liveblogging the high redshift universe

The first two billion years of galaxy formation: the reionization epoch and beyond.

The Aspen Center for Physics is a physics hostel of sorts, and a very nice one it is too.

The Aspen Center, which is located adjacent to the Aspen Institute on the edge of town, runs a series of (currently five) weekly workshop in january and february of each year, bringing in typically 50-100 physicists for intense workshops on a rotating series of "hot topics" in research.
In the summer there are four months of overlapping series of longer 3 week workshops where an effort is made to mingle the participants in different subfields.
The schedule is very intense, and in my experience, having attended a number of both winter and summer workshops, they are extremely productive.
In the past my experience has been that a summer workshop will likely lead to a long term shift in research emphasis on some topics, while the winter workshops lead to new papers and short term collaborations. And they are fun.

So, what of the science...

Via Lactea simulation from Diemand et al at UCSC showing the predicted dark matter substructure of a Milky Way sized galaxy

There is a long standing problem in structure formation, in that the standard cold dark matter model predicts far too many dwarf galaxies.
A galaxy roughly the size of the Milky Way ought to have thousands of dwarf galaxy companions, possibly tens of thousands, if stars form in low enough mass dark matter halos.
Simulations suggest that the dark matter halos are remarkable persistent, with a large fraction surviving as coherent structures despite the tidal forces of the "parent halo" in which they orbit, although some fraction is shredded completely through cosmic time.

But, the predicted dwarf galaxies are not there. Rather there are a few dozen at most (we don't know quite how many for sure, they are faint, diffuse and some are undoubtedly hidden behind the dust of the plane of the Milky Way).

So... how come?

Well, there are basically two explanations:
one is that the small dark matter halos never formed or were destroyed;
the second is that they formed, but for some reason never formed as many stars as would be expected.

Both possibilities are interesting.
If the halos never formed, then the small scale perturbations after the Big Bang were somehow suppressed, either because the perturbation spectrum is not as close to scale free as thought, and there is some new scale suppressing fluctuations - this would likely imply new physics; or because some process erased the small scale fluctuations as they formed - like warm dark matter.
If the halos formed (ie collapsed to non-linear regime and achieved high density) but were then erased as dark matter structures, this implies there is some cold dark matter physical process efficient at heating on those scales, which would be very interesting and might help us understand better the nature of dark matter.

Alternatively, the subhalos are there as predicted - the Milky Way halo is very lumpy - but there are not stars in them (there is the issue of the globular clusters - there are couple of hundred of them, and there might have been thousands more in the past - but for various reasons people don't fancy them as cold dark matter lump tracers - but, several of the known dwarf galaxies have nuclear clusters that look like pretty respectable globulars, and several globulars are suspected to be dwarf galaxy nuclei - and there has been some speculation that the globulars, which are compact, could be embedded in more extended bound dark matter halos - or not).
If the stars are not there, they most likely never formed (it is hard to get rid of stars once they form), this could be either due to external influences - like a radiation field ionizing the gas so it can't cool to form stars; or due to internal influences, like when these lumps start to make stars some internal feedback like supernova or ionizing radiation, very efficiently precludes further star formation.

So, on one hand new physics, on the other hand classical messy physics.

Each scenario leads to somewhat different detailed predictions - but, none currently are satisfactory, they don't fit in detail.
The scenarios are of course not mutually exclusive, there could be new physics and messy physics conspiring to suppress the dwarf galaxy formation.
This is, however, one of these problems that looks to be soluble in a finite time with some serious hard work and more constraining data. Then in a few years we might wonder what all the fuzz was about.