The Grand Observational Challenge for Galaxy Formation Simulations

After writing the commentary on the latest fin du MOND, it occurred to me that there are many issues that I consider to be obvious. But I’ve been thinking about them for a quarter century, so perhaps they may need to be clearly elucidated for those who don’t share that background. I am thinking, in particular, of galaxy formation modelers and theorists.

There are now many sophisticated galaxy formation simulations by many independent groups. They use different codes (sometimes with overlap) to implement the same physics with different algorithms. Well, sometimes it isn’t entirely clear that they’re talking about the same physics, or just using the same words to mean different things. But they all seek to form realistic galaxies in numerical simulations.

The observed radial acceleration relation (RAR) provides a strong test of simulated galaxy models. To claim that a suite of model galaxies is realistic, they must match the RAR. If they do, great. If they don’t, then they are not an adequate representation of observed reality.

What needs to happen now is for every group that performs these simulations to test their models against the data. We have provided the necessary observational data. All they need to do is make the same straightforward query of their simulation results. This is the Grand Observational Challenge for Galaxy Simulations.

Some requests:

  • Please be explicit: show your work.
    • Show us the RAR from your models. Don’t hide anything.
    • Show us the parameter space covered by your models (Mh, M*, R, Σ, etc.)
    • Show us the mass models of individual simulated galaxies.
    • Don’t just assert everything works out and expect me to believe it.
  • Start with what you’ve got.
    • I want to see what the current generation of models shows before you go and run more and more simulations until you find some that match the data.
    • When seeking models that do match, quantify the failure rate. How much parameter space do you have to hunt through before it works? How plausible were those not-quite-right parameters, independent of knowledge of the RAR?
  • Don’t claim more than you actually demonstrate.
    • If you have simulations that span only 0.03% of the observed mass range, then only claim to explain (at most) 0.03% of the problem.
  • Pay careful attention to the scatter.
    • How much intrinsic scatter should we expect?
    • What are the sources of scatter? Are they irreducible?
    • Are there residual correlations?
      • That is, at fixed mass (say) do galaxies fall systematically on one or the other side of the RAR depending on scale length or some other parameter?
  • Don’t fudge it.
    • I can tell.
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