Huge dwarfs or ghostly giants ?

Ultra-diffuse galaxies are enormous but have very few stars. That makes it particularly difficult to say whether their total mass is very high or very small with their stars being spread especially thin. What's especially annoying is that these things are found in large numbers, which has annoyed a lot of people who hoped they might be rare exceptions.

(For a longer introduction see https://astrorhysy.blogspot.com/2017/07/ultra-diffuse-galaxies-revenge-of-ghosts.html; couple of non-crucial missing images, will fix later)

Measuring the total mass directly is difficult, but it's relatively easy to compare them with "normal", brighter galaxies whose mass is more well-determined. In this paper, the authors compare the size vs. brightness relation of the UDGs. They find, unsurprisingly, that they're bigger and fainter than normal galaxies (duh !) but more interestingly they form a continuous relation with brighter galaxies : they aren't a distinctly different population. This contradicts previous studies which found that the size-luminosity relation didn't have much scatter. The authors argue that this isn't because anyone did anything wrong, but just because the previous studies wouldn't have been able to detect UDGs.

What this means for the mass of the UDGs is unclear. They also find that the structural properties of the UDGs and bright galaxies are different : the shape of the distribution of stars varies in a different way depending on their brightness. Even more confusingly, the UDGs appear to be different from both normal faint and bright galaxies. Which means the things are bloomin' complicated.

This paper is still under review and it's only a letter, but I think there are several parts here that could be explained a lot more clearly (especially the comparisons to normal galaxies). The two main ideas of UDGs have been either that they're basically low-mass galaxies that have been "inflated" by encounters with other galaxies, or that they formed exactly how they are and are as massive as other galaxies of comparable size (AFAIK, no-one has come up with a way for low-mass galaxies to form yet be so hugely extended from birth - the only way is for them to grow over time).

Unfortunately both scenarios could be compatible with the different relations. A dwarf galaxy that becomes much more extended might also be structurally affected in the process (detailed intro on galaxy structure : https://astrorhysy.blogspot.com/2017/11/the-dark-side-of-galaxy-evolution-ii.html). But a giant galaxy that's born with very few stars might also have a different light profile from a brighter object. It would have been nice if the UDGs had been clear outliers from the general trends rather than a continuous extension of existing relations, but the Universe isn't cooperating. On the other hand, these measurements give an extra constraint for anyone trying to simulate the formation of these ghostly irritants.

http://adsabs.harvard.edu/abs/2018arXiv181101962D