Back in October I summarised the discoveries of large numbers of extremely faint galaxies in the Coma and Virgo clusters. Since then there's been a steady stream of papers on these ultra-diffuse galaxies, at the rate of about one a week. They now seem to be a common feature of many galaxy clusters (e.g. http://adsabs.harvard.edu/abs/2016arXiv160200002V). 

Although low surface brightness galaxies have been known for many years, these galaxies are much larger - as extended as the Milky Way but a thousand times fainter. The important question is how massive they are. They could potentially be either huge dwarfs (very extended but not very massive) or so-called "crouching giants") (as massive as giant galaxies but with almost all of their mass in dark matter rather than stars). If they were crouching giants, they could be a huge challenge* to cosmological models, which don't predict anything like these objects in such numbers.

* As in, "holy crap what the hell happened ?"

A few very recent developments indicate that these objects are likely to be huge dwarfs. The difficulty has been that to measure the total mass you need to know how fast the stars are moving (to calculate how much mass you need to hold them together). For objects this faint, this is extremely difficult, especially since the little blighters don't seem to have any gas (which is much easier to use to measure motions).

A paper in February of this year (http://adsabs.harvard.edu/abs/2016ApJ...819L..20B) used a neat trick to make things easier, measuring the speeds of the globular star clusters of one of these galaxies rather than individual stars. Still difficult, but easier. They found that although the galaxy is extremely dominated by dark matter (around 3,000x as much mass in dark matter than stars - for normal galaxies it's more like 10x), it's just a dwarf galaxy after all. 

They also found that they could get a pretty accurate estimate of the total mass from the globular cluster mass - confirming that a relation known to work for ordinary galaxies works even on these very faint objects. Which means we can completely avoid measuring the motions of the stars altogether and essentially just measure the brightness of the globular clusters. Huzzah !

A paper published today (http://adsabs.harvard.edu/abs/2016arXiv160407496P) uses the globular cluster trick to confirm that another similar object is also a huge dwarf, while a paper from last month (http://adsabs.harvard.edu/abs/2016MNRAS.459L..51A) shows that these objects are broadly compatible with mainstream theoretical models of galaxy formation. So although we don't have too much data to work with, it seems likely that most of these objects are faint, low mass, but very extended.

Not that this means everything is hunky-dory. Far from it. Although the vast majority of these things don't have any measurable gas content, a very few are extremely gas rich (http://adsabs.harvard.edu/abs/2005MNRAS.357..819S). Why some should have gas (yet apparently are not currently forming any stars) while others have managed to lose their gas completely is a mystery. And still, as a hugely under-cited paper (http://adsabs.harvard.edu/abs/2016MNRAS.456.1607D) makes clear, the number of these objects is far lower than cosmological models predict. They may not be quite as dramatic as blowing all our ideas out of the water, but they're still bloody interesting little buggers even so.
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