Spitzer Space Telescope - Directors Discretionary Time Proposal #499


Confusion-Limited 16 Micron Imaging: Completing the Spitzer Legacy

Principal Investigator: James Colbert
           Institution: Spitzer Science Center

     Technical Contact: James Colbert, Spitzer Science Center

Co-Investigators: 
Harry Teplitz, SSC
Carrie Bridge, SSC
Claudia Scarlata, SSC
Brian Siana, Caltech
Mark Dickinson, NOAO
David Elbaz, CEA Saclay
David Koo, UCSC
Andrew Phillips, UCO/Lick
Delphine Marcillac, UA
Casey Papovich, UA
Emeric Le Floc'h, University of Hawaii
Ranga Chary, SSC

Science Category: high-z galaxies
 Observing Modes: IrsPeakUpImage
  Hours Approved: 15.2

Abstract:
We propose to obtain the first (and only) significant confusion-limited 16 micron 
imaging with the Spitzer Space Telescope.  We will achieve a sensitivity 3-4 times 
deeper than any previous 16 micron observation from any facility. We will target 
the ultradeep IRAC region within GOODS-N, creating a mid-IR legacy field where 
every Spitzer instrument observed down its respective confusion limit. We will 
measure galaxy number counts at suitable depth to compare with the deepest 
measurements at 24 microns. Given the interplay between the number density, 
luminosity evolution, and spectral properties which change with obscuring material 
and gas fraction, number counts are inherently complex in the mid-IR. The 
combination of 24 and 16 micron number counts are required to disentangle the 
many potential evolutionary scenarios of the LIRGs and ULIRGs at redshifts of 
z>1.5. Reaching 16 micron depths of 10 uJy, we will for the first time be able 
to detect MIR emission of a significant percentage of z$>$1.5 LIRGs in more than 
one waveband, critical for differentiating the many LIRG/ULIRG models as well 
as identifying silicate absorption. At the highest redshifts (z>2), deep 16 micron 
will also provide a measurement of the hot-dust component of Compton-thick 
AGN candidates.