can now be scored against a specific forecast of the Milky Way's building blocks, exposing which parts of the model's physics hold up.
- decision Astronomers searching for surviving Population III stars now have a predicted short list of places to look first in a galaxy like ours.
- cost A model this slow to run cannot be redone quickly with different physics, so chasing down any mismatch with Webb data would be a long and expensive process.
The design is a forward model. Katz, an assistant professor of astronomy and astrophysics at the University of Chicago [1], said the team put in "all of the physics we think is relevant", naming gravity, hydrodynamics, radiation and chemistry, "and then let it evolve and see if it reproduces what we actually see when we look around us today" [6]. The galaxy as it is now is the check on the physics. The early snapshots are the product, and Katz described them this way: "For the first time, we can directly predict what the early Milky Way would have looked like to telescopes like Hubble or the James Webb Space Telescope." [3]
Scale is what makes that kind of forecast possible. "We follow thousands of subsystems in the model and directly compute what they all would have looked like with our most powerful space telescopes, which is many orders of magnitude more than what had been simulated before," Katz said [7]. The comparison with earlier work is his. Phys.org calls the result "the most detailed model to date of how a galaxy like the Milky Way might have evolved over" its first several billion years [2]. Among those subsystems, Katz said, "Some of them are bursting out in star formation, others are dead, others are in the process of dying." [8] The six papers appear in The Open Journal of Astrophysics [4].
I would put the most weight on the iron result. It is aimed at a known observation, and until now simulations had been unable to reproduce it [10]. The model's account depends on gravity. A galaxy large enough keeps the iron from Population III explosions, and one too small loses it to space [11].
The thing this doesn't tell you is whether Population III stars actually did it. No telescope has directly seen one [12]. A model that reproduces the flat iron line shows the explanation is sufficient, though another process could in principle produce the same line. A surviving star found where the models say to look would test the idea directly [12].
The headline claim works the same way. The project began as an effort to fold what Webb has revealed into a model of the young galaxy. The team can now set its output beside Webb readings to see what matches and what is missing [13]. The phys.org account does not report the results of any such comparison yet.
What to watch
- A published comparison of MEGATRON's predicted images with Webb observations of early galaxies, showing which predictions match and which miss.
- Searches for surviving Population III stars in the places MEGATRON predicts; a detection would test the iron explanation directly.
- Further Milky Way-like runs from the team, which would show whether this assembly history is typical or a single case.