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101 Giant Galaxy Clusters Discovered
Porfiry says: "Astronomers behind the Massive Cluster Survey (MACS) have uncovered 101 giant galaxy clusters, many of them so distant and thus forming so early in the history of time that they challenge our current understanding of how quickly the Universe evolved into its current hierarchical structure of stars, galaxies and clusters. Galaxy clusters are the largest gravitationally bound structures in the Universe, typically containing a few hundred to thousands of galaxies, each of which in turn contains many billions of stars."
Scientific American had an interesting article titled "Is Space Finite?" which discusses the the possibility of the universe being quite small. They assert that it is theoretically possible that we are looking at echos of ourselves when we look way back into the past with our telescopes.
[RANT] I become increasingly frustrated with the way science (especially astronomy) is portrayed on /. Although I'm happy that ppl take interest in this field, I feel that creating hypes or suggesting breakthroughs in every little article is just not the way to go. It may be the american way... I dunno. For NASA, pumped PR is essential for its survival. I'm also amazed that whereas /. readers are in general critical and sceptical, when the subjects changes to science they believe everything without actually trying to understand what is being said.[/RANT]
Finding clusters @ z = 0.3 is no big deal and wont challenge our current understanding of how quickly the Universe evolved into its current hierarchical structure of stars, galaxies and clusters. The current theoretical (numerical) view of the deep universe comes from the Virgo Consortium and predicts the existence of clusters on much higher redshifts. Wat is interesting is that it appears to be relatively easy to image large amounts of cluster. Clusters have been found out to a redshift of 1.2 (universe 40% of current age) and protoclusters at z = 2.2 (universe 25% of current age). CAVEAT: this MACS sample are selected on basis of their X-RAY properties; they were snatched from the ROSAT source list. Only heavy clusters with lots of infalling gas will produce much X-RAY emission, therefore biasing against smaller/less gass rich clusters. It is completely unclear if the study of high density regions (ie clusters) is representative of global picture galaxy and cluster evolution.
There is also a program underway called the Sloan Digital Sky Survey; a huge project where they (amongst other things) try to find clusters by optical selection in an automated way.
Finally, the article states "The analysis is not yet complete, but it is already clear that our observations are in conflict with a high value of omega."
Translation: this does not mean that our current picture is challenged. To the contrary: this study very crudely confirms other analysis (spatial structure in cosmic microwave background) and arguments for low omega_matter. Low Omega_matter is the currently favoured model. Trying to present this study as a breakthrough in this respect is false.
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Angular momentum creates a barrier in terms of the effective potential. Basically, unless the material can find a way to dissipate angular momentum, it can't fall to the center of the cluster.
This is very interesting. Of course the popular article is short on details. Of interest to us astrophysicists is how many, how massive, and how large z, and how large a volume did they survey. We already know of a few clusters nearly 10^5 M_sol at z=0.5-0.8. I'll have to ask for details once they get back from the HEAD meeting...
But, in any case, the claim of hundreds is very exciting. In a universe with a large \Omega (both matter and dark matter, but not lambda or quintesence), massive clusters become extremely rare at large redshifts. That's why the person claims this will help measure omega very accurately. Unfortunately, there is a degeneracey (I believe the primary degeneracey is how much the luminous matter distribution is representative of the dark matter distributions, but I'd have to check to be sure.) and this observation alone can not determine Omega. However, when combined with other observations (such as supernovae and CMB), Omega can indeed be tightly constrained. We're closing in...