Spitzer Space Telescope - Directors Discretionary Time Proposal #80236

Spitzer-IRAC imaging of a Planck-selected Herschel-resolved candidate 
sub-mm galaxy proto-cluster at z = 3.25


Principal Investigator: Asantha Cooray
           Institution: UC Irvine

     Technical Contact: Asantha Cooray, UC Irvine

Co-Investigators: 
Steve  Eales, Cardiff
Loretta  Dunne, Nottingham
Hai Fu, UC Irvine
Sam  Kim, UC Irvine
Julie Wardlow, UC Irvine
Rob Ivison, Edinburgh
Dave Clements, Imperial
Guilia Rodighiero, Trieste 
Paul van der Werf, Leiden
Alain Omont, IAP
Gianfranco De Zotti, Trieste
Michal Michalowski, Edinburgh
Shane Bussmann, CfA

Science Category: cosmology
 Observing Modes: IRAC Post-Cryo Mapping
  Hours Approved: 4.0

Abstract:
Planck Early Release Compact Source Catalog (ERCSC) captures radio/sub-mm 
emission from Galactic cirrus structure to distant galaxy over densities. 
A joint study of Planck ERCSC and Herschel-SPIRE imaging overlapping in the 
GAMA survey fields of Herschel-ATLAS has led to the identification of one 
candidate proto-cluster of sub-millimeter galaxies. While ERCSC has this 
source detected with a flux of about 1 Jy, it is resolved into multiple 
sources in Herschel maps with an excess of about 20 submillimeter-bright 
galaxies over a region of 20 sq. arcmins.  The brightest source of this 
clump is gravitationally lensed and its redshift of 3.259 is now well 
determined from multiple CO lines. Most of the nearby sub-mm galaxies within 
Planck beam have similar sub-mm colors, indicating a close redshift. It is 
likely that we have detected the first example of an over-density in the 
cosmic infrared background (CIB). 

Such overdensities are expected in the hierarchical structure formation models 
and are likely to lead to proto-clusters, regions that will eventually collapse 
to form galaxy clusters seen at z < 2. Here we propose deep IRAC  imaging to 
identify the near-IR counterparts to the sub-mm sources and to identify an excess 
in the near-IR galaxy distribution, going to fainter galaxy fluxes that are expected 
below the Herschel-SPIRE confusion noise. The data will also be used to construct an 
over density map leading to a statistical study on the size, star-formation rate, 
gas consumption and other properties of this sub-mm galaxy proto-cluster. The IRAC 
imaging will be combined with planned optical and ground-based K-band imaging at 
Palomar to establish the photometric redshifts of galaxies in this region to separate 
foreground interlopers from the proto-cluster members and to reliably confirm the 
presence of a proto-cluster.