Yo Dawg, Herd You Like Missing Matter, So We Stole Some Of Your Missing Matter So You Can Miss Matter From Your Missing Matter
Some time ago you may remember I was going on about ultra diffuse galaxies (UDGs). These ghostly systems are comparable in size to the Milky Way but 100-1000x fainter. And you might remember I posted something like the plot shown below :
This is the baryonic Tully-Fisher relation (TFR). It plots the total mass of stars and gas as a function of how fast a galaxy is rotating (which is usually a good proxy for total mass of dark matter).
In the plot you can see that normal galaxies, in blue, lie on a nice neat straight line. It's possible to derive this line analytically. The problem is that the analysis predicts a population of galaxies which don't sit on the same line, which haven't hitherto been found.
The red points here show UDGs where it was possible to measure their rotation speed. Clearly they don't lie on the normal TFR. So good news, right ? Not necessarily. Those velocity measurements are uncertain because it's hard to estimate the viewing angle we're looking at, which can strongly affect the estimated rotation velocity. We can only get a direct measurement of rotation if we're lucky enough to see the galaxy edge-on; if we see them face-on, we can't measure the rotation at all. The fainter the galaxy, the harder it is to estimate the viewing (inclination) angle and the less accurate the correction will be. Which is bad news for things as faint as UDGs. Nevertheless, at least some UDGs do appear to deviate from the usual TFR.
The black points are the optically dark hydrogen clouds I've been investigating in the Virgo cluster. Their velocity widths are more secure, though their possible origins are more complicated.
Much to my delight, today's paper by Oman et al. is all about objects with those strange deviations from the TFR in both directions. They phrase things a bit differently, largely talking about galaxy formation efficiency. In essence, galaxies with higher baryonic (stars and gas) masses than expected have apparently high formation efficiency, in the sense that a small dark matter halo has accumulated more gas and stars than usual. Galaxies with lower baryonic masses than expected have correspondingly lower formation efficiencies. Or if you prefer, you can talk about galaxies with higher or lower rotation velocities than expected, it doesn't really matter.
Slightly annoyingly, Oman et al. don't cite either the Lesiman et al. UDG paper (red points) or my own dark clouds (black points)*. On the positive side, they discuss other systems previously unknown to me that also deviate from the TFR, in both directions. And these galaxies are not especially weird in other ways : they have much more normal levels of surface brightness. So it isn't just weirdly extreme objects that deviate from the TFR - more normal galaxies can do it too. And that's very reassuring.
* More oddly, they don't even mention that famous galaxy without dark matter so I guess it's nothing personal. :P
What could explain the deviations ? If I understand them correctly, there's not much problem explaining objects of low formation efficiency (fast rotators). Those would just be objects where the gas and stars are very extended. But cases of high formation efficiency (slow rotators), they say, are not compatible with the standard model. In fact, although the model does predict stronger scatter in the TFR in this regime, it would actually have the opposite effect to what the observations indicate.
The standard model could be wrong, of course, but let's leave that one on the "maybe" pile for now. Other options they suggest are that the gas and stars may not probe the full dark matter halo so their measurements underestimate the maximum rotation speed (this is also possible for some cluster galaxies which have experienced extreme amounts of gas loss, leaving behind only a remnant core of gas in their central regions - http://adsabs.harvard.edu/abs/2013MNRAS.428..459T). But that doesn't seem to work here because they have full rotation curves, and they're flat. So even if the gas and stars were more extended, the measured rotation would be the same. Another option could be that there's less dark matter than expected in the central regions of the galaxies, but simulations show that effect is far too weak.
Could it simply be a measurement error ? The distance estimates seem secure. Could they galaxies have been stripped, like those in Virgo ? No, they're too isolated.
What about the viewing angle ? It's hard to be sure, but this is definitely their favoured option. The measured rotational velocities of the deviant galaxies are very small, ~20 km/s (the Milky Way is more like 250 km/s), and a change to 30 km/s would be enough, in at least one case, to bring them back into agreement with normal galaxies. It only needs a very small error to explain this. The same problem could affect low-efficiency, fast-rotating galaxies too. If their viewing angle is estimated to be too low, then this will exaggerate the calculated rotation speed.
As far as I know this is entirely plausible for the systems they discuss here. But what about the ones in the plot ? I'm more skeptical. I went through the UDGs manually, and dang it, at least some of them really look like we're viewing them close to edge-on, so their velocities should be accurate. And for the dark clouds the velocity width is a lower limit, so they can only be wider than plotted here, not narrower.
What's the answer ? Dunno. Sorry.
https://arxiv.org/abs/1601.01026
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