These dynamic dimensions are too dynamic

Do Ultra Diffuse Galaxies rotate too slowly or is it all just a measurement error ? Last time I said, "I think this paper reasonably settles any concerns about the inclination angle measurements, though I won't say it's unquestionable."

This paper swings me back the other way. Their sample looks much more likely to be consistent with miniscule measurement errors that shift the velocity widths into an apparent realm of weirdy weirdness.

For those who haven't a clue what's going on, there were claims that certain galaxies are rotating much, much more slowly than expected given their mass. There are a variety of galaxies that do this. Most are so-called Ultra Diffuse Galaxies, meaning that their stars are unusually spread out, but there are a few brighter objects too. This would be extremely strange because the rotation-mass relation is normally quite tight. These oddballs are way off, in some cases as though they had no dark matter at all. And many of them are too isolated to explain by interactions with any other galaxies.

This latest paper is pilot for a big follow-up study of a sample of UDGs, getting gas measurements with the Green Bank Telescope to estimate their rotation. The GBT doesn't have the spatial resolution to get proper rotation curves - for galaxies this far away, it can only do line widths. So they use the optical images to estimate the inclination angle of the discs, and use that to correct the line widths into true rotation speed. (Gas measurements tell us about motion along the line of sight, so if we're viewing a galaxy face-one, it's line width is zero. If it's edge-on, we measure its rotation directly and no correction is needed.)

They observed 70 galaxies and detected 18. Half of these were confirmed to be UDGs while the other half were found to be foreground dwarfs, i.e. not as extended as previously thought. Interestingly, there's basically no morphological difference at all between a distant UDG and a much closer dwarf. The old "these cows are far away" problem is a tricky one indeed when it comes to galaxies.

Most of the rest of the results are not at all surprising, e.g. the gas rich UDGs tend to be bluer and more irregular than ones without detected gas. They have some low levels of star formation detectable via UV emission, but there's nothing much unexpected about that.

The dynamics are the interesting bit. Seven of their nine UDGs lie off the normal baryonic Tully-Fisher relation while the other two are well within the scatter of typical galaxies. For the outliers, they calculate how badly they'd have to have got the inclination angle wrong in order to bring them back to the standard TFR, and it's pretty substantial, ranging from 20 to 42 degrees. Large errors certainly aren't impossible, given that these are all very faint objects and the gas disc might be oriented differently to the observer than the stellar disc. But it sounds at first glance just too big to explain all of them.

Helpfully they also show what these inclination angles would look like in comparison to the ellipses they fitted to the actual data. What's really surprising is that these "corrected" ellipses are often very close indeed to the measured fit, sometimes almost perfectly overlapping despite an error of 20 degrees or more. I even had to manually plot the circles myself to convince myself that this is correct. It is. Should I ever give another course, I'll be sure to add a plot of what inclination angles look like in practise. Here's a simple version that uses thin discs - the thick disc formula is more complicated and I can't be bothered to show that right now.

Circles inclined from 0 (outer) to 80 degrees in steps of 10 degrees.

When you're dealing with faint fuzzy blobs, I can easily believe that errors of tens of degrees are possible. The authors stop short of saying that the previous results suffered from this problem, as they did have gas maps as well as the optical, but it's hard to see the obvious implication. So maybe UDGs don't have anything much unusual about their dynamics at all, which re-opens the whole controversy about galaxies devoid of dark matter. We'll see.

Systematically Measuring Ultra Diffuse Galaxies in HI: Results from the Pilot Survey

We present neutral hydrogen (HI) observations using the Robert C. Byrd Green Bank Telescope (GBT) of 70 optically-detected UDG candidates in the Coma region from the Systematically Measuring Ultra-Diffuse Galaxies survey (SMUDGes). We detect HI in 18 targets, confirming 9 to be gas-rich UDGs and the remainder to be foreground dwarfs.