Wide Field Imagers in Space and the Cluster Forbidden Zone Megan Donahue Space Telescope Science Institute Acknowledgements to: Greg Aldering (LBL) and Marc Postman (STScI)

Why Study High-Redshift Massive Clusters?  Clusters are the largest sites where we can “see” nearly all of the baryons that are there.  Clusters are thought to be “fair samples” of the universe.  Cluster evolution: predictable, hierarchical, and gravitationally-driven  Cluster evolution: sensitive to the overall density of the universe and the spectrum of initial density fluctuations (~8 Mpc)

Survey Questions  Are our fundamental assumptions about cluster formation and galaxy evolution valid?  What do the first clusters in the universe look like?  Is cluster formation related to the formation of quasars and radio galaxies?  When did galaxies and stars pollute the intracluster medium?  When did clusters acquire dense, hot atmospheres?

The Cluster Forbidden Zone z=1.5 and beyond  Old galaxies are difficult to detect in the optical at z>1.0  X-ray surface brightness fades  Ground-based infrared observations have high sky background.  Weak-lensing techniques require numerous background sources  SZ follow-up requires spectroscopy or photometry of spectral features prominent in the infrared (H-K break).

Go Wide  The most massive clusters have a space density of ~1 per cubic Gpc between z=0-1.  Cluster evolution makes rare clusters rarer at high redshift.

Evrard, et al. 2001, astro-ph/ Triangles -  CDM Circles -  CDM 5x10 13 h -1 M solar 3x

Go Red: Space-based infrared  Lower sky background (no OH emission)  Lower absorption (no H 2 O bands)  Wide-field, diffraction-limited image quality

Go Deep  Cluster evolution has been relatively modest since z~1 (constraining  m ).  Cluster formation models predict that cluster assembly was likely more rapid at earlier times.  Metal injection into cluster gas must have occurred at z>0.8.

Cluster Discovery in the Forbidden Zone  Wide-field: at least 1000 square degrees (see figure from Evrard)  Near-IR: H AB ~ 24 mag arcsec -2 enables detection of clusters out to z=2-2.5, 50% yield (matched filter experience).  X-ray: sensitivity for a 6 keV cluster at z=2.  t exp = 600 ksec for Chandra  60 ksec for XMM

Plan  Cluster discovery in the near-IR  PRIME (near-IR Discovery mission, Zheng JHU): PI science  Possible SNAP GO program (perhaps to follow up S-Z cluster candidates)  Cluster properties: velocity dispersions, temps  Multi-spec observations (R=100) with NGST (not possible with Keck)  Constellation-X (at 100x XMM collecting area, z=2 cluster temps could be obtained in about 25,000 seconds); iron abundances in ~100,000 seconds

Preparatory Theory Needed  Projection effects through full N-body simulations for weak-lensing surveys.  Intracluster medium evolution with feedback and entropy considerations for realistic X-ray and S-Z predictions.  Galaxy evolution in crowded environments: will all clusters at all redshifts have an old galaxy population?

Conclusions  Finding and studying high-redshift clusters are critical to understanding structure formation and the history of star and galaxy formation.  High-redshift clusters are rare: wide-area space-based surveys in the near IR are the best way to find them.  Coordinated multi-wavelength observations, SZ, near-IR, and X-ray, are required to reveal the properties of the clusters: mass, metallicity, galaxy content.