Aspen Summer Session
May 30, 2003
Afternoon Discussion: What's the Best Way to Find Clusters?

Methods:
Optical can reach >90% completeness w/ 10% contamination
 Donahue/Scharf 43 x-ray cand, 150 optical
  -large scatter betw. Lx and Lopt
  -Gilbank finds RCS technique,MF find all x-ray clusters; x-ray only find fraction of optical sources. Factor of 10 range in Lx at given luminosity
 -how to get around fact that galaxies form before cluster? How to get Rich-mass reln. at hi-z
 -if we can't measure DM halos, then throw it all out and just do correlation functions?
   --large payoff if you can observationally predict virial mass
  -can we use projected galaxy distribution?
  NO! (Risa) galaxies do not trace DM distribution
  -Weak lensing can get shape info if you fit a-priori profile (ie NFW). May  need to stack or average clusters.
 -what fraction of clusters are not NFW? depends on z

X-ray
10%  contamination, 100% complete above flux threshold. But not above mass threshold...
flux goes as dens^2, so high contrast, dark background
traces the virial region of DM halo which is nice for simulations
Optical surveys go lower in mass than will produce reasonable Xray luminosity


Weak lensing
extremely inefficient, but may be good for high mass cut
only way to find dark clusters

SZ
low resolution; galaxy radio emission can fill in SZ decrement
need multile frequencies, understanding of point source contributions

Bent Tail Radio Sources


Best:
Observer wants completeness in observable + purity
Theorist wants completeness in mass + purity
NASA admin wants to maximize clusters/dollar


Large scatter in Mass-observable reln will lead to large impurity
Is factor of 10 scatter in Lx-R compatible with observer Lx-M and R-M relations
NEED TO DERIVE Lx-M relation for large OPTICALLY SELECTED SAMPLE!!!!!



GILBANK THESIS
EBELING X-ray fluxes of Abell clusters
HENRY  500 sq. deg.

Increasing scatter at lower mass.
Kband light well correlated with mass