Spitzer Space Telescope - General Observer Proposal #40387


Unmasking the Strong Evolution of Cluster Starbursts

Principal Investigator: Alan Dressler
           Institution: Carnegie Institution of Washington

     Technical Contact: Alan Dressler, Carnegie Institution of Washington

Co-Investigators: 
Augustus Oemler, OCIW                                                 
Jane Rigby, OCIW                                                      
Lei Bai, Steward Observatory, Univ of Arizona                         
George Rieke, Steward Observatory, Univ of Arizona                    

Science Category: galaxy clusters and groups(high-z) 
 Observing Modes: MipsPhot 
  Hours Approved: 27.5 

Abstract:
We propose 24 um observations of galaxies in 3 rich clusters at 0.33 < z < 0.43.
We target large volumes in these clusters --- out to 5 Mpc from the cluster
center -- in order to study the star formation history of the infalling galaxies
that are building the clusters. Intermediate-redshift clusters are known to
include a large population of star-forming galaxies, unlike the passive galaxies
of today's clusters. More remarkably, these starforming galaxies are typically
star-bursting rather than continuously-star-forming, a behavior driven by either
the cluster environment, or by cosmic evolution, or both. The Spitzer MIPS
observations are essential for measuring how much star formation is hidden by
dust from our optical spectral diagnostics, and especially for identifying true
post-starburst galaxies. For the two principal clusters of our proposal, we
bring near-complete spectroscopic samples from IMACS/Magellan that provide ~2000
spectra with redshifts per field, ~250 of which are cluster or supercluster
members -- the infalling population. The complete sample allows us to compare
the cluster/supercluster population with similar ``field'' galaxies -- both
isolated and in groups. For these two cluster fields, ~1300 foreground and
background galaxies are in the redshift range of interest. These are targets for
24 um measurements as much as the cluster galaxies, since a comparison of
similar environments, in the outskirts of clusters and in the true field, will
test what effect, if any, the cluster is exerting on star forming history of
these galaxies.