Spitzer Space Telescope - Guaranteed Time Observer Proposal #40137


IRS Spectroscopy and MIPS Mapping of the Intracluster Medium in the Bullet
Cluster 1E0657-56

Principal Investigator: Philip Appleton
           Institution: California Institute of Technology

     Technical Contact: Philip Appleton, California Institute of Technology

Co-Investigators: 
James Houck, Cornell University                                       
Lee Armus, SSC-Caltech                                                
Patrick Ogle, SSC-Caltech                                             

Science Category: intermediate-z galaxies (0.05<z<0.5) 
 Observing Modes: IrsMap IrsStare MipsPhot 
  Hours Approved: 6.0 

Abstract:
We propose deep IRS and MIPS observations of the powerful X-ray cluster
1E0657-56 to map the distribution and properties of a likely cool intra-cluster
medium associated with the Bullet-like collision of a dark-matter/galaxy
sub-clump with the main cluster. Observations of 1E0657-56 (at z = 0.3) have
been interpreted as conclusive evidence that dark-matter associated with the
sub-clump interacts much more weakly with the cluster than the baryonic material
in the clump. Weak-lensing tomography shows the dark-matter (and associated
galaxies) lie ahead of a dissipative bow-shock structure seen in the
X-ray-emitting gas mapped recently by Chandra. Our proposal will test this
hypothesis further by studying the thermally important shock-coolants (large
dust grains and molecules) which we predict should be associated with the
shocked X-ray gas lagging the clump, and which is best detected in the mid and
far-IR. We exploit the large angular scale of the IRS low-res slits to perform
deep observations of 1E0657-56 to search for powerful emission from dust and
molecular gas which we expect to be important in such shocks spread over several
tens of arcsecs: the scale of the X-ray shock and filaments seen by Chandra.
The recent discovery of ultra-powerful (L > 6 x 1042 ergs/s) H2 emission in the
cluster Zwicky 3146 at a similar redshift to 1E0657-56 provides strong support
for the feasibility of this project. Our MIPS imaging of the entire cluster will
also test the idea that the dark matter/gaseous separation might affect the
process of accretion onto the most massive cluster galaxies: a process that
seems common in dynamically relaxed systems.