The problem is that it's highly questionable whether we really have that much better an understanding of physics. There are a lot of free parameters in these simulations because we simply don't have a handle on the values needed, and a lot of physics that isn't handled self-consistently. For example, simulations don't have anywhere near the resolution to predict when individual stars go supernova, so instead they might (for example) assume some supernovae rate and inject mass and energy into the gas accordingly. This can be done in quite sophisticated ways, but it's still a limitation. Given all this, it's not clear if the more recent simulations have really solved the missing satellite problem or just come up with one possible solution that happens to work but has no bearing on reality.
On the observational front things are a bit simpler. We can't un-find galaxies, we can only ever detect more of them. Often once a galaxy has been discovered we'll need more observations to say anything useful about it, but the nice thing about the Local Group is that we can get uniquely detailed information about our nearest neighbours.
This paper describes a discovery from an ongoing survey on the highly advanced 8m Subaru telescope. Such facilities allow extremely high sensitivity over relatively wide areas of the sky - still only a small fraction of the total, but nonetheless useful. The authors have already found two potential ultra faint galaxies with the survey and here they add a third, plus something that's probably a globular cluster.
The kind of objects surveys like this find are so faint that you can't just take a photograph and go,"Hurrah, a galaxy - let's call it Bob !". In fact you can't see them in the photographic images at all. Instead, they have to run statistical searches that look for stellar overdensities, removing any extended objects (i.e. background galaxies) and accounting for the colours of stars and their position on the colour-magnitude diagram. In this way, they can extract coherent structures consistent with stellar evolution models. The process won't find everything by any means, but it's a proven method for finding objects. Once you've got a target you can go and get spectra to find the velocities of the stars to see if they're moving coherently. That's already in progress for the objects here but they don't report any of those results yet.
While spectra would be better, what they report here is interesting by itself. Globular clusters and dwarf galaxies are quite well-separated when you plot their brightness as a function of radius. Based on this, one of their objects is very clearly a dwarf galaxy. The other they favour as a globular cluster, although looking at the trend I'd say it could be either (and they do say it could be a galaxy after all).
But their main result is the number of detections they have so far. They compare it with a standard model of galaxy formation that accounts for two effects : 1) it has some prescription to deal with its low resolution, which means it would otherwise underestimate how many satellite galaxies there should be; 2) it accounts for real galaxies having higher tidal forces than in the model, which would destroy more of the satellites than predicted. These two effects act against each other, though not necessarily equally.
That second effect is quite interesting. It's worth remembering that although dark matter is mass dominant overall, just how dominant it is can have strong local variation. In dwarf galaxies it can make the baryons negligible, but in more massive galaxies the stars and gas can be very substantial. So because they lack the massive discs of gas and stars of real galaxies, pure dark matter simulations underestimate the tidal forces acting to tear small satellites apart. The model they used here attempts (somehow) to account for this.
They then combine this with knowledge of how sensitive their own survey is to predict how many galaxies they might have missed. They find that they're actually detecting more galaxies than predicted, in complete contrast to just about everyone else ever.
This is certainly interesting, but it's not worth getting too excited just yet. They note that the Milky Way may have experienced a merger with another relatively large dwarf galaxy, which could have brought in its own satellites. But while it seems worth a remark, I think a far more likely explanation is simply their terribly small number statistics. Three galaxies just isn't enough to base a sound conclusion on, and they note the simulation they use doesn't account for galaxies as far away as the ones they've found. To their credit, they don't go nuts with this. They could have said ALL OF GALAXY PHYSICS WILL NEED TO BE RE-WRITTEN, and called a press conference declaring themselves to super-duper important. They didn't do that. Instead they just report the findings, note the uncertainties, and go home for a nice cup of tea. We'll have to wait and see how this one develops as the survey progresses, but in the meantime, kudos for them for not getting carried away.
Boötes IV: A New Milky Way Satellite Discovered in the Subaru Hyper Suprime-Cam Survey and Implications for the Missing Satellite Problem
We report on the discovery of a new Milky Way (MW) satellite in Boötes based on data from the on-going Hyper Suprime-Cam (HSC) Subaru Strategic Program (SSP). This satellite, named Boötes IV, is the third ultra-faint dwarf that we have discovered in the HSC-SSP.
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