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Comment by awb | Hacker News Reader

https://tritonstation.files.wordpress.com/2022/01/btfr_off.p...

> Baryonic Tully-Fisher relation showing the ultradiffuse galaxies discussed by Mancera Piña et al. (2019) as gray circles. These are all outliers from the relation; AGC 114905 is highlighted in orange. Placing much meaning in the outliers is a classic case of missing the forest for the trees. The outliers are trees. The Tully-Fisher relation is the forest.

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Calling them outliers isn't much of an explanation for something that shouldn't be able to exist.

The tully-fisher relation is an empirical relationship IIRC used to calculate distances to galaxies. You don't know that the tully-fisher relation applies in exactly the same way to a class of galaxies that are characteristically different from those galaxies which provide the dataset for calibrating the distance (IIRC, this is done by using cepheid variable stars). By necessity, UDGs are an extrapolation of the tully-fisher relationship. Suppose the tully-fisher is off by two digit percent due to UDGs being strange -- then it's possible that our calculations about the galaxies "dark matter content" could be completely wrong.

Afaik dark matter content is calculated by the rotation speed of the galaxy.

I'm not clear why the distance errors wouldn't average themselves out over the rotation

Only considering rotation speed for MOND seems like a simplist, poor approximation. One needs to take into account how wide/large is the galaxy. Its 3D shape. And a map of its local main density/sparsity points with estimated masses.

A failure to estimate accurately any of those metrics seems like the issue with this galaxy, not MOND itself.

Great pull quote.

For people new to the Tully-Fisher relation, the essence of it is that at galactic scale, gravity is not like v^2/r = GM/r^2.

It's more like v^2/r ~ sqrt(GM/r^2), so v^4 ~ M

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