Program Wrap Up---Discussion on Final Day, April 22 2011

On the final day of the program, participants met once again to discuss the "big questions" of cluster research, just as we had in the early heady days of the program. Everyone was asked to contribute a question or provocative statement written on a scrap of paper, and the bucket of questions was Monte-Carlo sampled by participants and then thrown out for discussion. The Michel de Nostradamus Prize for best contribution goes to:

In 10 years' time, when we all come back:
  • we will no longer talk about mass calibration
  • we will still talk about feedback
  • we wil no longer talk about 3 letter instabilities
  • the weather will still be great

Otherwise, in no particular order, the questions thrown out were:

As a cosmological simulator (hydrodynamic), which piece of physics should I add first, and can I finish that before the next important process is suggested?

What outcomes would observers like to see from theoretical work?

How much good or harm is the X-ray derived "Universal Pressure Profile" doing to cluster research? Dream for cluster research in 10 years: To see methods like non-equilibrium or non-thermal pressure on the cluster outskirts to be used as a tool for constraining growth of structure, and hence cosmology.

What signatures can we expect from AGN feedback toward low mass objects?

In regards to cosmological "galaxy formation" simulations, how do we define success? How do we define/diagnose convergence of such simulations?

What is the origin of giant radio halos?

How can we learn about the distant outskirts, close to the accretion shocks where most of the cluster mass is?

  • How is a cool core formed and fed with energy?
  • Can the global properties of ICM still be treated in a "simple", "scaling relation approach" mode, in particular for cosmological purposes?
  • Make stronger and stronger links between observers and simulations
  • Is cosmology with clusters possible at the 1-2% level given [illegible complications]?

Will eRosita take cluster cosmology to the next steps? What are some of the key challenges?

KITP talk I'd like to hear: A successful, clearly falsifiable solution to the cooling flow problem at group/coronae scales, with both simulation (w/out subgrid physics) and observational support. This seems likely to require an understanding of how AGN feedback works. How much physics do we ultimately need?

1) Will systematic errors in correlating halo mass to observables prevent us from using large statistical samples from X-ray, SZ, optical? To what extent will self-calibration help? What is needed to model systematics? Will sims help?
2) What is the sweet spot in mass where systematics are not bad and statistics is also good? How competitive is f_g method relative to the growth of structure?
3) What sets cool core/non-cool-core boundary? What is it so sharp? How does it depend on halo mass and redshift?
4) What is responsible for metal mixing? AGN or mergers, or pre-enrichment? How can it be tested?
5) How large is non-thermal pressure support globally? In bubbles, relics and halos? What's responsible for radio emission? Synchrotron, inverse compton or secondaries due to hadronic processes? Will GeV/TeV observations help?
6) Pressure/density clumping responsible for flattening of density/pressure profiles close to r200. Is it real?