The Radial Acceleration Relation is just not a thing so let's all shut up about it

A long time ago the Radial Acceleration Relation was quite the thing for galactic dynamic studies. There's a tight correlation between the acceleration predicted by the observed material and the actual observed acceleration, which is unexpected because of the enormous amount of invisible dark matter. MOND supporters danced with joy at a successful prediction, only for standard model advocates to shoot them down days later, pointing out that the exact same thing was observed in their own simulations.

It took quite some time, though, before anyone gave a credible explanation as to why this effect was actually seen in dark matter-dominated models, where naively one wouldn't expect a connection between normal and dark matter. Plenty of people waved their hands with loud cries of, "SCALING RELATIONS !", but no-one really understood what they were on about.

Finally some clever chaps managed to explain these scaling relations in a reasonably convincing way. In brief, galaxies can't form in the largest (because of feedback and other effects) or smallest (because they don't have enough gravity to accrete much gas) dark matter halos. And galactic dark matter halos have a characteristic density profile in the standard model, giving rise to the observed relation.

But the others of the current paper aren't satisfied. They want to know just what it is that's happening physically to cause this. What exactly is it about the observed baryonic matter that could lead to this behaviour ? Why does it match the MOND predictions ?  "This connection", they say, "must be in some sense “universal.” " I won't say I fully understand it, but their proposed solution feels quite elegant to me.

Normally astronomers think of star formation as being related to the gas density. More gas => more self-gravity => more stars. We normally assume that the density of the dark matter is low so that we can neglect it. Here the authors implicitly say, "yes, that's usually true, but it's not technically accurate, now is it ?". After all, what drives the collapse of the gas is the total gravitational field, not only the mass of gas. We can't possibly expect to discover a dark matter-baryon connection without including the dark matter.

Their model goes like this. Star formation proceeds efficiently if the total acceleration is above some threshold, whereas if it's below this threshold then there won't be much star formation at all. In regions initially dominated by dark matter, and below the threshold, few baryons will ever be accreted : the feedback from the few stars which do form will easily suppress further star formation by blowing the gas away. So these regions remain dominated by dark matter, i.e. dwarf galaxies and galaxy outskirts. In the opposite case of regions initially dominated by baryons and above the threshold, star formation will proceed much more efficiently. Feedback will be less effective at disrupting the gas inflow due to the stronger gravitational field, so these regions will always be dominated by baryons. Thus the acceleration threshold marks the boundary between regions dominated by baryons or dark matter.

This means there's a physical reason to expect the observed change in the RAR at low masses. They go further, saying that this also predicts the characteristic scaling behaviour at the different extremes, and even why galaxies generally have flat rotation curves : it's not a "conspiracy", as MONDian advocates worry, but an entirely natural effect of stellar feedback in regions dominated by dark matter - a flat rotation curve (if I understand them correctly) is the natural form of a dark matter halo, and it's only the presence of baryons that disrupts this. They also say that this all in no way implies that they expect the relation to be perfect or perfectly universal : variations in the stellar mass function will alter the feedback, so there could certainly be deviations from galaxy to galaxy; they can't make detailed predictions with this hugely simplified model - the point is to predict basic scalings, not develop a new and exact model of galaxy formation.

Again, I need to think a lot more about this. At first glance it seems like a neat way to universally connect the microphysics of star formation with the more global parameter of acceleration. I wonder how far this can be pushed to explain star formation despite its limitations : what happens in the other cases, if dark matter dominates but the acceleration is above the threshold, or if baryons dominate but the acceleration is low ? What, if anything, does this suggest for ultra diffuse and dark galaxies ? Definitely interesting, and this is a case where I wholly support the authors putting the paper on arXiv and asking for feedback* before publications.

* "Feedback is welcome", they say, leaving it to the reader to decide if that's a pun or not.

Stellar feedback sets the universal acceleration scale in galaxies

It has been established for decades that rotation curves deviate from the Newtonian gravity expectation given baryons alone below a characteristic acceleration scale $g_\dagger \sim 10^{-8}\,\rm{cm\,s^{-2}}$, a scale promoted to a new fundamental constant in MOND-type theories.